Panspermia
Encyclopedia
Panspermia is the hypothesis
that life exists throughout the Universe, distributed by meteoroids, asteroids and planetoids
.
Panspermia proposes that life that can survive the effects of space, such as extremophile
bacteria, become trapped in debris that is ejected into space after collisions between planets that harbor life and Small Solar System Bodies (SSSB). Bacteria may travel dormant for an extended amount of time before colliding randomly with other planets or intermingling with protoplanetary disk
s. If met with ideal conditions on a new planet's surfaces, the bacteria become active and the process of evolution begins. Panspermia is not meant to address how life began, just the method that may cause its sustenance.
The related but distinct idea of exogenesis is a more limited hypothesis that proposes life on Earth was transferred from elsewhere in the Universe but makes no prediction about how widespread it is. Because the term "exogenesis" is more well-known, it tends to be used in reference to what should strictly speaking be called panspermia.
philosopher Anaxagoras
. In the nineteenth century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834), Kelvin
(1871), Hermann von Helmholtz
(1879) and, somewhat later, by Svante Arrhenius
(1903).
There is as yet no evidence to support or contradict panspermia, although the majority view holds that panspermia – especially in its interstellar form – is unlikely given the challenges of survival and transport in space.
Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe
(born 1939) were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution
.
Panspermia does not necessarily suggest that life originated only once and subsequently spread through the entire Universe, but instead that once started, it may be able to spread to other environments suitable for replication.
s) or interplanetary (between planets in the same star system
); its transport mechanisms may include radiation pressure
and lithopanspermia (microorganisms in rocks). Deliberate directed panspermia from space to seed Earth or sent from Earth to seed other solar systems have also been proposed. One new twist to the hypothesis by engineer Thomas Dehel (2006), proposes that plasmoid
magnetic fields ejected from the magnetosphere may move the few spores lifted from the Earth's atmosphere with sufficient speed to cross interstellar space to other systems before the spores can be destroyed.
Interplanetary transfer of material is well documented, as evidenced by meteorites of Martian origin
found on Earth.
Space probe
s may also be a viable transport mechanism for interplanetary cross-pollination in our solar system or even beyond. However, space agencies have implemented sterilization
procedures to avoid planetary contamination.
Stardust Space Probe - The link between Comets and Panspermia was investigated further with a NASA Launch performed by NASA beginning in 2004, entitled "The Stardust Mission". Ion Propulsion spacecraft was loaded with machinery to bring back lab samples from the tail of a comet. This published document from NASA entitled "NASA Researchers Make First Discovery of Life's Building Blocks in Comet". This article refers to the Glycine and other building blocks that have been found in comets. Comets travel through space with these frozen potentially reproductive materials, and the tail of the comets appear when gases melt in the presence of our sun.
The Precambrian
fossil
record indicates that life appeared soon after the Earth was formed. This would imply that life appeared within several hundred million years when conditions became favourable. Generally accepted scientific estimates of the age of the Earth
place its formation (along with the rest of the Solar system
) at about 4.55 billion years old. The oldest known sedimentary rock
s are somewhat altered Hadean
formations from the southern tip of Akilia island
, West Greenland
. These rocks have been dated as no younger than 3.85 billion years.
The oldest known fossilized stromatolite
s or bacterial aggregates, are dated at 3.5 billion years old. The bacteria that form stromatolites, cyanobacteria, are photosynthetic. Most models of the origin of life have the earliest organisms obtaining energy from reduced
chemicals, with the more complex mechanisms of photosynthesis evolving later. During the Late Heavy Bombardment
of the Earth's Moon
about 3.9 billion years (as evidenced by Apollo
lunar samples) impact intensities may have been up to 100x those immediately before. From analysis of lunar melts and observations of similar cratering on Mars' highlands, Kring and Cohen suggest that the Late Heavy Bombardment was caused by asteroid
impacts that affected the entire inner solar system
. This is likely to have effectively sterilised
Earth's entire planetary surface, including submarine hydrothermal systems
that would be otherwise protected.
are particularly interested in studying extremophile
s as many organisms of this type are capable of surviving in environments similar to those known to exist on other planets. Some organisms have been shown to be more resistant to extreme conditions than previously recognized, and may be able to survive for very long periods of time, probably even in deep space and, hypothetically, could travel in a dormant state between environments suitable for ongoing life
.
Some bacteria and animals have been found to thrive in oceanic hydrothermal vents above 100 °C
; a study revealed that a fraction of bacteria survive heating pulses up to 250°C in vacuum, while similar heating at normal atmospheric pressure leads to the total sterilization of samples. Other bacteria can thrive in strongly caustic environment
s, others at extreme pressures 11 km under the ocean, while others survive in extremely dry, desiccating conditions, frigid cold, vacuum or acid environments. Survival in space is not limited to bacteria, lichens or archea: the animal Tardigrade
has been proven to survive the vacuum of space.
Recent experiments suggest that if bacteria were somehow sheltered from the radiation of space, perhaps inside a thick meteoroid or an icy comet, they could survive dormant for millions of years. Deinococcus radiodurans
is a radioresistant bacterium that can survive high radiation levels.
Duplicating the harsh conditions of cold interstellar space in their laboratory, NASA scientists have created primitive vesicles
that mimic some aspects of the membraneous structures found in all living things. These chemical compounds may have played a part in the origin of life.
s, algae
, fungi
and some protozoans, and some bacteria produce endospores or cysts
during times of stress. These structures may be highly resilient to ultraviolet and gamma radiation, desiccation
, lysozyme
, temperature
, starvation
and chemical disinfectants, while metabolically inactive. Spores germinate
when favourable conditions are restored after exposure to conditions fatal to the parent organism. According to astrophysicist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth - and we know they're very hardened to radiation."
missions. In December 2006, Michael C. Malin
of Malin Space Science Systems
published a paper in the journal Science
which argued that his camera (the Mars Observer Camera) had found evidence suggesting water was occasionally flowing on the surface of Mars within the last five years.
Water oceans might exist on Europa
, Enceladus
, Triton
and perhaps other moons in the Solar system. Even moons that are now frozen ice balls might earlier have been melted internally by heat from radioactive rocky cores. Bodies like this may be common throughout the universe. Living bacteria found in core samples retrieved from 3700 metres (12,139.1 ft) deep at Lake Vostok
in Antarctica, have provided data for extrapolations to the likelihood of microorganisms surviving frozen in extraterrestrial habitats or during interplanetary transport. Also, bacteria have been discovered living within warm rock deep in the Earth's crust.
is the only place known by human beings to harbor life in the observed universe. Today's estimates of values for the Drake Equation
suggest the probability of intelligent life in a single galaxy like our own Milky Way
may be much smaller than once was thought, while the sheer number of galaxies make it seem probable that life has arisen somewhere else in the Universe. According to current theories of physics, space travel over such vast distances would take an incredibly long time to the outside observer, with vast amounts of energy required. Nevertheless, small groups of researchers like the Search for Extra-Terrestrial Intelligence (SETI
) continue to monitor the skies for transmissions from within our own galaxy at least.
Astrobiological proponents like the Rare Earth hypothesis
ts recognise that the conditions required for the evolution of intelligent life might be exceedingly rare in the Universe, while simultaneously noting that simple single-celled microorganisms may well be abundant.
indicates a non-terrestrial origin for these molecules rather than terrestrial contamination. Biologically relevant molecules identified so far include uracil
, an RNA nucleobase
, and xanthine
. These results demonstrate that many organic compounds which are components of life on Earth were already present in the early solar system and may have played a key role in life's origin.
In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life (the amino acid glycine
) in a comet for the first time.
On August 8, 2011, a report, based on NASA
studies with meteorites found on Earth
, was published suggesting building blocks of DNA
(adenine
, guanine
and related organic molecules) may have been formed extraterrestrially in outer space
. In October 2011, scientists reported that cosmic dust
contains complex organic
matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. One of the scientists suggested that these complex organic compounds may have been related to the development of life on earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
bacteria living inside the camera of the Surveyor 3
spacecraft when it was brought back to Earth by Apollo 12
. They believed that the bacteria survived since the time of the craft's launch to the Moon. However, these reports are disputed by Leonard D. Jaffe, who was Surveyor program scientist and custodian of the Surveyor 3 parts brought back from the Moon, stated in a letter to the Planetary Society that an unnamed member of his staff reported that a "breach of sterile procedure" took place at just the right time to produce a false positive result. NASA was funding an archival study in 2007 that was trying to locate the film of the camera-body microbial sampling to confirm the report of a breach in sterile technique. NASA currently stands by its original assessment: see Reports of Streptococcus mitis on the moon
.
or Rush plant that had been glued into the fragment and camouflaged using coal dust
. The outer "fusion layer" was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought he sought to influence the 19th century debate on spontaneous generation
— rather than panspermia — by demonstrating the transformation of inorganic to biological matter.
a professor of astronomy
suggested a "garbage theory" for the origin of life, the theory says that life on Earth might have spread from a pile of waste
products accidentally dumped on Earth long ago by extraterrestrials.
, who along with Leslie Orgel
proposed the hypothesis of 'directed panspermia'. This proposes that the seeds of life may have been purposely spread by an advanced extraterrestrial civilization. Later, after biologists had proposed that an "RNA world" might be involved in the origin of life, Crick noted that he had been overly pessimistic about the chances of life originating on Earth.
The application of directed panspermia has been proposed as a way to spread life from Earth to other solar systems. For example, microbial payloads launched by solar sails at speeds up to 0.0001 c (30,000 m/s) would reach targets at 10 to 100 light-years in 0.1 million to 1 million years. Fleets of microbial capsules can be aimed at clusters of new stars in star-forming clouds where they may land on planets, or captured by asteroids and comets and later delivered to planets. Payloads may contain extremophile
s for diverse environments and cyanobacteria similar to early microorganisms. Hardy multicellular organisms (rotifer cysts) may be included to induce higher evolution.
The probability of hitting the target zone can be calculated from
where A(target) is the cross-section of the target area, dy is the positional uncertainty at arrival; a - constant (depending on units), r(target) is the radius of the target area; v the velocity of the probe; (tp) the targeting precision (arcsec/yr); and d the distance to the target (all units in SIU). Guided by high-resolution astrometry of 1×10−5 arcsec/yr, almost nearby target stars (Alpha PsA, Beta Pictoris) can be seeded by milligrams of launched microbes; while seeding the Rho Ophiochus star-forming cloud requires hundreds of kilograms of dispersed capsules.
Theoretically, terrestrial spacecraft travelling to other celestial bodies such as the Moon, could carry with them microorganisms or other organic materials ubiquitous on Earth, thus raising the possibility that we can seed life on other planetary bodies. This is a concern among space researchers who try to prevent Earth contamination
from distorting data, especially in regards to finding possible extraterrestrial life. Even the best sterilization techniques cannot guarantee that potentially invasive biologic or organic materials will not be unintentionally carried along. The harsh environments encountered throughout the rest of the solar system so far do not seem to support complex terrestrial life. However, matter exchange in form of meteor impacts
has existed and will exist in the solar system even without human intervention.
was retrieved from a field in Kazakhstan. The 5,500-pound capsule, seven-feet in diameter, carried a payload of 43 European experiments in a range of scientific disciplines – including fluid physics, biology, crystal growth, radiation exposure and astrobiology
. The capsule contained, among other things, lichen
that were exposed to the radiation of space. Scientists also strapped basalt and granite disks riddled with cyanobacteria to the capsule's heat shield to see if the microorganisms could survive the brutal conditions of reentry. Some bacteria, lichens, spores, and even one animal (Tardigrades) were found to have survived the outer space environment and cosmic radiation.
, which is being developed by the Planetary Society
, will consist of sending selected microorganisms on a three-year interplanetary round-trip in a small capsule aboard the Russian Phobos-Grunt
spacecraft
in 2011. The goal is to test whether organisms can survive a few years in deep space. The experiment will test one aspect of transpermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another.
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
that life exists throughout the Universe, distributed by meteoroids, asteroids and planetoids
Small Solar System body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...
.
Panspermia proposes that life that can survive the effects of space, such as extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...
bacteria, become trapped in debris that is ejected into space after collisions between planets that harbor life and Small Solar System Bodies (SSSB). Bacteria may travel dormant for an extended amount of time before colliding randomly with other planets or intermingling with protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
s. If met with ideal conditions on a new planet's surfaces, the bacteria become active and the process of evolution begins. Panspermia is not meant to address how life began, just the method that may cause its sustenance.
The related but distinct idea of exogenesis is a more limited hypothesis that proposes life on Earth was transferred from elsewhere in the Universe but makes no prediction about how widespread it is. Because the term "exogenesis" is more well-known, it tends to be used in reference to what should strictly speaking be called panspermia.
Hypothesis
The first known mention of the term was in the writings of the 5th century BC GreekAncient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
philosopher Anaxagoras
Anaxagoras
Anaxagoras was a Pre-Socratic Greek philosopher. Born in Clazomenae in Asia Minor, Anaxagoras was the first philosopher to bring philosophy from Ionia to Athens. He attempted to give a scientific account of eclipses, meteors, rainbows, and the sun, which he described as a fiery mass larger than...
. In the nineteenth century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834), Kelvin
William Thomson, 1st Baron Kelvin
William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, PRSE, was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging...
(1871), Hermann von Helmholtz
Hermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...
(1879) and, somewhat later, by Svante Arrhenius
Svante Arrhenius
Svante August Arrhenius was a Swedish scientist, originally a physicist, but often referred to as a chemist, and one of the founders of the science of physical chemistry...
(1903).
There is as yet no evidence to support or contradict panspermia, although the majority view holds that panspermia – especially in its interstellar form – is unlikely given the challenges of survival and transport in space.
Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe
Chandra Wickramasinghe
Vidya Jothi Nalin Chandra Wickramasinghe , FIMA, FRAS, FRSA is Professor at Cardiff University and Honorary Professor at the University of Buckingham. He is the Director of the Buckingham Centre for Astrobiology...
(born 1939) were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution
Macroevolution
Macroevolution is evolution on a scale of separated gene pools. Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes within a species or population.The process of speciation may fall...
.
Panspermia does not necessarily suggest that life originated only once and subsequently spread through the entire Universe, but instead that once started, it may be able to spread to other environments suitable for replication.
Proposed mechanisms
The mechanisms proposed for interstellar panspermia are hypothetical and currently unproven. Panspermia can be said to be either interstellar (between star systemStar system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
s) or interplanetary (between planets in the same star system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
); its transport mechanisms may include radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
and lithopanspermia (microorganisms in rocks). Deliberate directed panspermia from space to seed Earth or sent from Earth to seed other solar systems have also been proposed. One new twist to the hypothesis by engineer Thomas Dehel (2006), proposes that plasmoid
Plasmoid
A plasmoid is a coherent structure of plasma and magnetic fields. Plasmoids have been proposed to explain natural phenomena such as ball lightning, magnetic bubbles in the magnetosphere, and objects in cometary tails, in the solar wind, in the solar atmosphere, and in the heliospheric current sheet...
magnetic fields ejected from the magnetosphere may move the few spores lifted from the Earth's atmosphere with sufficient speed to cross interstellar space to other systems before the spores can be destroyed.
Interplanetary transfer of material is well documented, as evidenced by meteorites of Martian origin
Mars meteorite
A martian meteorite is a rock that formed on the planet Mars, was ejected from Mars by the impact of an asteroid or comet, and landed on the Earth. Of over 53000 meteorites that have been found on Earth, 99 are martian...
found on Earth.
Space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
s may also be a viable transport mechanism for interplanetary cross-pollination in our solar system or even beyond. However, space agencies have implemented sterilization
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...
procedures to avoid planetary contamination.
Stardust Space Probe - The link between Comets and Panspermia was investigated further with a NASA Launch performed by NASA beginning in 2004, entitled "The Stardust Mission". Ion Propulsion spacecraft was loaded with machinery to bring back lab samples from the tail of a comet. This published document from NASA entitled "NASA Researchers Make First Discovery of Life's Building Blocks in Comet". This article refers to the Glycine and other building blocks that have been found in comets. Comets travel through space with these frozen potentially reproductive materials, and the tail of the comets appear when gases melt in the presence of our sun.
Research
Until a large portion of the galaxy is surveyed for signs of life or contact is made with hypothetical extraterrestrial civilizations, the panspermia hypothesis in its fullest meaning will remain difficult to test.Early life on Earth
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
record indicates that life appeared soon after the Earth was formed. This would imply that life appeared within several hundred million years when conditions became favourable. Generally accepted scientific estimates of the age of the Earth
Age of the Earth
The age of the Earth is 4.54 billion years This age is based on evidence from radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples...
place its formation (along with the rest of the Solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
) at about 4.55 billion years old. The oldest known sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s are somewhat altered Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
formations from the southern tip of Akilia island
Akilia island
Akilia Island is an island in southwestern Greenland, about 22 kilometers south of Nuuk , at . Akilia is the location of a rock formation that has been proposed to contain the oldest known sedimentary rocks on Earth,...
, West Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
. These rocks have been dated as no younger than 3.85 billion years.
The oldest known fossilized stromatolite
Stromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....
s or bacterial aggregates, are dated at 3.5 billion years old. The bacteria that form stromatolites, cyanobacteria, are photosynthetic. Most models of the origin of life have the earliest organisms obtaining energy from reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
chemicals, with the more complex mechanisms of photosynthesis evolving later. During the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
of the Earth's Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
about 3.9 billion years (as evidenced by Apollo
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...
lunar samples) impact intensities may have been up to 100x those immediately before. From analysis of lunar melts and observations of similar cratering on Mars' highlands, Kring and Cohen suggest that the Late Heavy Bombardment was caused by asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
impacts that affected the entire inner solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. This is likely to have effectively sterilised
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...
Earth's entire planetary surface, including submarine hydrothermal systems
Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...
that would be otherwise protected.
Extremophiles
AstrobiologistsAstrobiology
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...
are particularly interested in studying extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...
s as many organisms of this type are capable of surviving in environments similar to those known to exist on other planets. Some organisms have been shown to be more resistant to extreme conditions than previously recognized, and may be able to survive for very long periods of time, probably even in deep space and, hypothetically, could travel in a dormant state between environments suitable for ongoing life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
.
Some bacteria and animals have been found to thrive in oceanic hydrothermal vents above 100 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
; a study revealed that a fraction of bacteria survive heating pulses up to 250°C in vacuum, while similar heating at normal atmospheric pressure leads to the total sterilization of samples. Other bacteria can thrive in strongly caustic environment
Environment (biophysical)
The biophysical environment is the combined modeling of the physical environment and the biological life forms within the environment, and includes all variables, parameters as well as conditions and modes inside the Earth's biosphere. The biophysical environment can be divided into two categories:...
s, others at extreme pressures 11 km under the ocean, while others survive in extremely dry, desiccating conditions, frigid cold, vacuum or acid environments. Survival in space is not limited to bacteria, lichens or archea: the animal Tardigrade
Tardigrade
Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...
has been proven to survive the vacuum of space.
Recent experiments suggest that if bacteria were somehow sheltered from the radiation of space, perhaps inside a thick meteoroid or an icy comet, they could survive dormant for millions of years. Deinococcus radiodurans
Deinococcus radiodurans
Deinococcus radiodurans is an extremophilic bacterium, one of the most radioresistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and has been listed as the world's toughest bacterium in The Guinness Book Of World Records.-Name...
is a radioresistant bacterium that can survive high radiation levels.
Duplicating the harsh conditions of cold interstellar space in their laboratory, NASA scientists have created primitive vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
that mimic some aspects of the membraneous structures found in all living things. These chemical compounds may have played a part in the origin of life.
Spores
Spores are another potential vector for transporting life through inhospitable and inimical environments, such as the depths of interstellar space. Spores are produced as part of the normal life cycle of many plantPlant
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, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
and some protozoans, and some bacteria produce endospores or cysts
Microbial cyst
A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed down and...
during times of stress. These structures may be highly resilient to ultraviolet and gamma radiation, desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
, lysozyme
Lysozyme
Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, are glycoside hydrolases, enzymes that damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between...
, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, starvation
Starvation
Starvation is a severe deficiency in caloric energy, nutrient and vitamin intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, death...
and chemical disinfectants, while metabolically inactive. Spores germinate
Germination
Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
when favourable conditions are restored after exposure to conditions fatal to the parent organism. According to astrophysicist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth - and we know they're very hardened to radiation."
Potential habitats for life
The presence of past liquid water on Mars, suggested by river-like formations on the red planet, was confirmed by the Mars Exploration RoverMars Exploration Rover
NASA's Mars Exploration Rover Mission is an ongoing robotic space mission involving two rovers, Spirit and Opportunity, exploring the planet Mars...
missions. In December 2006, Michael C. Malin
Michael C. Malin
Michael C. Malin is an American astronomer, space-scientist, and CEO of Malin Space Science Systems. His cameras have been important scientific instruments in the Exploration of Mars....
of Malin Space Science Systems
Malin Space Science Systems
Malin Space Science Systems is a San Diego, California company that designs, develops, and operates instruments to fly on unmanned spacecraft. MSSS is headed by chief scientist and CEO Michael C. Malin....
published a paper in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
which argued that his camera (the Mars Observer Camera) had found evidence suggesting water was occasionally flowing on the surface of Mars within the last five years.
Water oceans might exist on Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
, Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
, Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
and perhaps other moons in the Solar system. Even moons that are now frozen ice balls might earlier have been melted internally by heat from radioactive rocky cores. Bodies like this may be common throughout the universe. Living bacteria found in core samples retrieved from 3700 metres (12,139.1 ft) deep at Lake Vostok
Lake Vostok
Lake Vostok is the largest of more than 140 subglacial lakes found under the surface of Antarctica. The overlying ice provides a continuous paleoclimatic record of 400,000 years, although the lake water itself may have been isolated for 15 to 25 million years. The lake is named after the...
in Antarctica, have provided data for extrapolations to the likelihood of microorganisms surviving frozen in extraterrestrial habitats or during interplanetary transport. Also, bacteria have been discovered living within warm rock deep in the Earth's crust.
Extraterrestrial life
EarthEarth
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...
is the only place known by human beings to harbor life in the observed universe. Today's estimates of values for the Drake Equation
Drake equation
The Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...
suggest the probability of intelligent life in a single galaxy like our own Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
may be much smaller than once was thought, while the sheer number of galaxies make it seem probable that life has arisen somewhere else in the Universe. According to current theories of physics, space travel over such vast distances would take an incredibly long time to the outside observer, with vast amounts of energy required. Nevertheless, small groups of researchers like the Search for Extra-Terrestrial Intelligence (SETI
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
) continue to monitor the skies for transmissions from within our own galaxy at least.
Astrobiological proponents like the Rare Earth hypothesis
Rare Earth hypothesis
In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...
ts recognise that the conditions required for the evolution of intelligent life might be exceedingly rare in the Universe, while simultaneously noting that simple single-celled microorganisms may well be abundant.
Spaceborne organic molecules
A 2008 analysis of 12C/13C isotopic ratios of organic compounds found in the Murchison meteoriteMurchison meteorite
The Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....
indicates a non-terrestrial origin for these molecules rather than terrestrial contamination. Biologically relevant molecules identified so far include uracil
Uracil
Uracil is one of the four nucleobases in the nucleic acid of RNA that are represented by the letters A, G, C and U. The others are adenine, cytosine, and guanine. In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine.Uracil is a common and...
, an RNA nucleobase
Nucleobase
Nucleobases are a group of nitrogen-based molecules that are required to form nucleotides, the basic building blocks of DNA and RNA. Nucleobases provide the molecular structure necessary for the hydrogen bonding of complementary DNA and RNA strands, and are key components in the formation of stable...
, and xanthine
Xanthine
Xanthine , is a purine base found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine....
. These results demonstrate that many organic compounds which are components of life on Earth were already present in the early solar system and may have played a key role in life's origin.
In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life (the amino acid glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
) in a comet for the first time.
On August 8, 2011, a report, based on NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
studies with meteorites found on 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...
, was published suggesting building blocks of 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...
(adenine
Adenine
Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...
, guanine
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
and related organic molecules) may have been formed extraterrestrially in outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
. In October 2011, scientists reported that cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
contains complex organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. One of the scientists suggested that these complex organic compounds may have been related to the development of life on earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
Still under investigation/undetermined
- Of the four Viking biological experimentsViking biological experimentsThe two Viking spacecraft each carried four types of biological experiments to the surface of Mars in the late 1970s. These were the first Mars landers to carry out experiments to look for biosignatures of life on Mars. The landers used a robotic arm to put soil samples into sealed test containers...
performed by the Mars lander VikingViking programThe Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...
in 1976, only the LR (Labeled Release) experiment gave results that were initially indicative of life (metabolism). However, the similar results from heated controls, how the release of indicative gas tapered off, and the lack of organic molecules in soil samples all suggest that the results were the result of a non-living chemical reaction rather than biological metabolismMetabolismMetabolism 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...
. Later experiments showed that existing oxidizers in the Martian soil could reproduce the results of the positive LR Viking experiment. Despite this, the Viking's LR experiment designer remains convinced that it is diagnostic for life on Mars.
- A meteoriteMeteoriteA meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
originating from MarsMarsMars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
known as ALH84001ALH84001Allan Hills 84001 is a meteorite that was found in Allan Hills, Antarctica on December 27, 1984 by a team of U.S. meteorite hunters from the ANSMET project. Like other members of the group of SNCs , ALH 84001 is thought to be from Mars. However, it does not fit into any of the previously...
was shown in 1996 to contain microscopicMicroscopicThe microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
structures resembling small terrestrial nanobacteria. When the discovery was announced, many immediately conjectured that these were fossilFossilFossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s and were the first evidence of extraterrestrial lifeExtraterrestrial lifeExtraterrestrial life is defined as life that does not originate from Earth...
— making headlines around the world. Public interest soon started to dwindle as most experts started to agree that these structures were not indicative of life, but could instead be formed abiotically from organic molecules. However, in November 2009, a team of scientists at Johnson Space Center, including David McKay, reasserted that there was "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite and finding magnetite crystals.
- On May 11, 2001, two researchers from the University of NaplesUniversity of Naples Federico IIThe University of Naples Federico II is a university located in Naples, Italy. It was founded in 1224 and is organized into 13 faculties. It is the world's oldest state university and one of the oldest academic institutions in continuous operation...
claimed to have found live extraterrestrial bacteria inside a meteorite. Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci claim the bacteria were wedged inside the crystal structure of minerals, but were resurrected when a sample of the rock was placed in a culture medium. They believe that the bacteria were not terrestrial because they survived when the sample was sterilized at very high temperature and washed with alcohol. They also claim that the bacteria's DNA is unlike any on Earth. They presented a report on May 11, 2001, concluding that this is the first evidence of extraterrestrial life, documented in its genetic and morphological properties. Some of the bacteria they discovered were found inside meteorites that have been estimated to be over 4.5 billion years old, and were determined to be related to modern day Bacillus subtilisBacillus subtilisBacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...
and Bacillus pumilis bacteria on Earth but appears to be a different strain.
- An Indian and British team of researchers led by Chandra Wickramasinghe reported on 2001 that air samples over Hyderabad, India, gathered from the stratosphere by the Indian Space Research Organization, contained clumps of living cells. Wickramasinghe calls this "unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 km, above which no air from lower down would normally be transported". Two bacterial and one fungal species were later independently isolated from these filters which were identified as Bacillus simplex, Staphylococcus pasteuri and Engyodontium album respectively. The experimental procedure suggested that these were not the result of laboratory contamination, although similar isolation experiments at separate laboratories were unsuccessful.
- A reaction report at NASA AmesNASA Ames Research CenterThe Ames Research Center , is one of the United States of America's National Aeronautics and Space Administration 10 major field centers.The centre is located in Moffett Field in California's Silicon Valley, near the high-tech companies, entrepreneurial ventures, universities, and other...
indicated skepticism towards the premise that Earth life cannot travel to and reside at such altitudes. Max Bernstein, a space scientist associated with SETISETIThe search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
and Ames, argues the results should be interpreted with caution, noting that "it would strain one's credulity less to believe that terrestrial organisms had somehow been transported upwards than to assume that extraterrestrial organisms are falling inward". Pushkar Ganesh Vaidya from the Indian Astrobiology Research Centre reported in his 2009 paper that "the three microorganisms captured during the balloon experiment do not exhibit any distinct adaptations expected to be seen in microorganisms occupying a cometary niche".
- In 2005 an improved experiment was conducted by ISRO. On April 10, 2005 air samples were collected from six places at different altitudes from the earth ranging from 20 km to more than 40 km. Adequate precautions were taken to rule out any contamination from any microorganisms already present in the collection tubes. The samples were tested at two labs in India. The labs found 12 bacterial and 6 fungal colonies in these samples. The fungal colonies were PenicilliumPenicilliumPenicillium is a genus of ascomycetous fungi of major importance in the natural environment as well as food and drug production. Members of the genus produce penicillin, a molecule that is used as an antibiotic, which kills or stops the growth of certain kinds of bacteria inside the body...
decumbens, Cladosporium cladosporioidesCladosporium cladosporioidesCladosporium cladosporioides is a fungal plant pathogen that affects wheat. It is the source of the series of chemical compounds known as calphostins.- External links :* *...
, AlternariaAlternariaAlternaria is a genus of ascomycete fungi. Alternaria species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma...
sp. and Tilletiopsis albescens. Out of the 12 bacterial samples, three were identified as new species and named Janibacter hoyeli.sp.nov (after Fred HoyleFred HoyleSir 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...
), Bacillus isronensis.sp.nov (named after ISRO) and Bacillus aryabhati (named after the ancient Indian mathematician, AryabhataAryabhataAryabhata was the first in the line of great mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy...
). These three new species showed that they were more resistant to UV radiation than similar bacteria found on Earth. For any organism living so far up the Earth's atmosphere or having come from outside Earth, the UV radiation resistance would be extremely critical for survival.
Disputed
A NASA research group found a small number of Streptococcus mitisStreptococcus mitis
Streptococcus mitis is a mesophilic alpha-hemolytic species of Streptococcus that inhabits the human mouth. It is a Gram positive, coccus, facultative anaerobe and catalase negative. It can cause endocarditis...
bacteria living inside the camera of the Surveyor 3
Surveyor 3
Surveyor 3 was the third lander of the American unmanned Surveyor program sent to explore the surface of the Moon. Launched on April 17, 1967, Surveyor 3 landed on April 20, 1967 at the Mare Cognitum portion of the Oceanus Procellarum...
spacecraft when it was brought back to Earth by Apollo 12
Apollo 12
Apollo 12 was the sixth manned flight in the American Apollo program and the second to land on the Moon . It was launched on November 14, 1969 from the Kennedy Space Center, Florida, four months after Apollo 11. Mission commander Charles "Pete" Conrad and Lunar Module Pilot Alan L...
. They believed that the bacteria survived since the time of the craft's launch to the Moon. However, these reports are disputed by Leonard D. Jaffe, who was Surveyor program scientist and custodian of the Surveyor 3 parts brought back from the Moon, stated in a letter to the Planetary Society that an unnamed member of his staff reported that a "breach of sterile procedure" took place at just the right time to produce a false positive result. NASA was funding an archival study in 2007 that was trying to locate the film of the camera-body microbial sampling to confirm the report of a breach in sterile technique. NASA currently stands by its original assessment: see Reports of Streptococcus mitis on the moon
Reports of Streptococcus mitis on the moon
As part of the Apollo 12 mission, the camera from the Surveyor 3 probe was brought back to Earth. On analyzing the camera it was found that the common bacterium Streptococcus mitis was alive on the camera...
.
Hoaxes
A separate fragment of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seed capsule embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, the seed was found to be that of a European JuncaceaeJuncaceae
Juncaceae, the rush family, are a monocotyledonous family of flowering plants. There are eight genera and about 400 species. Members of the Juncaceae are slow-growing, rhizomatous, herbaceous plants, and they may superficially resemble grasses. They often grow on infertile soils in a wide range...
or Rush plant that had been glued into the fragment and camouflaged using coal dust
Coal dust
Coal dust is a fine powdered form of coal, which is created by the crushing, grinding, or pulverizing of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal.-Explosions:...
. The outer "fusion layer" was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought he sought to influence the 19th century debate on spontaneous generation
Spontaneous generation
Spontaneous generation or Equivocal generation is an obsolete principle regarding the origin of life from inanimate matter, which held that this process was a commonplace and everyday occurrence, as distinguished from univocal generation, or reproduction from parent...
— rather than panspermia — by demonstrating the transformation of inorganic to biological matter.
Objections to panspermia and exogenesis
- Life as we know it requires the elements hydrogenHydrogenHydrogen 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...
, 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...
, 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...
, oxygenOxygenOxygen 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...
, 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...
, phosphorusPhosphorusPhosphorus 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...
and 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...
(H, C, N, O, Fe, P, and S respectively) to exist at sufficient densities and temperatures for the chemical reactions between them to occur. These conditions are not widespread in the Universe, so this limits the distribution of life as an ongoing process. First, the elements C, N and O are only created after at least one cycle of star birth/death: this is a limit to the earliest time life could have arisen. Second, densities of elements sufficient for the formation of more complex molecules necessary to life (such as amino acids) only occur in molecular dust clouds (109–1012 particles/m3), and (following their collapse) in solar systems. Third, temperatures must be lower than those in stars (elements are stripped of electrons: 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...
state) but higher than in the interstellar mediumInterstellar mediumIn 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...
(reaction rates are too low). This restricts ongoing life to planetary environments where heavy elements are present at high densities, so long as temperatures are sufficient for plausible reaction rates. Note this does not restrict dormant forms of life to these environments, so this argument only contradicts the widest interpretation of panspermia — that life is ongoing and is spread across many different environments throughout the Universe — and presupposes that any life needs those elements, which the proponents of alternative biochemistriesAlternative biochemistryHypothetical 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...
do not consider certain. Recent findings have boosted or even confirmed these alternative biochemistry theories with the discovery that arsenicArsenicArsenic 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...
can act as a building block of life. However, the principal researcher for that finding is no longer sure that it is correct.
- Space is a damaging environment for life, as it would be exposed to radiationRadiationIn physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
, cosmic rayCosmic rayCosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s and stellar windStellar windA stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s. Studies of bacteria frozen in Antarctic glacierGlacierA glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s have shown that DNA has a half-lifeHalf-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 1.1 million years under such conditions, suggesting that while life may have potentially moved around within the Solar System it is unlikely that it could have arrived from an interstellar source. Environments may exist within meteors or cometCometA 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...
s that are somewhat shielded from these hazards. However, the extreme resistance of Deinococcus radioduransDeinococcus radioduransDeinococcus radiodurans is an extremophilic bacterium, one of the most radioresistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and has been listed as the world's toughest bacterium in The Guinness Book Of World Records.-Name...
to radiation, cold, dehydration and vacuum shows that at least one known organism is capable of surviving the hazards of space without need for special protection.
- Bacteria would not survive the immense heat and forces of an impact on Earth — no conclusions (whether positive or negative) have yet been reached on this point. However, most of the heat generated when a meteorMETEORMETEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
enters the Earth's atmosphere is carried away by ablationAblationAblation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
and the interiors of freshly landed meteorites are rarely heated much and are often cold. For example, a sample of hundreds of nematodeNematodeThe nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
worms on the space shuttle ColumbiaSpace Shuttle ColumbiaSpace Shuttle Columbia was the first spaceworthy Space Shuttle in NASA's orbital fleet. First launched on the STS-1 mission, the first of the Space Shuttle program, it completed 27 missions before being destroyed during re-entry on February 1, 2003 near the end of its 28th, STS-107. All seven crew...
survived its crash landing from 63 kmMesosphereThe mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
inside a 4 kg locker, and samples of already dead moss were not damaged. Though this is not a very good example, being protected by the man-made locker and possibly pieces of the shuttle, it lends some support to the idea that life could survive a trip through the atmosphere. The existence of Martian meteorites and Lunar meteoriteLunar meteoriteA Lunar meteorite is a meteorite that is known to have originated on the Moon.-Discovery:In January 1982, John Schutt, leading an expedition in Antarctica for the ANSMET program, found a meteorite that he recognized to be unusual...
s on Earth suggests material transfer from other celestial bodies to Earth happens regularly.
- Supporters of exogenesis also argue that on a larger scale, for life to emerge in one place in the Universe and subsequently spread to other planets would be simpler than similar life emerging separately on different planets. Thus, finding any evidence of extraterrestrial life similar to ours would lend credibility to exogenesis. However, this again assumes that the emergence of life in the entire Universe is rare enough as to limit it to one or few events or origination sites. Exogenesis still requires life to have originated from somewhere, most probably some form of geogenesis. Given the immense expanse of the entire Universe, it has been argued that there is a higher probability that there exists (or has existed) another Earth-like planet that has yielded life (geogenesis) than not. This explanation is more preferred under Occam's Razor than exogenesis since it theorizes that the creation of life is a matter of probability and can occur when the correct conditions are met rather than in exogenesis that assumes it is a singular event or that Earth did not meet those conditions on its own. In other words, exogenesis theorizes only one or few origins of life in the Universe, whereas geogenesis theorizes that it is a matter of probability depending on the conditions of the celestial body. Consider that even the most rare events on Earth can happen multiple times and independent of one another. However, since to date no extraterrestrial life has been confirmed, both theories still suffer from lack of information and too many unidentified variables.
- Even if life were to survive the hardships of space, or further, was being created in space, these would be very enduring life forms, as the theory itself proposes, and they would have already visibly populated and altered Venus and Mars as well as other moons in the solar system. The absence of life in Venus, with a somewhat similar composition to Earth's primitive conditions or the absence of life on Mars, given the proposed "resilience" of life in space, also negates this as a viable theory, at least from what can be observed in our own Solar System: Life would be hyper abundant all around our Solar System if this theory were true, and we observe the opposite, except on Earth.
Accidental panspermia
Thomas GoldThomas Gold
Thomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...
a professor of astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
suggested a "garbage theory" for the origin of life, the theory says that life on Earth might have spread from a pile of waste
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
products accidentally dumped on Earth long ago by extraterrestrials.
Directed panspermia
A second prominent proponent of panspermia was the late Nobel prize winner Professor Francis CrickFrancis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
, who along with Leslie Orgel
Leslie Orgel
Leslie Eleazer Orgel FRS was a British chemist.Born in London, England, Orgel received his B.A. in chemistry with first class honours from Oxford University in 1949...
proposed the hypothesis of 'directed panspermia'. This proposes that the seeds of life may have been purposely spread by an advanced extraterrestrial civilization. Later, after biologists had proposed that an "RNA world" might be involved in the origin of life, Crick noted that he had been overly pessimistic about the chances of life originating on Earth.
The application of directed panspermia has been proposed as a way to spread life from Earth to other solar systems. For example, microbial payloads launched by solar sails at speeds up to 0.0001 c (30,000 m/s) would reach targets at 10 to 100 light-years in 0.1 million to 1 million years. Fleets of microbial capsules can be aimed at clusters of new stars in star-forming clouds where they may land on planets, or captured by asteroids and comets and later delivered to planets. Payloads may contain extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...
s for diverse environments and cyanobacteria similar to early microorganisms. Hardy multicellular organisms (rotifer cysts) may be included to induce higher evolution.
The probability of hitting the target zone can be calculated from
where A(target) is the cross-section of the target area, dy is the positional uncertainty at arrival; a - constant (depending on units), r(target) is the radius of the target area; v the velocity of the probe; (tp) the targeting precision (arcsec/yr); and d the distance to the target (all units in SIU). Guided by high-resolution astrometry of 1×10−5 arcsec/yr, almost nearby target stars (Alpha PsA, Beta Pictoris) can be seeded by milligrams of launched microbes; while seeding the Rho Ophiochus star-forming cloud requires hundreds of kilograms of dispersed capsules.Theoretically, terrestrial spacecraft travelling to other celestial bodies such as the Moon, could carry with them microorganisms or other organic materials ubiquitous on Earth, thus raising the possibility that we can seed life on other planetary bodies. This is a concern among space researchers who try to prevent Earth contamination
Interplanetary contamination
Interplanetary Contamination is the hypothetical contamination of a sterile planetary body by human spacecraft, either deliberate or unintentional. This is considered a potential form of directed panspermia...
from distorting data, especially in regards to finding possible extraterrestrial life. Even the best sterilization techniques cannot guarantee that potentially invasive biologic or organic materials will not be unintentionally carried along. The harsh environments encountered throughout the rest of the solar system so far do not seem to support complex terrestrial life. However, matter exchange in form of meteor impacts
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...
has existed and will exist in the solar system even without human intervention.
Foton-M3 spacecraft
In September 2007, after enduring a 12-day orbital mission and a fiery reentry, the European unmanned spacecraft Foton-M3Foton-M
Foton-M is series of robotic spacecraft used by Russia and the European Space Agency for research conducted in the microgravity environment of Earth orbit. The Foton-M design is based on the design of the Foton, with several improvements including a new telemetry and telecommand unit for increased...
was retrieved from a field in Kazakhstan. The 5,500-pound capsule, seven-feet in diameter, carried a payload of 43 European experiments in a range of scientific disciplines – including fluid physics, biology, crystal growth, radiation exposure and astrobiology
Astrobiology
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...
. The capsule contained, among other things, lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
that were exposed to the radiation of space. Scientists also strapped basalt and granite disks riddled with cyanobacteria to the capsule's heat shield to see if the microorganisms could survive the brutal conditions of reentry. Some bacteria, lichens, spores, and even one animal (Tardigrades) were found to have survived the outer space environment and cosmic radiation.
The Living Interplanetary Flight Experiment
The Living Interplanetary Flight ExperimentLiving Interplanetary Flight Experiment
The Living Interplanetary Flight Experiment is an interplanetary mission developed by the Planetary Society. It consists of sending selected microorganisms on a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft in 2011, which is a sample-return...
, which is being developed by the Planetary Society
Planetary Society
The Planetary Society is a large, publicly supported, non-government and non-profit organization that has many research projects related to astronomy...
, will consist of sending selected microorganisms on a three-year interplanetary round-trip in a small capsule aboard the Russian Phobos-Grunt
Phobos-Grunt
Fobos-Grunt or Phobos-Grunt was an attempted Russian sample return mission to Phobos, one of the moons of Mars. It was launched on 9 November 2011 at 02:16 local time from the Baikonur Cosmodrome, but subsequent rocket burns intended to set the craft on a course for Mars failed, leaving it...
spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
in 2011. The goal is to test whether organisms can survive a few years in deep space. The experiment will test one aspect of transpermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another.
See also
External links
- Francis Crick's notes for a lecture on directed panspermia, dated 5 November 1976.