Alternative biochemistry
Encyclopedia
Hypothetical types of biochemistry are forms of biochemistry
speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on earth
commonly use carbon
for basic structural and metabolic
functions, water
as a solvent
and DNA
or RNA
to define and control their form, it is possible that undiscovered life-forms could exist that differ radically in their basic structures and biochemistry from that known to science.
The possibility of extraterrestrial life
being based on these "alternative" biochemistries is a common subject in science fiction
, but is also discussed in a non-fiction scientific context. A recent example of the non-fiction discussion is a 2007 report on life's limiting conditions prepared by a committee of scientists under the United States National Research Council
. The committee, chaired by John A. Baross, considers "hypothetical alternative chemistries of life", including a range of solvents other than water. The committee begins its discussion by raising the concern that a space agency might conduct a well-resourced search for life on other worlds "and then fail to recognize it if it is encountered".
s are almost universally of the L form and sugar
s are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids or L sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. It is questionable, however, whether such a biochemistry would be truly alien; while it is certainly an alternative stereochemistry
, molecules that are overwhelmingly found in one enantiomer
throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal
) organisms such as in comparisons between members of Archea and other domains
, making it an open topic whether an alternative stereochemistry
is truly novel.
s other than carbon
to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the molecular machinery necessary for life. Still, since human
s are carbon-based beings and have never encountered life outside the earth’s environment, excluding the possibility of all other elements may be considered carbon chauvinism
.
The most commonly proposed basis for an alternative biochemical system is the silicon
atom, since silicon has many chemical properties
similar to carbon and is in the same periodic table group
, the carbon group
. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.
However, silicon has several drawbacks as a carbon alternative. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms, which permits the chemical versatility necessary for metabolism
. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms. Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules". This is because silicon atoms are much bigger, having a larger mass
and atomic radius
, and so have difficulty forming double or triple covalent bond
s, which are important for a biochemical system.
Silane
s, which are chemical compound
s of hydrogen
and silicon that are analogous to the alkane
hydrocarbon
s, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymer
s of alternating silicon and oxygen
atoms instead of direct bonds between silicon, known collectively as silicone
s, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. Complex long-chain silicone molecules are still less stable than their carbon counterparts, though.
Another obstacle is that silicon dioxide
(a common ingredient of many sands), the analog of carbon dioxide
, is a non-soluble solid at the temperature range where water is liquid, making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. Another problem with silicon dioxide is that it would be the product of aerobic respiration. If a silicon-based life form were to breathe using oxygen, as life on Earth does, it would possibly produce silicon dioxide as a by-product
of this, assuming that the only difference between the two types of life is silicon in place of carbon. This implies that the exhaled product, silicon dioxide, would be a solid, thus filling the respiratory organs of the organism with sand. This however would be solved if the organism lives in temperatures of several hundred to thousand degrees, where the silicon dioxide becomes a liquid. Oxygen-breathing silicon life, if it exists, is therefore most likely to exist in environments with very high temperatures or pressure.
Finally, of the varieties of molecules identified in the interstellar medium
, 84 are based on carbon while only 8 are based on silicon. Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets.
Also, even though Earth
and other terrestrial planet
s are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in the Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon, though rare, has proven to be much more successful as a life base than the much more abundant silicon, may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. For example: silicon is less versatile than carbon in forming compounds; the compounds formed by silicon are unstable and it blocks the flow of heat. Even so, biogenic silica
is used by some Earth life, such as the silicate skeletal structure of diatom
s. This suggests that extraterrestrial life forms may have silicon-based structure molecules and carbon-based proteins for metabolic purposes, therefore enabling the ability to feed on a common resource on a terrestrial planet like Earth for building up the silicon-based part of their body.
Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon.
A. G. Cairns-Smith
has proposed that the first living organisms to exist on Earth were clay minerals—which were probably based on silicon.
In cinematic and literary science fiction
, a moment when man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor
in the late 1960s, these machines are often classed as computer
s (or computer-guided robot
s) and filed under "silicon
-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".
and phosphorus
also offer possibilities as the basis for biochemical molecules. Like carbon, phosphorus can form long chain molecules on its own, which would potentially allow it to form complex macromolecule
s were it not so reactive. However, in combination with nitrogen, it can form much more stable covalent bonds and create a wide range of molecules, including rings (a class of compounds called phosphazene
s).
Earth's atmosphere is approximately 78% nitrogen, but this would probably not be of much use to a phosphorus-nitrogen (P-N) life-form since molecular nitrogen (N2) is nearly inert and energetically expensive to "fix
" due to its triple bond. On the other hand, one could say that some Earth plant
s such as legumes can fix nitrogen using symbiotic bacteria
contained in their root nodules, but those bacteria have to exist before the nitrogen fixation process they perform can actually take place. On Earth, the intense temperatures created by lightning split atmospheric nitrogen in order to make it available for the first nitrogen containing organisms to use. A nitrogen dioxide
(NO2) or ammonia
(NH3) atmosphere would be more useful. Nitrogen also forms several oxides, such as nitric oxide
, nitrous oxide
, and dinitrogen tetroxide
, and all would be present in a nitrogen-dioxide-rich atmosphere.
Debate continues, as several aspects of a phosphorus-nitrogen cycle biology would be energy deficient. Also, nitrogen and phosphorus are unlikely to occur in the ratios and quantity required in the universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.
An ammoniated atmosphere would be possible and stable at first view (in a reductive environment), and this type of environment would be preferencially present on massive planets which are more likely to retain hydrogen slowing down its escape to space, and have thick atmosphere that better protect ammonia from radiations; like super-Earth
with mass in range between the Earth and the little giant planets like Uranus and Neptune. But it is doubtful that an atmosphere rich in nitrogen dioxide could even exist. Since the nitrogen oxides are all endoenergetic
compared with molecular nitrogen and oxygen; and they are oxidizing, they would decompose by stellar radiation and by catalysis on the surface of rocks when they are produced. Unlike nitrogen dioxide, the chemically similar gas nitrogen trifluoride
is not endoenergetic and is more stable, but the relative rarity of fluorine means that NF3 is unlikely to be present in large enough concentrations in any atmosphere.
s (such as mellitic anhydride
or similar) as its building blocks, instead of hydrocarbons.
as a terminal electron acceptor are known from anaerobic life forms
on Earth. However, it has been proposed that chlorine
might serve as a more general biological alternative to oxygen
, either in carbon-based biologies or hypothetical non-carbon-based ones. But chlorine is much less abundant than oxygen in the universe, and so planets with a sufficiently chlorine-rich atmosphere are likely to be rare, if they exist at all. Chlorine will instead likely be bound up as salt
s and other inert compounds.
, which is chemically similar to phosphorus
, while poisonous for most life form
s on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaine
s. Fungi and bacteria
can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryote
s can use arsenate as a terminal electron acceptor during anaerobic growth
and some can utilize arsenite as an electron donor to generate energy.
It has been speculated that the earliest life forms on Earth may have used arsenic
in place of phosphorus in the backbone of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis
than corresponding phosphate ester
s that arsenic would not be suitable for this function. The authors of a 2010 geomicrobiology
study supported in part by NASA have postulated that a bacterium, named GFAJ-1
, collected in the sediments of Mono Lake
in eastern California
, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate
or other means to reduce the effective concentration
of water and stabilize its arsenate esters.
This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls Science writer Carl Zimmer
contacted several scientists for an assessment: "I reached out to a dozen experts ... Almost unanimously, they think the NASA scientists have failed to make their case".
s, however.
as a solvent
. This has led to discussion about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia
, sulfuric acid
,
formamide
, hydrocarbon
s,
and (at temperatures much lower than Earth's) liquid nitrogen
, or hydrogen
in the form of a supercritical fluid
.
Carl Sagan
once described himself as both a carbon chauvinist and a water chauvinist;
however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist". He considered hydrocarbons, hydrofluoric acid
, and ammonia as possible alternatives to water.
Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity
useful for temperature regulation, a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion
allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water is the only compound listed here that is less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds) then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.
There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Since boiling points can vary based on the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus
, with 92 bars (9.2 MPa) of pressure, it can indeed exist in liquid form over a wide temperature range.
is perhaps the most commonly proposed alternative. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible.
However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension
to be three times smaller, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. For these reasons, some members of the scientific community question how well ammonia could hold prebiotic molecules together to allow the emergence of a self-reproducing system. Ammonia is also flammable and can be oxidized and could not exist sustainably in a biosphere that oxidizes it. It would, however, be stable in a reducing environment.
A biosphere
based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life on Earth. Terrestrial life on Earth usually exists within the melting point and boiling point
of water at normal pressure, between 0 °C (273 K
) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Such extremely cold temperatures create problems, as they slow biochemical reactions tremendously and may cause biochemical precipitation
out of solution due to high melting point
s. Ammonia could be a liquid at normal temperatures, but at much higher pressures; for example, at 60 atm
, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).
Ammonia and ammonia-water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn
's largest moon Titan
.
s, such as the methane/ethane lakes detected on Titan
by the Cassini spacecraft, could act as a solvent over a wide range of temperatures but would lack polarity
. There is debate about the effectiveness of methane as a medium for life compared to water or ammonia. While water is a far better solvent than methane, enabling easier transport of substances in a cell, methane's lesser chemical reactivity allows for the easier formation of large structures akin to proteins. Isaac Asimov
, the biochemist
and science fiction
writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or liquid hydrogen
.
Possible evidence for this form of life on Titan
was identified in 2010 by Darrell Strobel of Johns Hopkins University
; an over-abundance of molecular hydrogen in Titan's upper atmospheric layers, which leads to a downward flow at a rate of roughly 1025 molecules per second. Near the surface the hydrogen apparently disappears, which may imply its consumption by methanogenic life forms. Another paper released the same month showed little evidence of acetylene on Titan's surface, where scientists had expected the compound to accumulate; according Strobel, this is consistent with the hypothesis that acetylene is being consumed by methanogens. Chris McKay, while agreeing that presence of life is a possible explanation for the findings about hydrogen and acetylene, has cautioned that other explanations are currently more likely: namely the possibility that the results are due to human error, or to the presence of some as-yet unknown catalyst in the soil. He noted that such a catalyst, effective at 95 Kelvin, would in itself be a startling discovery.
, like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C and its boiling point is 19.54 °C (at atmospheric pressure
); the difference between the two is little more than 100 °C. HF also makes hydrogen bonds with its neighbor molecules as do water and ammonia. These would make HF a candidate to host life on other planets.
Not much research has been done on liquid HF in regards to its ability to dissolve and react with non-polar molecules. The biota in an HF ocean could use the fluorine
as an electron acceptor to photosynthesize energy.
HF is dangerous to the systems of molecules that earth-life is made of, but the paraffin
s are stable with it.
But presence of great amounts of free HF on a planetary scale, like water on Earth, is only possible outside the temperature range from liquid water and water vapour. Any free water would be react with the polar HF and would form a solution of hydrofluoric acid
. The hypothetical planet would also surely contain silicate
s that would react with HF to form inert compounds as silicon fluorides, as soon as HF would be present; thus preventing its concentration in great quantities in a hypothetical planetary environment.
The cosmic abundance of fluorine
is low, and it forms chemically inert compounds rapidly in interstellar nebulae, because it is the most reactive element.
, methanol
, hydrogen sulfide
and hydrogen chloride
. Hydrogen chloride suffers from the low cosmic abundance of chlorine
, while hydrogen sulfide suffers from its high reactivity. Moreover, the first two could not be expected to be found in vast quantities on a planetary scale, and would only be part of the internal physiology of organisms.
A proposal has been made that life on Mars
may exist and be using a mixture of water and hydrogen peroxide
as its solvent. A 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.
, or a eutectic mixture of such salts, could remain liquid at extremely high temperatures, and could theoretically act as a solvent. Life in such circumstances may be restricted to refractory materials like metal oxides. The participation of metal oxide compounds in a biochemical system is unlikely, as their molecular structures are simpler compared to organic compounds and they are largely inert.
s from organisms on Earth.
The first plants on Earth may have been a slightly different colour, because the Sun
was, in the first geological eons, a little less bright, and its light was filtered by passing through an atmosphere with a different composition.
Black is the optimum color for converting all available light to energy as efficiently as possible.
It is as of yet unclear exactly why plants on Earth are green and not black.
. Modern oxygen breathing animals would have been biochemically impossible until earlier photosynthetic life transformed Earth's atmosphere. The first dramatic rise in atmospheric oxygen on Earth, to about a tenth of its present-day value, occurred approximately 2.5 billion years ago, and that level did not change significantly until the Cambrian era approximately 600 million years ago.
Changes in the gas mixture in the atmosphere, even in an atmosphere made up predominantly of the same molecules of Earth's atmosphere, impacts the biochemistry and morphology of life. For example, periods of high oxygen concentrations determined from ice core samples have been associated with fauna of a larger scale in the fossil record, while periods associated with of low oxygen concentrations have been associated with fauna of a smaller scale in the fossil record.
Also, while it is customary to think of plants on one side of the oxygen and nitrogen cycles as being sessile, and of animals on the other side as being motile, this is not a biological imperative. There are animals which are sessile for all or most of their lives (such as corals), and there are plants (such as tumbleweed
s, and venus fly traps) that exhibit more mobility than is customarily associated with plants. On a slowly rotating planet, for example, it might be adaptive for photosynthesis to be performed by "plants" that can move to remain in the light, like Earth's sunflower
s; while non-photosynthetic "animals", much like Earth's fungi, might have a lesser need to move from place to place on their own. This would be a mirror image of Earth's ecology
.
or hibernation
state that can be sustained for years or even millennia between more active life stages. Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.
For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, while desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.
, under conditions that might exist in space. Computer models showed that when the dust became charged the particles could self-organize into microscopic helical structures capable of replicating themselves, interacting with other neighboring structures,
and evolving into more stable forms. Similar forms of life were described in Fred Hoyle
's classic novel The Black Cloud
.
's short story "Technical Error" there is an example of differing chirality
. This is not a case of alien life, rather it is an accident. A technical error in a new power plant changes the chirality of a worker. He cannot read because he sees all type as reversed, and he believes his left side is his right. The amusement ends when it is discovered that he is dying from malnutrition because his new biology cannot utilize normal food.
The concept of reversed chirality also figured prominently in the plot of James Blish
's Star Trek novel Spock Must Die!
, where a transporter experiment gone awry ends up creating a duplicate Spock who turns out to be a perfect mirror-image of the original all the way down to the atomic level.
An example of silicon based life forms takes place in the Alan Dean Foster
novel Sentenced to Prism
in which the protagonist Evan Orgell is trapped on a planet whose entire ecosystem is mostly silicon-based.
Perhaps the most extreme example in science fiction is James White
's Sector General
: a series of novels and short stories about multienvironment hospital for the strangest life-forms imaginable, some of them breathing methane, chlorine, water and sometimes also oxygen. Some of the species metabolise directly hard radiation and their environment doesn't differ much from the atmosphere of a star, while others live in near absolute zero temperatures. All life forms are classified according to their metabolism, internal and external features, and more extreme abilities (telepathy, empathy, hive mind, etc.) with four letter codes. Humans from Earth share the DBDG specification with small furry beings called Nidians.
One of the major sentient species in Terry Pratchett
's Discworld
universe are the "earth"-based (ranging from Detritus to Diamond) Trolls
.
Pratchett has also written the science fiction novel The Dark Side of the Sun
which features a range of extraordinary life-forms, including a telepathic body of water, creatures called "Sundogs", which are capable of interstellar travel from birth, and a sentient planet: effectively a giant silicon-based computer.
Fred Hoyle
's classic novel The Black Cloud
features a life form consisting of a vast cloud of interstellar dust, the individual particles of which interact via electromagnetic signalling analogous to how the individual cells of multicellular terrestrial life interact. Outside of science-fiction, life in interstellar dust has been proposed as part of the panspermia hypothesis. The low temperatures and densities of interstellar clouds would seem to imply that life processes would operate much more slowly there than on Earth. Inorganic dust-based life has been speculated upon based on recent computer simulations.
Similarly, Arthur C. Clarke's "Crusade
" revolves around a planetwide life-form based on silicon and superfluid helium located in deep intergalactic space, processing its thoughts slowly by human standards, that sends probes to look for similar life in nearby galaxies. It concludes that it needs to make planets more habitable for similar life-forms, and sends out other probes to foment supernova
e to do so. Clarke implies that this is what accounts for most supernovae having occurred in the same region of space and warns that the effort will eventually reach Earth.
Robert L. Forward's Camelot 30K
describes an ecosystem on the surface of Kuiper belt objects that is based on a fluorocarbon
chemistry with OF2
as the principal solvent instead of H2O. The organisms in this ecology keep warm by secreting a pellet of uranium-235
inside themselves and then moderating its nuclear fission using a boron
-rich carapace around it. Kuiper belt objects are known to be rich in organic compounds such as tholin
s, so some form of life existing on their surfaces is not entirely implausible–though perhaps not going so far as to develop natural internal nuclear reactors, as have Forward's. Fluorine is also of low cosmic abundance, so its use in this manner is unlikely.
In Forward's Rocheworld
series, an Earth-like biochemistry is proposed that uses a mixture of water and ammonia as its solvent. In Dragon's Egg
and Starquake
, Forward proposes life on the surface of a neutron star
utilizing "nuclear chemistry" in the degenerate matter
crust. Since such life utilised strong nuclear forces instead of electromagnetic interactions, it was posited that life might function millions of times faster than typical on Earth.
Gregory Benford
and David Brin
's Heart of the Comet
features a comet
with a conventional carbon-and-water-based ecosystem that becomes active near the perihelion when the Sun warms it. Brin's own novel Sundiver
is an example of science fiction proposing a form of life existing within the plasma atmosphere of a star
using complex self-sustaining magnetic field
s. Similar sorts of plasmoid life have sometimes been proposed to exist in other places, such as planetary ionosphere
s or the interstellar medium
, but usually only by fringe theorists (see ball lightning
for some additional discussion). Gregory Benford had a form of plasma-based life exist in the accretion disk of a primordial black hole
in his novel Eater
.
The suggestion that life could even occur within the plasma of a star has been picked up by other science fiction writers, as in David Brin's
Uplift Saga
or Frederik Pohl's
novel The World at the End of Time
. The idea is that places where reactions occur–even an incredible environment as a star–presents a possible medium for some chain of events that could produce a system able to replicate.
The Outsiders
in Larry Niven
's Known Space
universe are cryogenic
creatures based on liquid helium
. They derive thermoelectric energy from a temperature gradient by basking half their body in sunlight, keeping the other half in shadow and exposed to interstellar vacuum.
Stephen Baxter
has imagined perhaps some of the most unusual exotic life-forms in his Xeelee
series of novels and stories, including supersymmetric photino-based life that congregate in the gravity wells of stars, entities composed of quantum
wave functions, and the Qax, who thrive in any form of convection cell
s, from swamp gas to the atmospheres of gas giant
s. In his book Manifold: Space
, he also proposes natural robots, life forms made of iron
, called the Gaijin, evolving from creatures in oceans of iron carbonyl.
In his novel Diaspora
, Greg Egan
posits entire virtual universes implemented on Turing Machine
s encoded by Wang Tiles
in gargantuan polysaccharide
'carpets.' The sentient ocean that covers much of the surface of Solaris in Stanislaw Lem
's eponymous novel
also seems, from much of the fictional research quoted and discussed in the book, to be based on some element other than carbon. In the same novel Egan describes lifeforms in the 6-D 'macrosphere' which use a collapsed atom chemistry with energetic processes of the same order as nuclear reactions, due to the peculiarities of higher dimensional physics.
In her novel Brain Plague, Joan Slonczewski describes a species of intelligent microrganisms with arsenic
based chemistries that live symbiotically with human hosts.
Sergeant Schlock is one of the lead characters in the webcomic Schlock Mercenary
. His species, Carbosilicate Amorphs, evolved from self-repairing distributed data storage devices, and as such, redundantly distribute their 'brain' throughout their body. They are highly resistant to hard vacuum, explosive decompression
, projectile weapons, chemical-based explosives, and dismemberment. Their only specialty organ is their eyes, which they harvest as fruit from the Ghanj-Rho eye-tree on their home planet. While the Amorphs have the ability to move fast, quietly, and sprout appendages at will, they excel at 'closer-than-melee-range combat, primarily "meme-toxins" against other Amorphs.
A more farcical example comes from The Hitchhiker's Guide to the Galaxy
, where the Hooloovoo are a hyperintelligent shade of the colour blue.
Alien warriors recruited by the god Klael in David Eddings
' "Tamuli
" trilogy are noted by their human opponents to breathe marsh-gas (methane). Within Eddings' universe, this limits their capacity for exertion in an oxygen atmosphere, and also determines the tactics used to fight them and eventually to destroy them in their encampments.
The eponymous organism in Michael Crichton
's The Andromeda Strain
is described as reproducing via the direct conversion of energy into matter.
A well-known example of a non–carbon-based life-form in science fiction is the Horta in the original Star Trek
episode "Devil in the Dark
". A highly intelligent silicon-based creature made almost entirely of pure rock, it tunnels through rock as easily as humans move through air. The entire species dies out every 50,000 years except for one who tends for all the eggs, which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet, Janus VI. The inadvertent destruction of many of these eggs by a human mining colony led the mother Horta to respond by killing the colonists and sabotaging their equipment; it was only through a Vulcan mind meld that the race's benevolence and intelligence were discovered and peaceful relations established.
Star Trek would later offer other corporeal life-forms with an alternative biochemistry. The Tholian
s of "The Tholian Web" are depicted and described, in that episode and later in the Star Trek: Enterprise
episode "In a Mirror, Darkly
" as being primarily of mineral-based composition and thriving only in superheated conditions. Another episode from TOSs third season, "The Savage Curtain", depicted another rock creature called an Excalbian, which is believed in fanon to also have been silicon-based.
In Star Trek: The Next Generation
, the Crystalline Entity appeared in two episodes, "Datalore" and "Silicon Avatar". This was an enormous spacefaring crystal lattice
that had taken thousands of lives in its quest for energy. It may have been unaware of this, however, but it was destroyed before communications could be established at a level sufficient to ascertain it.
In another episode, "Home Soil", intelligent crystals that formed a "microbrain" were discovered during a terraforming mission, and they described the humans they encountered as "ugly bags of mostly water."
"The Disease", an episode of Star Trek: Voyager
featured some artificially-engineered silicon-based parasites, and an Enterprise episode, "Observer Effect
", also presented a lethal silicon-based virus.
In another Voyager episode, "Hope and Fear", a xenon
-based life-form was mentioned. In the Enterprise episode "The Communicator", an alien species is encountered whose blood chemistry, while not explicitly stated, is sufficiently different from terrestrial organisms that it is not red and iron is toxic to it. Various Star Trek series also had episodes featuring photonic lifeforms.
movie The Empire Strikes Back, two life-forms were encountered by the characters that were non-carbon based entities. Although details of their physiology were not mentioned on screen, the space slug, (a giant worm-like creature that lived on asteroids in the vacuum of space), and the Mynock, (pesky bat-like vermin that would attach to spaceship hulls and chew through power conduits to feed off the raw energy), are said to be silicon-based organisms in Star Wars Expanded Universe
sources. Also from The Empire Strikes Back, the bounty hunter
Zuckuss is a member of the Gand race, an ammonia-based life-form. However, it is worth noting that the Gand are divided into two subspecies, only one of which breathes at all, the other drawing all their required sustenance from food intake and producing speech by essentially modulated flatulence
.
Appearing only in the Star Wars Expanded Universe
is the spice spider of Kessel, a creature made of glitterstim spice and silicon that spun crystalline webs harvested by miners as glitterstim spice, an illegal psychoactive narcotic
. The spider used the webs to catch bogeys, tiny energy creatures that it consumed for energy.
real-time strategy
games, both the gameplay and storyline revolve heavily around the introduction to Earth of an extraterrestrial mutagen called Tiberium
via meteor, which displays strikingly lifelike behaviours such as self-replication, evolution, and homeostasis, without undergoing anything like common carbon-based metabolic cycles, and which appears to be colonising the Earth, converting it into an environment unsuited to carbon-based biology. Earth creatures (such as animals, plants and even humans) exposed to Tiberium can either be killed because of the radiation or be transformed into Tiberium-based life-forms, to whom Tiberium radiation is curative rather than toxic. It is later revealed that Tiberium was introduced to earth by the Scrin, an extremely advanced race of Tiberium-based aliens bent on mining the planet after the Tiberium deposits have reached maturity.
In the Halo
video game series, a race of Covenant aliens named "Grunts" by humans require a breathing apparatus while fighting the humans in an Earth-like atmosphere. According to the novelizations of the video game, the Grunts' apparatus allows them to breathe the methane they need to survive.
In the Master of Orion
series of space strategy games, there exists an extraterrestrial race called Silicoids, whose appearance (and presumably composition) is similar to crystalline mineral structures. The game posits that this grants them immunity to the effects of hostile environments and pollution and they require no sustenance, at the expense of impeding their reproductive rate and their ability to interact with other intelligent species.
In the Metroid Prime
series, Phazon is a highly radioactive, self-regenerating mineral with organic properties that is generated by the sentient planet Phaaze.
In Metroid Prime Hunters
, Spire is a rock-like, silicon-based alien. He is the last Diamont (presumably a play on the word diamond
, which is composed of carbon
).
In the Submarine TITANS
strategy game, the alien race in the game are called "the Silicons" because they are silicon-based life forms.
In the Star Control
series, the Chenjesu, are hyperintelligent, peaceful silicon-based life-forms that were the backbone of the Alliance of Free Stars. Their crystalline biology apparently gives them the ability to send and receive hyperwave transmissions. Also, there are the Slylandro, who are gas beings residing in the upper atmosphere of a gas giant
. As well, there are evidences of another silicon-based race, the Taalo who are described by the xenophobic
Ur-Quan as the only race to have not awakened their territorial instincts. The Taalo were also immune to mind control
In the game of Xenosaga
, artificial life forms known as Realians have been created using silicon-based chemistry. They resemble humans in every aspect, except they are considered to be lower than humans on the social ladder
.
In Mass Effect
the alien Turians and Quarians, are both based on dextro-amino acids, as opposed to the all other sentient species of the galaxy based on levo-amino acids. There are also the Volus, an ammonia based species that must wear pressure suits to survive in environments suited to the other races.
In Spore
, the Grox
refer to the player and to other alien empires as "slow thinking carbon-based lifeforms" and "carbon wads", implying that the Grox (which are at least partly machine life) are not carbon-based. Also, the Grox can only exist on barren planets which cannot support other life, and when a planet is terraformed the Grox inhabiting it die immediately. The Grox seem to gather sustenance from the radiation from the galactic core, as the Grox colonies are larger the closer they are to the galactic core.
In Muv Luv, the BETA which calls itself the "higher/superior existence" says they were created by a silicon-based being simply called "The Creator". As such, they don't consider any non-silicon-based creature to be alive, not even themselves. Its reasoning was that only silicon-based beings occur naturally and have the ability to reproduce and disperse. When the human main character, Takeru, argues that humans also have the ability to reproduce and disperse, the higher existence says carbon too easily mingles with other elements and therefore it would be impossible for a carbon-based existence to have evolved on its own. Thus, humans must be other biological machines created by a life form just as the BETA are.
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...
commonly use carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
for basic structural and metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
functions, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
as a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
and DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
or RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
to define and control their form, it is possible that undiscovered life-forms could exist that differ radically in their basic structures and biochemistry from that known to science.
The possibility of extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
being based on these "alternative" biochemistries is a common subject in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
, but is also discussed in a non-fiction scientific context. A recent example of the non-fiction discussion is a 2007 report on life's limiting conditions prepared by a committee of scientists under the United States National Research Council
United States National Research Council
The National Research Council of the USA is the working arm of the United States National Academies, carrying out most of the studies done in their names.The National Academies include:* National Academy of Sciences...
. The committee, chaired by John A. Baross, considers "hypothetical alternative chemistries of life", including a range of solvents other than water. The committee begins its discussion by raising the concern that a space agency might conduct a well-resourced search for life on other worlds "and then fail to recognize it if it is encountered".
Alternative-chirality biomolecules
Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acidAmino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s are almost universally of the L form and sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids or L sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. It is questionable, however, whether such a biochemistry would be truly alien; while it is certainly an alternative stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....
, molecules that are overwhelmingly found in one enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
) organisms such as in comparisons between members of Archea and other domains
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...
, making it an open topic whether an alternative stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....
is truly novel.
Non-carbon-based biochemistries
Scientists have speculated about the pros and cons of using atomAtom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s other than carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the molecular machinery necessary for life. Still, since human
Human
Humans are the only living species in the Homo genus...
s are carbon-based beings and have never encountered life outside the earth’s environment, excluding the possibility of all other elements may be considered carbon chauvinism
Carbon chauvinism
Carbon chauvinism is a neologism meant to disparage the assumption that the chemical processes of hypothetical extraterrestrial life must be constructed primarily from carbon , as carbon's chemical and thermodynamic properties render it far superior to all other elements.-Concept:The term was used...
.
Silicon biochemistry
The most commonly proposed basis for an alternative biochemical system is the silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
atom, since silicon has many chemical properties
Chemical property
A chemical property is any of a material's properties that becomes evident during a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity...
similar to carbon and is in the same periodic table group
Periodic table group
In chemistry, a group is a vertical column in the periodic table of the chemical elements. There are 18 groups in the standard periodic table, including the d-block elements, but excluding the f-block elements....
, the carbon group
Carbon group
The carbon group is a periodic table group consisting of carbon , silicon , germanium , tin , lead , and ununquadium ....
. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.
However, silicon has several drawbacks as a carbon alternative. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms, which permits the chemical versatility necessary for metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms. Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules". This is because silicon atoms are much bigger, having a larger mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and atomic radius
Atomic radius
The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the nucleus to the boundary of the surrounding cloud of electrons...
, and so have difficulty forming double or triple covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s, which are important for a biochemical system.
Silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
s, which are chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and silicon that are analogous to the alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s of alternating silicon and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atoms instead of direct bonds between silicon, known collectively as silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
s, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. Complex long-chain silicone molecules are still less stable than their carbon counterparts, though.
Another obstacle is that silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
(a common ingredient of many sands), the analog of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, is a non-soluble solid at the temperature range where water is liquid, making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. Another problem with silicon dioxide is that it would be the product of aerobic respiration. If a silicon-based life form were to breathe using oxygen, as life on Earth does, it would possibly produce silicon dioxide as a by-product
By-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....
of this, assuming that the only difference between the two types of life is silicon in place of carbon. This implies that the exhaled product, silicon dioxide, would be a solid, thus filling the respiratory organs of the organism with sand. This however would be solved if the organism lives in temperatures of several hundred to thousand degrees, where the silicon dioxide becomes a liquid. Oxygen-breathing silicon life, if it exists, is therefore most likely to exist in environments with very high temperatures or pressure.
Finally, of the varieties of molecules identified in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
, 84 are based on carbon while only 8 are based on silicon. Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets.
Also, even though Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and other terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
s are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in the Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon, though rare, has proven to be much more successful as a life base than the much more abundant silicon, may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. For example: silicon is less versatile than carbon in forming compounds; the compounds formed by silicon are unstable and it blocks the flow of heat. Even so, biogenic silica
Biogenic silica
Biogenic silica , also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. Silica is an amorphous metal oxide formed by complex inorganic polymerization processes. This is opposed to the other major biogenic minerals, comprising...
is used by some Earth life, such as the silicate skeletal structure of diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s. This suggests that extraterrestrial life forms may have silicon-based structure molecules and carbon-based proteins for metabolic purposes, therefore enabling the ability to feed on a common resource on a terrestrial planet like Earth for building up the silicon-based part of their body.
Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon.
A. G. Cairns-Smith
Graham Cairns-Smith
Alexander Graham Cairns-Smith is an organic chemist and molecular biologist at the University of Glasgow, most famous for his controversial 1985 book, Seven Clues to the Origin of Life...
has proposed that the first living organisms to exist on Earth were clay minerals—which were probably based on silicon.
In cinematic and literary science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
, a moment when man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
in the late 1960s, these machines are often classed as computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s (or computer-guided robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
s) and filed under "silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".
Nitrogen and phosphorus biochemistry
NitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
also offer possibilities as the basis for biochemical molecules. Like carbon, phosphorus can form long chain molecules on its own, which would potentially allow it to form complex macromolecule
Macromolecule
A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
s were it not so reactive. However, in combination with nitrogen, it can form much more stable covalent bonds and create a wide range of molecules, including rings (a class of compounds called phosphazene
Phosphazene
Phosphazenes are a class of chemical compounds in which a phosphorus atom is covalently linked to a nitrogen atom by a double bond and to three other atoms or radicals by single bonds. While other substitutions produce relatively persistent compounds, in organic synthesis the term largely refers to...
s).
Earth's atmosphere is approximately 78% nitrogen, but this would probably not be of much use to a phosphorus-nitrogen (P-N) life-form since molecular nitrogen (N2) is nearly inert and energetically expensive to "fix
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
" due to its triple bond. On the other hand, one could say that some Earth plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s such as legumes can fix nitrogen using symbiotic bacteria
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...
contained in their root nodules, but those bacteria have to exist before the nitrogen fixation process they perform can actually take place. On Earth, the intense temperatures created by lightning split atmospheric nitrogen in order to make it available for the first nitrogen containing organisms to use. A nitrogen dioxide
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...
(NO2) or ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3) atmosphere would be more useful. Nitrogen also forms several oxides, such as nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
, nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
, and dinitrogen tetroxide
Dinitrogen tetroxide
Dinitrogen tetroxide is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide; some call this mixture dinitrogen tetroxide, while some call it nitrogen dioxide.Dinitrogen tetroxide is a powerful oxidizer, making it highly...
, and all would be present in a nitrogen-dioxide-rich atmosphere.
- In a nitrogen dioxide atmosphere, P-N plant analogues could absorb nitrogen dioxide from the air and phosphorus from the ground. The nitrogen dioxide would be reduced, with analogues to sugarSugarSugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
being produced in the process, and waste oxygen would be released into the atmosphere. Animals based on phosphorus and nitrogen would consume the plants, use atmospheric oxygen to metabolize the sugar analogues, exhaling nitrogen dioxide and depositing phosphorus, or phosphorus-rich material, as solid waste. - In an ammonia atmosphere, P-N plants would absorb ammonia from the air and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and from the known biochemistry of Earth. It would be analogous to Earth's atmospheric carbon supply being in the form of methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
instead of carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
.
Debate continues, as several aspects of a phosphorus-nitrogen cycle biology would be energy deficient. Also, nitrogen and phosphorus are unlikely to occur in the ratios and quantity required in the universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.
An ammoniated atmosphere would be possible and stable at first view (in a reductive environment), and this type of environment would be preferencially present on massive planets which are more likely to retain hydrogen slowing down its escape to space, and have thick atmosphere that better protect ammonia from radiations; like super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...
with mass in range between the Earth and the little giant planets like Uranus and Neptune. But it is doubtful that an atmosphere rich in nitrogen dioxide could even exist. Since the nitrogen oxides are all endoenergetic
Bond energy
In chemistry, bond energy is the measure of bond strength in a chemical bond. It is the heat required to break one Mole of molecules into their individual atoms. For example, the carbon-hydrogen bond energy in methane E is the enthalpy change involved with breaking up one molecule of methane into...
compared with molecular nitrogen and oxygen; and they are oxidizing, they would decompose by stellar radiation and by catalysis on the surface of rocks when they are produced. Unlike nitrogen dioxide, the chemically similar gas nitrogen trifluoride
Nitrogen trifluoride
Nitrogen trifluoride is the inorganic compound with the formula NF3. This nitrogen-fluorine compound is a colorless, toxic, odourless, nonflammable gas. It finds increasing use as an etchant in microelectronics.-Applications:...
is not endoenergetic and is more stable, but the relative rarity of fluorine means that NF3 is unlikely to be present in large enough concentrations in any atmosphere.
Other exotic element-based biochemistries
- BoronBoronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
's chemistry is possibly even more variable than that of carbonCarbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, since it has the ability to form polyhedral clusters and three-center two-electron bondThree-center two-electron bondA three-center two-electron bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a...
s. Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. However, boron's low cosmic abundance makes it less likely as a base for life than carbon. - Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds; the heteropoly acidHeteropoly acidA heteropoly acid is a class of acid made up of a particular combination of hydrogen and oxygen with certain metals and non-metals. This type of acid is frequently used as a re-usable acid catalyst in chemical reactions....
s are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconiaCubic zirconiaCubic zirconia is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate...
). TitaniumTitaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, aluminum, magnesiumMagnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and ironIronIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
are all more abundant in the Earth's crust than carbon. Metal oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely. - SulfurSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
is also able to form long-chain molecules, but suffers from the same high reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely theoretical, especially since sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen. Sulfur-reducing bacteriaSulfur-reducing bacteriaSulfur-reducing bacteria get their energy by reducing elemental sulfur to hydrogen sulfide. They couple this reaction with the oxidation of acetate, succinate or other organic compounds....
can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfideHydrogen sulfideHydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
.)
Carbon-based alternatives to hydrocarbons
In hydrogen-depleted or strongly oxidizing environments, a carbon-based biology in which little to no hydrogen is used might be a possibility. Such "hydrogenless life" would use oxocarbonOxocarbon
An oxocarbon or oxide of carbon is an inorganic compound consisting only of carbon and oxygen.The simplest and most common oxocarbons are carbon monoxide and carbon dioxide...
s (such as mellitic anhydride
Mellitic anhydride
Mellitic anhydride, anhydride of mellitic acid, is organic compound with formula C12O9.Mellitic anhydride is oxide of carbon , like CO2, CO, and C3O2. It is white sublimable solid, apparently obtained by Liebig and Wöhler in 1830 in their study of mellite , who assigned it the formula C4O3. The...
or similar) as its building blocks, instead of hydrocarbons.
Chlorine as an alternative to oxygen
A number of alternatives to molecular oxygenAnaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...
as a terminal electron acceptor are known from anaerobic life forms
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...
on Earth. However, it has been proposed that chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
might serve as a more general biological alternative to oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, either in carbon-based biologies or hypothetical non-carbon-based ones. But chlorine is much less abundant than oxygen in the universe, and so planets with a sufficiently chlorine-rich atmosphere are likely to be rare, if they exist at all. Chlorine will instead likely be bound up as salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s and other inert compounds.
Arsenic as an alternative to phosphorus
ArsenicArsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, which is chemically similar to phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, while poisonous for most life form
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaine
Arsenobetaine
Arsenobetaine is an organoarsenic compound that is the main source of arsenic found in fish. It is the arsenic analog of trimethylglycine, commonly known as betaine...
s. Fungi and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s can use arsenate as a terminal electron acceptor during anaerobic growth
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...
and some can utilize arsenite as an electron donor to generate energy.
It has been speculated that the earliest life forms on Earth may have used arsenic
Arsenic biochemistry
Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element on the earth's crust, and although many arsenic compounds are often considered highly toxic, a wide variety of organoarsenic compounds are produced...
in place of phosphorus in the backbone of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
than corresponding phosphate ester
Organophosphate
An organophosphate is the general name for esters of phosphoric acid. Phosphates are probably the most pervasive organophosphorus compounds. Many of the most important biochemicals are organophosphates, including DNA and RNA as well as many cofactors that are essential for life...
s that arsenic would not be suitable for this function. The authors of a 2010 geomicrobiology
Geomicrobiology
Geomicrobiology is the result of the combination of geology and microbiology. The field of geomicrobiology concerns the role of microbe and microbial processes in geological and geochemical processes and vice-versa...
study supported in part by NASA have postulated that a bacterium, named GFAJ-1
GFAJ-1
GFAJ-1 is a strain of rod-shaped bacterium in the family Halomonadaceae. The extremophile was isolated from the hypersaline and alkaline Mono Lake in eastern California by a research team led by NASA astrobiologist Felisa Wolfe-Simon...
, collected in the sediments of Mono Lake
Mono Lake
Mono Lake is a large, shallow saline lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in a basin that has no outlet to the ocean...
in eastern California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate
Polyhydroxybutyrate
Polyhydroxybutyrate is a polyhydroxyalkanoate , a polymer belonging to the polyesters class that was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne. PHB is produced by microorganisms apparently in response to conditions of physiological stress...
or other means to reduce the effective concentration
Activity (chemistry)
In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...
of water and stabilize its arsenate esters.
This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls Science writer Carl Zimmer
Carl Zimmer
Carl Zimmer is a popular science writer and blogger, especially regarding the study of evolution and parasites. He has written several books and contributes science essays to publications such as The New York Times and Discover...
contacted several scientists for an assessment: "I reached out to a dozen experts ... Almost unanimously, they think the NASA scientists have failed to make their case".
Selenium or tellurium as an alternative to sulfur
Some organisms are already known to feature selenoproteins, in which sulfur is replaced by selenium. Some fungi also can produce telluro-methionine and telluro-cysteine. No organism has yet been shown to completely replace sulfur with the heavier chalcogenChalcogen
The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...
s, however.
Non-water solvents
In addition to carbon compounds, all currently known terrestrial life also requires waterWater
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
as a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
. This has led to discussion about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
,
formamide
Formamide
Formamide, also known as methanamide, is an amide derived from formic acid. It is a clear liquid which is miscible with water and has an ammonia-like odor. It is used primarily for manufacturing sulfa drugs and synthesizing vitamins and as a softener for paper and fiber...
, hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s,
and (at temperatures much lower than Earth's) liquid nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, or hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
in the form of a supercritical fluid
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...
.
Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...
once described himself as both a carbon chauvinist and a water chauvinist;
however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist". He considered hydrocarbons, hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
, and ammonia as possible alternatives to water.
Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...
useful for temperature regulation, a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water is the only compound listed here that is less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds) then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.
There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Since boiling points can vary based on the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, with 92 bars (9.2 MPa) of pressure, it can indeed exist in liquid form over a wide temperature range.
Ammonia
AmmoniaAmmonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
is perhaps the most commonly proposed alternative. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible.
However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
to be three times smaller, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. For these reasons, some members of the scientific community question how well ammonia could hold prebiotic molecules together to allow the emergence of a self-reproducing system. Ammonia is also flammable and can be oxidized and could not exist sustainably in a biosphere that oxidizes it. It would, however, be stable in a reducing environment.
A biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life on Earth. Terrestrial life on Earth usually exists within the melting point and boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of water at normal pressure, between 0 °C (273 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Such extremely cold temperatures create problems, as they slow biochemical reactions tremendously and may cause biochemical precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
out of solution due to high melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
s. Ammonia could be a liquid at normal temperatures, but at much higher pressures; for example, at 60 atm
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).
Ammonia and ammonia-water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's largest moon Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
.
Methane
A mixture of hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s, such as the methane/ethane lakes detected on Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
by the Cassini spacecraft, could act as a solvent over a wide range of temperatures but would lack polarity
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
. There is debate about the effectiveness of methane as a medium for life compared to water or ammonia. While water is a far better solvent than methane, enabling easier transport of substances in a cell, methane's lesser chemical reactivity allows for the easier formation of large structures akin to proteins. Isaac Asimov
Isaac Asimov
Isaac Asimov was an American author and professor of biochemistry at Boston University, best known for his works of science fiction and for his popular science books. Asimov was one of the most prolific writers of all time, having written or edited more than 500 books and an estimated 90,000...
, the biochemist
Biochemist
Biochemists are scientists who are trained in biochemistry. Typical biochemists study chemical processes and chemical transformations in living organisms. The prefix of "bio" in "biochemist" can be understood as a fusion of "biological chemist."-Role:...
and science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
.
Possible evidence for this form of life on Titan
Life on Titan
Whether or not there is life on Titan, the largest moon of Saturn, is at present an open question and a topic of scientific discussion and research....
was identified in 2010 by Darrell Strobel of Johns Hopkins University
Johns Hopkins University
The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...
; an over-abundance of molecular hydrogen in Titan's upper atmospheric layers, which leads to a downward flow at a rate of roughly 1025 molecules per second. Near the surface the hydrogen apparently disappears, which may imply its consumption by methanogenic life forms. Another paper released the same month showed little evidence of acetylene on Titan's surface, where scientists had expected the compound to accumulate; according Strobel, this is consistent with the hypothesis that acetylene is being consumed by methanogens. Chris McKay, while agreeing that presence of life is a possible explanation for the findings about hydrogen and acetylene, has cautioned that other explanations are currently more likely: namely the possibility that the results are due to human error, or to the presence of some as-yet unknown catalyst in the soil. He noted that such a catalyst, effective at 95 Kelvin, would in itself be a startling discovery.
Hydrogen fluoride
Hydrogen fluorideHydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
, like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C and its boiling point is 19.54 °C (at atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
); the difference between the two is little more than 100 °C. HF also makes hydrogen bonds with its neighbor molecules as do water and ammonia. These would make HF a candidate to host life on other planets.
Not much research has been done on liquid HF in regards to its ability to dissolve and react with non-polar molecules. The biota in an HF ocean could use the fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
as an electron acceptor to photosynthesize energy.
HF is dangerous to the systems of molecules that earth-life is made of, but the paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
s are stable with it.
But presence of great amounts of free HF on a planetary scale, like water on Earth, is only possible outside the temperature range from liquid water and water vapour. Any free water would be react with the polar HF and would form a solution of hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
. The hypothetical planet would also surely contain silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s that would react with HF to form inert compounds as silicon fluorides, as soon as HF would be present; thus preventing its concentration in great quantities in a hypothetical planetary environment.
The cosmic abundance of fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
is low, and it forms chemically inert compounds rapidly in interstellar nebulae, because it is the most reactive element.
Other solvents or cosolvents
Other solvents sometimes proposed include formamideFormamide
Formamide, also known as methanamide, is an amide derived from formic acid. It is a clear liquid which is miscible with water and has an ammonia-like odor. It is used primarily for manufacturing sulfa drugs and synthesizing vitamins and as a softener for paper and fiber...
, methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
and hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
. Hydrogen chloride suffers from the low cosmic abundance of chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, while hydrogen sulfide suffers from its high reactivity. Moreover, the first two could not be expected to be found in vast quantities on a planetary scale, and would only be part of the internal physiology of organisms.
A proposal has been made that life on Mars
Life on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...
may exist and be using a mixture of water and hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
as its solvent. A 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.
Molten salts
A molten saltSalt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
, or a eutectic mixture of such salts, could remain liquid at extremely high temperatures, and could theoretically act as a solvent. Life in such circumstances may be restricted to refractory materials like metal oxides. The participation of metal oxide compounds in a biochemical system is unlikely, as their molecular structures are simpler compared to organic compounds and they are largely inert.
Non-green photosynthesizers
Physicists have noted that, while photosynthesis on Earth generally involves green plants, a variety of other colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than that received on Earth. These studies indicate that while blue photosynthetic plants would be unlikely (because absorbed blue light provides some of the highest photosynthetic yields in the light spectrum), yellow or red plants are plausible. These conclusions are based in part on the luminosity spectra of different types of stars, the transmission characteristics of hypothetical planetary atmospheres, and the absorption spectra of various photosynthetic pigmentPhotosynthetic pigment
A photosynthetic pigment is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis.- Plants :...
s from organisms on Earth.
The first plants on Earth may have been a slightly different colour, because the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
was, in the first geological eons, a little less bright, and its light was filtered by passing through an atmosphere with a different composition.
Black is the optimum color for converting all available light to energy as efficiently as possible.
It is as of yet unclear exactly why plants on Earth are green and not black.
Alternative atmospheres
The gases present in the atmosphere on Earth have varied greatly over its history. Traditional plant photosynthesis has terraformed the atmosphere by sequestering carbon from carbon dioxide, increasing the proportion of molecular oxygen, and by participating in the nitrogen cycleNitrogen cycle
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...
. Modern oxygen breathing animals would have been biochemically impossible until earlier photosynthetic life transformed Earth's atmosphere. The first dramatic rise in atmospheric oxygen on Earth, to about a tenth of its present-day value, occurred approximately 2.5 billion years ago, and that level did not change significantly until the Cambrian era approximately 600 million years ago.
Changes in the gas mixture in the atmosphere, even in an atmosphere made up predominantly of the same molecules of Earth's atmosphere, impacts the biochemistry and morphology of life. For example, periods of high oxygen concentrations determined from ice core samples have been associated with fauna of a larger scale in the fossil record, while periods associated with of low oxygen concentrations have been associated with fauna of a smaller scale in the fossil record.
Also, while it is customary to think of plants on one side of the oxygen and nitrogen cycles as being sessile, and of animals on the other side as being motile, this is not a biological imperative. There are animals which are sessile for all or most of their lives (such as corals), and there are plants (such as tumbleweed
Tumbleweed
A tumbleweed is the above-ground part of a plant that, once mature and dry, disengages from the root and tumbles away in the wind. Usually, the tumbleweed is the entire plant apart from the roots, but in a few species it is a flower cluster. The tumbleweed habit is most common in steppe and desert...
s, and venus fly traps) that exhibit more mobility than is customarily associated with plants. On a slowly rotating planet, for example, it might be adaptive for photosynthesis to be performed by "plants" that can move to remain in the light, like Earth's sunflower
Sunflower
Sunflower is an annual plant native to the Americas. It possesses a large inflorescence . The sunflower got its name from its huge, fiery blooms, whose shape and image is often used to depict the sun. The sunflower has a rough, hairy stem, broad, coarsely toothed, rough leaves and circular heads...
s; while non-photosynthetic "animals", much like Earth's fungi, might have a lesser need to move from place to place on their own. This would be a mirror image of Earth's ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
.
Variable environments
Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a sporeSpore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
or hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
state that can be sustained for years or even millennia between more active life stages. Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.
For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, while desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.
Dust and plasma-based
In 2007 V. N. Tsytovich and colleagues proposed that life-like behaviors could be exhibited by dust particles suspended in a plasmaPlasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, under conditions that might exist in space. Computer models showed that when the dust became charged the particles could self-organize into microscopic helical structures capable of replicating themselves, interacting with other neighboring structures,
and evolving into more stable forms. Similar forms of life were described in Fred Hoyle
Fred Hoyle
Sir Fred Hoyle FRS was an English astronomer and mathematician noted primarily for his contribution to the theory of stellar nucleosynthesis and his often controversial stance on other cosmological and scientific matters—in particular his rejection of the "Big Bang" theory, a term originally...
's classic novel The Black Cloud
The Black Cloud
The Black Cloud is a science fiction novel written by astrophysicist Sir Fred Hoyle. Published in 1957, the book details the arrival of an enormous cloud of gas that enters the solar system and threatens to destroy most of the life on Earth by blocking the Sun's radiation.-Plot summary:In 1964,...
.
Usage in fiction
In the realm of science fiction, there have occasionally been forms of life proposed that, while often highly speculative and unsupported by rigorous theoretical examination, are nevertheless interesting and in some cases even plausible.Novels, short stories and comics
In Arthur C. ClarkeArthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...
's short story "Technical Error" there is an example of differing chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
. This is not a case of alien life, rather it is an accident. A technical error in a new power plant changes the chirality of a worker. He cannot read because he sees all type as reversed, and he believes his left side is his right. The amusement ends when it is discovered that he is dying from malnutrition because his new biology cannot utilize normal food.
The concept of reversed chirality also figured prominently in the plot of James Blish
James Blish
James Benjamin Blish was an American author of fantasy and science fiction. Blish also wrote literary criticism of science fiction using the pen-name William Atheling, Jr.-Biography:...
's Star Trek novel Spock Must Die!
Spock Must Die!
Spock Must Die! is a Star Trek novel by James Blish released in 1970. It was published by Bantam Books. It is notable as the second of hundreds of original novels to be based upon the Star Trek franchise....
, where a transporter experiment gone awry ends up creating a duplicate Spock who turns out to be a perfect mirror-image of the original all the way down to the atomic level.
An example of silicon based life forms takes place in the Alan Dean Foster
Alan Dean Foster
Alan Dean Foster is an American author of fantasy and science fiction. He currently resides in Prescott, Arizona, with his wife, and is also known for his novelizations of film scripts...
novel Sentenced to Prism
Sentenced to Prism
Sentenced to Prism is a science fiction novel written by Alan Dean Foster, and is a stand-alone entry in his Humanx Commonwealth series of books...
in which the protagonist Evan Orgell is trapped on a planet whose entire ecosystem is mostly silicon-based.
Perhaps the most extreme example in science fiction is James White
James White (author)
James White was a Northern Irish author of science fiction novellas, short stories and novels. He was born in Belfast and returned there after spending his early years in Canada. After a few years in the clothing industry, he worked at Short Brothers Ltd. from 1965 until taking early retirement in...
's Sector General
Sector General
Sector General is a series of twelve science fiction books and various short stories by the Northern Irish author James White. The series derives its name from the setting of the majority of the books, the Sector 12 General Hospital, a huge hospital space station located in deep space, designed to...
: a series of novels and short stories about multienvironment hospital for the strangest life-forms imaginable, some of them breathing methane, chlorine, water and sometimes also oxygen. Some of the species metabolise directly hard radiation and their environment doesn't differ much from the atmosphere of a star, while others live in near absolute zero temperatures. All life forms are classified according to their metabolism, internal and external features, and more extreme abilities (telepathy, empathy, hive mind, etc.) with four letter codes. Humans from Earth share the DBDG specification with small furry beings called Nidians.
One of the major sentient species in Terry Pratchett
Terry Pratchett
Sir Terence David John "Terry" Pratchett, OBE is an English novelist, known for his frequently comical work in the fantasy genre. He is best known for his popular and long-running Discworld series of comic fantasy novels...
's Discworld
Discworld (world)
The Discworld is the fictional setting for all of Terry Pratchett's Discworld fantasy novels. It consists of a large disc resting on the backs of four huge elephants which are in turn standing on the back of an enormous turtle, named Great A'Tuin as it slowly swims...
universe are the "earth"-based (ranging from Detritus to Diamond) Trolls
Troll (Discworld)
Trolls in Terry Pratchett's Discworld novels, unlike the monstrous trolls of folklore and J. R. R. Tolkien, have been subverted into a moderately civilised race. Trolls on the Discworld are, essentially, living, mobile rocks...
.
Pratchett has also written the science fiction novel The Dark Side of the Sun
The Dark Side of the Sun
The Dark Side of the Sun is a science fiction novel by Terry Pratchett, first published in 1976. It is similar to the work of Isaac Asimov. According to Don D'Ammassa, both this and Pratchett's 1981 sci-fi novel Strata are spoofing parts of Larry Niven's Ringworld...
which features a range of extraordinary life-forms, including a telepathic body of water, creatures called "Sundogs", which are capable of interstellar travel from birth, and a sentient planet: effectively a giant silicon-based computer.
Fred Hoyle
Fred Hoyle
Sir Fred Hoyle FRS was an English astronomer and mathematician noted primarily for his contribution to the theory of stellar nucleosynthesis and his often controversial stance on other cosmological and scientific matters—in particular his rejection of the "Big Bang" theory, a term originally...
's classic novel The Black Cloud
The Black Cloud
The Black Cloud is a science fiction novel written by astrophysicist Sir Fred Hoyle. Published in 1957, the book details the arrival of an enormous cloud of gas that enters the solar system and threatens to destroy most of the life on Earth by blocking the Sun's radiation.-Plot summary:In 1964,...
features a life form consisting of a vast cloud of interstellar dust, the individual particles of which interact via electromagnetic signalling analogous to how the individual cells of multicellular terrestrial life interact. Outside of science-fiction, life in interstellar dust has been proposed as part of the panspermia hypothesis. The low temperatures and densities of interstellar clouds would seem to imply that life processes would operate much more slowly there than on Earth. Inorganic dust-based life has been speculated upon based on recent computer simulations.
Similarly, Arthur C. Clarke's "Crusade
Crusade (short story)
"Crusade" is a short story by Arthur C. Clarke published in 1968 and republished in The Wind from the Sun as well as The Collected Stories of Arthur C. Clarke....
" revolves around a planetwide life-form based on silicon and superfluid helium located in deep intergalactic space, processing its thoughts slowly by human standards, that sends probes to look for similar life in nearby galaxies. It concludes that it needs to make planets more habitable for similar life-forms, and sends out other probes to foment supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e to do so. Clarke implies that this is what accounts for most supernovae having occurred in the same region of space and warns that the effort will eventually reach Earth.
Robert L. Forward's Camelot 30K
Camelot 30K
Camelot 30K is a hard science fiction novel written by the United States physicist Robert L. Forward. It was published in 1993 by Tor Books. The story mainly deals with the concept of human contact and interaction with a kingdom of intelligent alien life that dwells on a frozen world where the...
describes an ecosystem on the surface of Kuiper belt objects that is based on a fluorocarbon
Fluorocarbon
Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...
chemistry with OF2
Oxygen difluoride
Oxygen difluoride is the chemical compound with the formula F2O. As predicted by VSEPR theory, the molecule adopts a "V" shaped structure like H2O, but it has very different properties, being a strong oxidizer.-Preparation:...
as the principal solvent instead of H2O. The organisms in this ecology keep warm by secreting a pellet of uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
inside themselves and then moderating its nuclear fission using a boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
-rich carapace around it. Kuiper belt objects are known to be rich in organic compounds such as tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...
s, so some form of life existing on their surfaces is not entirely implausible–though perhaps not going so far as to develop natural internal nuclear reactors, as have Forward's. Fluorine is also of low cosmic abundance, so its use in this manner is unlikely.
In Forward's Rocheworld
Rocheworld
Rocheworld is a science fiction novel by Robert Forward in which he uses a light sail propulsion system to set the crew on an interstellar mission. The spaceship and crew of 20 have to travel 5.9 light-years Rocheworld (first published in serial form in 1982; first book publication, under the...
series, an Earth-like biochemistry is proposed that uses a mixture of water and ammonia as its solvent. In Dragon's Egg
Dragon's Egg
Dragon's Egg is a hard science fiction novel written by Robert L. Forward and published in 1980. In the story, Dragon's Egg is a neutron star with a surface gravity 67 billion times that of Earth, and inhabited by cheela, intelligent creatures that have the volume of sesame seeds and live a...
and Starquake
Starquake (book)
Starquake is a science fiction novel written by Robert L. Forward and published in 1989 . The novel is about the life of the Cheela civilization, creatures who live on a neutron star named Dragon's Egg, struggling to recover from a disastrous starquake.-Plot introduction:This story begins at the...
, Forward proposes life on the surface of a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
utilizing "nuclear chemistry" in the degenerate matter
Degenerate matter
Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle...
crust. Since such life utilised strong nuclear forces instead of electromagnetic interactions, it was posited that life might function millions of times faster than typical on Earth.
Gregory Benford
Gregory Benford
Gregory Benford is an American science fiction author and astrophysicist who is on the faculty of the Department of Physics and Astronomy at the University of California, Irvine...
and David Brin
David Brin
Glen David Brin, Ph.D. is an American scientist and award-winning author of science fiction. He has received the Hugo, Locus, Campbell and Nebula Awards.-Biography:...
's Heart of the Comet
Heart of the Comet
Heart of the Comet is a novel by David Brin and Gregory Benford about human space travel to Comet Halley published in 1986. Its publication coincided with the comet's 1986 approach to the Earth....
features a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
with a conventional carbon-and-water-based ecosystem that becomes active near the perihelion when the Sun warms it. Brin's own novel Sundiver
Sundiver
Sundiver is a 1980 science fiction novel by David Brin. It is the first book of his Uplift trilogy, and was followed by the Hugo and Nebula award winning novel Startide Rising in 1983.-Plot summary:...
is an example of science fiction proposing a form of life existing within the plasma atmosphere of a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
using complex self-sustaining magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s. Similar sorts of plasmoid life have sometimes been proposed to exist in other places, such as planetary ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
s or the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
, but usually only by fringe theorists (see ball lightning
Ball lightning
Ball lightning is an unexplained atmospheric electrical phenomenon. The term refers to reports of luminous, usually spherical objects which vary from pea-sized to several metres in diameter. It is usually associated with thunderstorms, but lasts considerably longer than the split-second flash of a...
for some additional discussion). Gregory Benford had a form of plasma-based life exist in the accretion disk of a primordial black hole
Primordial black hole
A primordial black hole is a hypothetical type of black hole that is formed not by the gravitational collapse of a large star but by the extreme density of matter present during the universe's early expansion....
in his novel Eater
Eater (novel)
Eater is a hard science fiction novel written by UC- Irvine physics professor Gregory Benford. It was published in May 2000 by Eos. Heavy on the physics information, Eater describes humankind's encounter with a cosmic intelligence that comes in the form of a small black hole.-Plot summary:In the...
.
The suggestion that life could even occur within the plasma of a star has been picked up by other science fiction writers, as in David Brin's
David Brin
Glen David Brin, Ph.D. is an American scientist and award-winning author of science fiction. He has received the Hugo, Locus, Campbell and Nebula Awards.-Biography:...
Uplift Saga
Uplift Universe
The Uplift Universe is a fictional universe created by science fiction writer David Brin. A central feature in this universe is the process of biological uplift.His books which take place in this universe:* Sundiver...
or Frederik Pohl's
Frederik Pohl
Frederik George Pohl, Jr. is an American science fiction writer, editor and fan, with a career spanning over seventy years — from his first published work, "Elegy to a Dead Planet: Luna" , to his most recent novel, All the Lives He Led .He won the National Book Award in 1980 for his novel Jem...
novel The World at the End of Time
The World at the End of Time
World at the End of Time is a 1990 hard science fiction novel by Frederik Pohl. It tells the parallel stories of a human and a plasma-based intelligence who manage to survive to the time near the heat death of the universe...
. The idea is that places where reactions occur–even an incredible environment as a star–presents a possible medium for some chain of events that could produce a system able to replicate.
The Outsiders
Outsider (Known Space)
The Outsiders are a fictional alien race in Larry Niven's Known Space series. They are many-limbed beings that are invariably described as a cat o'nine tails with a fattened handle...
in Larry Niven
Larry Niven
Laurence van Cott Niven / ˈlæri ˈnɪvən/ is an American science fiction author. His best-known work is Ringworld , which received Hugo, Locus, Ditmar, and Nebula awards. His work is primarily hard science fiction, using big science concepts and theoretical physics...
's Known Space
Known Space
Known Space is the fictional setting of some dozen science fiction novels and several collections of short stories written by author Larry Niven. It has also in part been used as a shared universe in the Man-Kzin Wars spin-off anthologies sub-series....
universe are cryogenic
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
creatures based on liquid helium
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
. They derive thermoelectric energy from a temperature gradient by basking half their body in sunlight, keeping the other half in shadow and exposed to interstellar vacuum.
Stephen Baxter
Stephen Baxter
Stephen Baxter is a prolific British hard science fiction author. He has degrees in mathematics and engineering.- Writing style :...
has imagined perhaps some of the most unusual exotic life-forms in his Xeelee
Xeelee
The Xeelee are a fictional hyperadvanced species from Stephen Baxter's Xeelee Sequence. They were first remotely mentioned in the 1994 novel Timelike Infinity and were later central actors of several novels and a substantial number of short stories...
series of novels and stories, including supersymmetric photino-based life that congregate in the gravity wells of stars, entities composed of quantum
Quantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...
wave functions, and the Qax, who thrive in any form of convection cell
Convection cell
A convection cell is a phenomenon of fluid dynamics that occurs in situations where there are density differences within a body of liquid or gas. The convection usually requires a gravitational field but in microgravity experiments, thermal convection has been observed without gravitational effects...
s, from swamp gas to the atmospheres of gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s. In his book Manifold: Space
Manifold: Space
Manifold: Space is a science fiction book by author Stephen Baxter, first published in the United Kingdom in 2000, then released in the United States in 2001. It is the second book of the Manifold series and examines another possible solution to the Fermi paradox...
, he also proposes natural robots, life forms made of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, called the Gaijin, evolving from creatures in oceans of iron carbonyl.
In his novel Diaspora
Diaspora (novel)
Diaspora, a hard science fiction novel by the Australian writer Greg Egan, first appeared in print in 1997.-Plot introduction:This novel's setting is a posthuman future, in which transhumanism long ago became the default philosophy embraced by the vast majority of human cultures.The novel began as...
, Greg Egan
Greg Egan
Greg Egan is an Australian science fiction author.Egan published his first work in 1983. He specialises in hard science fiction stories with mathematical and quantum ontology themes, including the nature of consciousness...
posits entire virtual universes implemented on Turing Machine
Turing machine
A Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...
s encoded by Wang Tiles
Wang tile
Wang tiles , first proposed by mathematician, logician, and philosopher Hao Wang in 1961, are a class of formal systems...
in gargantuan polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
'carpets.' The sentient ocean that covers much of the surface of Solaris in Stanislaw Lem
Stanislaw Lem
Stanisław Lem was a Polish writer of science fiction, philosophy and satire. He was named a Knight of the Order of the White Eagle. His books have been translated into 41 languages and have sold over 27 million copies. He is perhaps best known as the author of the 1961 novel Solaris, which has...
's eponymous novel
Solaris (novel)
Solaris is a 1961 Polish science fiction novel by Stanisław Lem. It is about the ultimate inadequacy of communication between human and non-human species....
also seems, from much of the fictional research quoted and discussed in the book, to be based on some element other than carbon. In the same novel Egan describes lifeforms in the 6-D 'macrosphere' which use a collapsed atom chemistry with energetic processes of the same order as nuclear reactions, due to the peculiarities of higher dimensional physics.
In her novel Brain Plague, Joan Slonczewski describes a species of intelligent microrganisms with arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
based chemistries that live symbiotically with human hosts.
Sergeant Schlock is one of the lead characters in the webcomic Schlock Mercenary
Schlock Mercenary
Schlock Mercenary is a comedic webcomic written and drawn by Howard Tayler. It follows the tribulations of a star-travelling mercenary company in a satiric, mildly dystopian 31st-century space opera setting...
. His species, Carbosilicate Amorphs, evolved from self-repairing distributed data storage devices, and as such, redundantly distribute their 'brain' throughout their body. They are highly resistant to hard vacuum, explosive decompression
Explosive decompression
Uncontrolled decompression refers to an unplanned drop in the pressure of a sealed system, such as an aircraft cabin and typically results from human error, material fatigue, engineering failure or impact causing a pressure vessel to vent into its lower-pressure surroundings or fail to pressurize...
, projectile weapons, chemical-based explosives, and dismemberment. Their only specialty organ is their eyes, which they harvest as fruit from the Ghanj-Rho eye-tree on their home planet. While the Amorphs have the ability to move fast, quietly, and sprout appendages at will, they excel at 'closer-than-melee-range combat, primarily "meme-toxins" against other Amorphs.
A more farcical example comes from The Hitchhiker's Guide to the Galaxy
The Hitchhiker's Guide to the Galaxy
The Hitchhiker's Guide to the Galaxy is a science fiction comedy series created by Douglas Adams. Originally a radio comedy broadcast on BBC Radio 4 in 1978, it was later adapted to other formats, and over several years it gradually became an international multi-media phenomenon...
, where the Hooloovoo are a hyperintelligent shade of the colour blue.
Alien warriors recruited by the god Klael in David Eddings
David Eddings
David Eddings was an American author who wrote several best-selling series of epic fantasy novels.-Biography:...
' "Tamuli
The Tamuli
The Tamuli is a series of fantasy novels by David Eddings. The series consists of three volumes:# Domes of Fire# The Shining Ones# The Hidden CityThe Tamuli is the sequel to The Elenium...
" trilogy are noted by their human opponents to breathe marsh-gas (methane). Within Eddings' universe, this limits their capacity for exertion in an oxygen atmosphere, and also determines the tactics used to fight them and eventually to destroy them in their encampments.
The eponymous organism in Michael Crichton
Michael Crichton
John Michael Crichton , best known as Michael Crichton, was an American best-selling author, producer, director, and screenwriter, best known for his work in the science fiction, medical fiction, and thriller genres. His books have sold over 200 million copies worldwide, and many have been adapted...
's The Andromeda Strain
The Andromeda Strain
The Andromeda Strain , by Michael Crichton, is a techno-thriller novel documenting the efforts of a team of scientists investigating a deadly extraterrestrial microorganism that rapidly and fatally clots human blood, while in other people inducing insanity...
is described as reproducing via the direct conversion of energy into matter.
Star Trek
Star Trek: The Original Series
Star Trek is an American science fiction television series created by Gene Roddenberry, produced by Desilu Productions . Star Trek was telecast on NBC from September 8, 1966, through June 3, 1969...
episode "Devil in the Dark
The Devil in the Dark (TOS episode)
"The Devil in the Dark" is a first-season episode Star Trek: The Original Series which first aired on March 9, 1967. It was repeated on June 15, 1967. It is episode 25 of the first season and was written by Gene L. Coon and directed by Joseph Pevney. William Shatner writes in his memoirs that "The...
". A highly intelligent silicon-based creature made almost entirely of pure rock, it tunnels through rock as easily as humans move through air. The entire species dies out every 50,000 years except for one who tends for all the eggs, which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet, Janus VI. The inadvertent destruction of many of these eggs by a human mining colony led the mother Horta to respond by killing the colonists and sabotaging their equipment; it was only through a Vulcan mind meld that the race's benevolence and intelligence were discovered and peaceful relations established.
Star Trek would later offer other corporeal life-forms with an alternative biochemistry. The Tholian
Tholian
Tholians are a fictional starfaring and extremely territorial race in the Star Trek fictional universe. The Tholian home nation is the Tholian Assembly, which inhabits an area of space in the Alpha Quadrant, near the Cardassian Union and the Badlands....
s of "The Tholian Web" are depicted and described, in that episode and later in the Star Trek: Enterprise
Star Trek: Enterprise
Star Trek: Enterprise is a science fiction television series. It follows the adventures of humanity's first warp 5 starship, the Enterprise, ten years before the United Federation of Planets shown in previous Star Trek series was formed.Enterprise premiered on September 26, 2001...
episode "In a Mirror, Darkly
In a Mirror, Darkly (Enterprise episode)
"In a Mirror, Darkly" is a two-part episode of Star Trek: Enterprise. "In a Mirror, Darkly " was the 700th live-action Star Trek episode broadcast...
" as being primarily of mineral-based composition and thriving only in superheated conditions. Another episode from TOSs third season, "The Savage Curtain", depicted another rock creature called an Excalbian, which is believed in fanon to also have been silicon-based.
In Star Trek: The Next Generation
Star Trek: The Next Generation
Star Trek: The Next Generation is an American science fiction television series created by Gene Roddenberry as part of the Star Trek franchise. Roddenberry, Rick Berman, and Michael Piller served as executive producers at different times throughout the production...
, the Crystalline Entity appeared in two episodes, "Datalore" and "Silicon Avatar". This was an enormous spacefaring crystal lattice
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
that had taken thousands of lives in its quest for energy. It may have been unaware of this, however, but it was destroyed before communications could be established at a level sufficient to ascertain it.
In another episode, "Home Soil", intelligent crystals that formed a "microbrain" were discovered during a terraforming mission, and they described the humans they encountered as "ugly bags of mostly water."
"The Disease", an episode of Star Trek: Voyager
Star Trek: Voyager
Star Trek: Voyager is a science fiction television series set in the Star Trek universe. Set in the 24th century from the year 2371 through 2378, the series follows the adventures of the Starfleet vessel USS Voyager, which becomes stranded in the Delta Quadrant 70,000 light-years from Earth while...
featured some artificially-engineered silicon-based parasites, and an Enterprise episode, "Observer Effect
Observer Effect (Enterprise episode)
Observer Effect is the title of the 87th episode from the television series Star Trek: Enterprise. "Observer Effect" first aired on January 21, 2005 on the American television network UPN.-Plot:...
", also presented a lethal silicon-based virus.
In another Voyager episode, "Hope and Fear", a xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
-based life-form was mentioned. In the Enterprise episode "The Communicator", an alien species is encountered whose blood chemistry, while not explicitly stated, is sufficiently different from terrestrial organisms that it is not red and iron is toxic to it. Various Star Trek series also had episodes featuring photonic lifeforms.
Star Wars
In the Star WarsStar Wars
Star Wars is an American epic space opera film series created by George Lucas. The first film in the series was originally released on May 25, 1977, under the title Star Wars, by 20th Century Fox, and became a worldwide pop culture phenomenon, followed by two sequels, released at three-year...
movie The Empire Strikes Back, two life-forms were encountered by the characters that were non-carbon based entities. Although details of their physiology were not mentioned on screen, the space slug, (a giant worm-like creature that lived on asteroids in the vacuum of space), and the Mynock, (pesky bat-like vermin that would attach to spaceship hulls and chew through power conduits to feed off the raw energy), are said to be silicon-based organisms in Star Wars Expanded Universe
Star Wars Expanded Universe
The Star Wars Expanded Universe encompasses all of the officially licensed, fictional background of the Star Wars universe, outside of the six feature films produced by George Lucas. The expanded universe includes books, comic books, video games, spin-off films like Star Wars: The Clone Wars,...
sources. Also from The Empire Strikes Back, the bounty hunter
Bounty hunter
A bounty hunter captures fugitives for a monetary reward . Other names, mainly used in the United States, include bail enforcement agent and fugitive recovery agent.-Laws in the U.S.:...
Zuckuss is a member of the Gand race, an ammonia-based life-form. However, it is worth noting that the Gand are divided into two subspecies, only one of which breathes at all, the other drawing all their required sustenance from food intake and producing speech by essentially modulated flatulence
Flatulence
Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply gas...
.
Appearing only in the Star Wars Expanded Universe
Star Wars Expanded Universe
The Star Wars Expanded Universe encompasses all of the officially licensed, fictional background of the Star Wars universe, outside of the six feature films produced by George Lucas. The expanded universe includes books, comic books, video games, spin-off films like Star Wars: The Clone Wars,...
is the spice spider of Kessel, a creature made of glitterstim spice and silicon that spun crystalline webs harvested by miners as glitterstim spice, an illegal psychoactive narcotic
Narcotic
The term narcotic originally referred medically to any psychoactive compound with any sleep-inducing properties. In the United States of America it has since become associated with opioids, commonly morphine and heroin and their derivatives, such as hydrocodone. The term is, today, imprecisely...
. The spider used the webs to catch bogeys, tiny energy creatures that it consumed for energy.
Other film and television
- In the movie Titan A.E.Titan A.E.Titan A.E. is an American animated post-apocalyptic science fiction film directed by Don Bluth and Gary Goldman released in 2000. The title refers to the spacecraft that is central to the plot, with A.E. meaning "After Earth."...
, an alien race called the Drej were composed of a coherent plasma. - In the movie The Monolith MonstersThe Monolith MonstersThe Monolith Monsters is a science fiction film directed by John Sherwood and starring Grant Williams and Lola Albright. It is based on a story by Jack Arnold and Robert M...
, a silicon meteor reproduces, draining silicates from everything it touches. It needs water to start its cycle and contains molecular structures typical of many kinds of rocks, mixed together. A geologist says that its structure is nearly impossible. The meteor is killed by salt water, which can stop the cycle. - In FirewalkerFirewalker (The X-Files)"Firewalker" is the ninth episode of the second season of The X-Files television series, broadcast by the Fox Broadcasting Company on November 18, 1994...
, a second-season episodeThe X-Files (season 2)The second season of the science fiction television series The X-Files commenced airing on Fox in the United States on September 16, 1994, concluded on the same channel on May 19, 1995, and contained 25 episodes.- Production :...
of The X-FilesThe X-FilesThe X-Files is an American science fiction television series and a part of The X-Files franchise, created by screenwriter Chris Carter. The program originally aired from to . The show was a hit for the Fox network, and its characters and slogans became popular culture touchstones in the 1990s...
, a silicon-based plant that infects humans parasitically through its sporeSporeIn biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s is discovered living deep in a volcanoVolcano2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
. - Also from The X-Files, the first-season episodeThe X-Files (season 1)The first season of the science fiction television series The X-Files commenced airing on the Fox network in the United States on September 10, 1993 and concluded on the same channel on May 13, 1994 after airing all 24 episodes....
"Ice" deals with an ammonia-based vermiform parasite. - A key plot point in the comedyComedyComedy , as a popular meaning, is any humorous discourse or work generally intended to amuse by creating laughter, especially in television, film, and stand-up comedy. This must be carefully distinguished from its academic definition, namely the comic theatre, whose Western origins are found in...
EvolutionEvolution (film)Evolution is a 2001 American science fiction comedy film directed by Ivan Reitman and starring David Duchovny, Orlando Jones, Seann William Scott, Julianne Moore and Ted Levine...
involves nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
-based life forms, and using seleniumSeleniumSelenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
-based shampooShampooShampoo is a hair care product used for the removal of oils, dirt, skin particles, dandruff, environmental pollutants and other contaminant particles that gradually build up in hair...
to poison them (with the bonus of a product placement for Head & ShouldersHead & ShouldersHead & Shoulders is a brand of anti-dandruff shampoo produced by Procter & Gamble. Head & Shoulders Classic Clean Shampoo is the top-selling shampoo in the United States by dollar sales.- History :...
). - In the Stargate SG-1Stargate SG-1Stargate SG-1 is a Canadian-American adventure and military science fiction television series and part of Metro-Goldwyn-Mayer's Stargate franchise. The show, created by Brad Wright and Jonathan Glassner, is based on the 1994 feature film Stargate by Dean Devlin and Roland Emmerich...
fourth season episode "Scorched EarthScorched earthA scorched earth policy is a military strategy or operational method which involves destroying anything that might be useful to the enemy while advancing through or withdrawing from an area...
", a Human society known as the Enkarans are threatened on their new homeworld by an alien ship that is terraforming the planet to be suitable for the sulfurSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-based Gadmeer species. - In the Stargate AtlantisStargate AtlantisStargate Atlantis is a Canadian-American adventure and military science fiction television series and part of MGM's Stargate franchise. The show was created by Brad Wright and Robert C. Cooper as a spin-off series of Stargate SG-1, which was created by Wright and Jonathan Glassner and was itself...
fifth season episode "Remnants", a device is found whose purpose was to seed a planet with siliconSiliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-based life. - In Ben 10Ben 10The Omnitrix was originally created by a Galvan named Azmuth. The Omnitrix was intended to allow beings to experience life as other species in order to bring understanding and foster peace in the universe....
, both the Omnitrix alien Diamondhead and the alien bounty hunterBounty hunterA bounty hunter captures fugitives for a monetary reward . Other names, mainly used in the United States, include bail enforcement agent and fugitive recovery agent.-Laws in the U.S.:...
Tetrax Shard are members of the Petrosapien species, which are a form of silicon-based life. Other silicon-based lifeforms on the show include the Omnitrix aliens Chromostone (who is crystalline), and Echo Echo and Upgrade (who are both biomechanical). Other member's of Upgrade's species have appeared, including the shape-shifting "Ship," a pet of Ben's girlfriend, Julie. - Indiana Jones and the Kingdom of the Crystal Skull (2008) introduces thirteen "extra dimensional beings" with crystal skeletonsCrystal skullThe crystal skulls are a number of human skull hardstone carvings made of clear or milky quartz rock, known in art history as "rock crystal", claimed to be pre-Columbian Mesoamerican artifacts by their alleged finders. However, none of the specimens made available for scientific study have been...
, who founded a city that became the basis of the El DoradoEl DoradoEl Dorado is the name of a Muisca tribal chief who covered himself with gold dust and, as an initiation rite, dived into a highland lake.Later it became the name of a legendary "Lost City of Gold" that has fascinated – and so far eluded – explorers since the days of the Spanish Conquistadors...
myth. Though their flesh has died and rotted away, their minds still live on within their skeletons, which communicate telepathically. - An episode ("The Chrysalids") of the series Space: 1999Space: 1999Space: 1999 is a British science-fiction television series that ran for two seasons and originally aired from 1975 to 1977. In the opening episode, nuclear waste from Earth stored on the Moon's far side explodes in a catastrophic accident on 13 September 1999, knocking the Moon out of orbit and...
presents humanoidHumanoidA humanoid is something that has an appearance resembling a human being. The term first appeared in 1912 to refer to fossils which were morphologically similar to, but not identical with, those of the human skeleton. Although this usage was common in the sciences for much of the 20th century, it...
s on a planet protected by a electric waves system, which are living in a chlorine atmosphere. - The episode "The Stones of Blood", of the 16th season of Doctor WhoDoctor WhoDoctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...
, the Fourth Doctor encounters the Ogri, a silicon-based life form, and in the same sub-plot, the Megara, who are made entirely out of an unknown substance, possibly energy, and they uphold the word of the law, and execute all who break the law with a beam of energy.
Computer and video games
In the Command & ConquerCommand & Conquer series
Command & Conquer is a video game franchise, mostly of the real-time strategy style as well as a first-person shooter game based on the former...
real-time strategy
Real-time strategy
Real-time strategy is a sub-genre of strategy video game which does not progress incrementally in turns. Brett Sperry is credited with coining the term to market Dune II....
games, both the gameplay and storyline revolve heavily around the introduction to Earth of an extraterrestrial mutagen called Tiberium
Tiberium
Tiberium or Ichor is a fictional substance that is central to the plot of much of the Command & Conquer series of real-time strategy video games...
via meteor, which displays strikingly lifelike behaviours such as self-replication, evolution, and homeostasis, without undergoing anything like common carbon-based metabolic cycles, and which appears to be colonising the Earth, converting it into an environment unsuited to carbon-based biology. Earth creatures (such as animals, plants and even humans) exposed to Tiberium can either be killed because of the radiation or be transformed into Tiberium-based life-forms, to whom Tiberium radiation is curative rather than toxic. It is later revealed that Tiberium was introduced to earth by the Scrin, an extremely advanced race of Tiberium-based aliens bent on mining the planet after the Tiberium deposits have reached maturity.
In the Halo
Halo (video game series)
Halo is a multi-million dollar science fiction video game franchise created by Bungie and now managed by 343 Industries and owned by Microsoft Studios. The series centers on an interstellar war between humanity and a theocratic alliance of aliens known as the Covenant...
video game series, a race of Covenant aliens named "Grunts" by humans require a breathing apparatus while fighting the humans in an Earth-like atmosphere. According to the novelizations of the video game, the Grunts' apparatus allows them to breathe the methane they need to survive.
In the Master of Orion
Master of Orion
Master of Orion is a turn-based, 4X science fiction computer strategy game released in 1993 by MicroProse on the MS-DOS and Mac OS operating systems. The purpose of the game is to lead one of ten races to dominate the galaxy through a combination of diplomacy and conquest while developing...
series of space strategy games, there exists an extraterrestrial race called Silicoids, whose appearance (and presumably composition) is similar to crystalline mineral structures. The game posits that this grants them immunity to the effects of hostile environments and pollution and they require no sustenance, at the expense of impeding their reproductive rate and their ability to interact with other intelligent species.
In the Metroid Prime
Metroid Prime
Metroid Prime is a video game developed by Retro Studios and Nintendo for the Nintendo GameCube, released in North America on November 17, 2002...
series, Phazon is a highly radioactive, self-regenerating mineral with organic properties that is generated by the sentient planet Phaaze.
In Metroid Prime Hunters
Metroid Prime Hunters
Metroid Prime Hunters is a first-person shooter and adventure game for the Nintendo DS handheld video game console. It was developed by American video game developers Nintendo Software Technology, and was released by Nintendo in North America in March 2006, in Europe and Australia in May 2006, and...
, Spire is a rock-like, silicon-based alien. He is the last Diamont (presumably a play on the word diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
, which is composed of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
).
In the Submarine TITANS
Submarine TITANS
Submarine TITANS is a real time strategy video game for Microsoft Windows rendered in 2D isometric perspective graphics. It was developed by Ellipse Studios, published by Strategy First, and released in North America on August 9, 2000. The game takes place in the 22nd century in which a comet has...
strategy game, the alien race in the game are called "the Silicons" because they are silicon-based life forms.
In the Star Control
Star Control
Star Control is a science fiction computer game that was developed by Toys for Bob and published by Accolade in the early 1990s. Star Control still enjoys a cult following...
series, the Chenjesu, are hyperintelligent, peaceful silicon-based life-forms that were the backbone of the Alliance of Free Stars. Their crystalline biology apparently gives them the ability to send and receive hyperwave transmissions. Also, there are the Slylandro, who are gas beings residing in the upper atmosphere of a gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
. As well, there are evidences of another silicon-based race, the Taalo who are described by the xenophobic
Xenophobia
Xenophobia is defined as "an unreasonable fear of foreigners or strangers or of that which is foreign or strange". It comes from the Greek words ξένος , meaning "stranger," "foreigner" and φόβος , meaning "fear."...
Ur-Quan as the only race to have not awakened their territorial instincts. The Taalo were also immune to mind control
In the game of Xenosaga
Xenosaga
is a series of science fiction video games developed by Monolith Soft and published by Namco Bandai. Xenosaga's main story is in the form of a trilogy of PlayStation 2 video games. There have been three spin-off games and an anime adaptation. The Xenosaga series serves as a spiritual successor to...
, artificial life forms known as Realians have been created using silicon-based chemistry. They resemble humans in every aspect, except they are considered to be lower than humans on the social ladder
Ladder
A ladder is a vertical or inclined set of rungs or steps. There are two types: rigid ladders that can be leaned against a vertical surface such as a wall, and rope ladders that are hung from the top. The vertical members of a rigid ladder are called stringers or stiles . Rigid ladders are usually...
.
In Mass Effect
Mass Effect
Mass Effect is an action role-playing game developed by BioWare for the Xbox 360 and Microsoft Windows by Demiurge Studios. The Xbox 360 version was released worldwide in November 2007 published by Microsoft Game Studios...
the alien Turians and Quarians, are both based on dextro-amino acids, as opposed to the all other sentient species of the galaxy based on levo-amino acids. There are also the Volus, an ammonia based species that must wear pressure suits to survive in environments suited to the other races.
In Spore
Spore (2008 video game)
Spore is a multi-genre single-player god game developed by Maxis and designed by Will Wright. The game was released for the Microsoft Windows and Macintosh operating systems in September 2008 as Spore...
, the Grox
Grox (Spore)
The Grox are an aggressive and vicious race of mechanically based cyborg carnivores that rule the center of the Galaxy in the video game, Spore. They are commonly thought to be based on the Borg of the Star Trek Series because of the striking physical similarities between the two. The Grox are...
refer to the player and to other alien empires as "slow thinking carbon-based lifeforms" and "carbon wads", implying that the Grox (which are at least partly machine life) are not carbon-based. Also, the Grox can only exist on barren planets which cannot support other life, and when a planet is terraformed the Grox inhabiting it die immediately. The Grox seem to gather sustenance from the radiation from the galactic core, as the Grox colonies are larger the closer they are to the galactic core.
In Muv Luv, the BETA which calls itself the "higher/superior existence" says they were created by a silicon-based being simply called "The Creator". As such, they don't consider any non-silicon-based creature to be alive, not even themselves. Its reasoning was that only silicon-based beings occur naturally and have the ability to reproduce and disperse. When the human main character, Takeru, argues that humans also have the ability to reproduce and disperse, the higher existence says carbon too easily mingles with other elements and therefore it would be impossible for a carbon-based existence to have evolved on its own. Thus, humans must be other biological machines created by a life form just as the BETA are.