Life on Titan
Encyclopedia
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,...
, is at present an open question and a topic of scientific discussion and research.
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....
is far colder than 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 its surface seems to lack liquid 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...
; factors which have led some scientists to consider life there unlikely. On the other hand, the following points have been made in favor of Titan's suitability to sustain some form of life:
- Titan appears to have lakesLakes of TitanThe Lakes of Titan, a moon of Saturn, are bodies of liquid ethane and methane that have been detected by the Cassini–Huygens space probe, and had been suspected long before. The large ones are known as maria and the small ones as lacūs .-History:The possibility that there were hydrocarbon seas on...
of liquid ethaneEthaneEthane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
and/or liquid 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...
on its surface, as well as rivers and seas, which some scientific models (still tentative and debated) suggest could support non-water-based life. - It has also been suggested that life may exist in a sub-surface ocean consisting of water and ammoniaAmmoniaAmmonia 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...
. - Titan's atmosphereAtmosphere of TitanThe atmosphere of Titan is known as the only fully developed atmosphere that exists on a natural satellite in our solar system.-History:The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas Solà, who observed distinct limb darkening on Titan in 1903, and...
is thick, chemically active, and is known to be rich in organic compoundOrganic compoundAn 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...
s; this has led to speculation about whether chemical precursors of life may have been generated there. - The atmosphere also contains 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...
gas, which is cycling through the atmosphere and the surface environment, and which living things comparable to Earth methanogenMethanogenMethanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria...
s could combine with some of the organic compounds (such as acetyleneAcetyleneAcetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
) to obtain energy.
In June 2010, scientists analysing data from the Cassini–Huygens mission reported anomalies in the atmosphere near the surface which are consistent with the presence of methane-producing organisms, but may alternatively be due to non-living chemical or meteorological processes. The Cassini–Huygens mission was not equipped to provide direct evidence for biology or complex organics.
Surface temperature
Due to its distance from the Sun, Titan is much colder than the Earth. Its surface temperature is about 94 K (−179 °C, or −290 °F). At these temperatures, water ice does not melt, evaporate or sublime, but remains solid.Because of the extreme cold and also because of lack 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...
(CO2) in the atmosphere, scientists such as Jonathan Lunine
Jonathan Lunine
Jonathan I. Lunine is an American planetary scientist and physicist. Lunine teaches at Cornell University, where he is the David C. Duncan Professor in the Physical Sciences. Having published more than 200 research papers, Lunine is at the forefront of research into planet formation, evolution, and...
have viewed Titan less as a likely habitat for extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
, than as an experiment for examining theories on the conditions that prevailed prior to the appearance of life on Earth. However Lunine does not rule out life in an environment of liquid methane and ethane, and has written about what discovery of such a life form (even if very primitive) would imply about the prevalence of life in the universe.
Past theories about the temperature
In the 1970s, astronomers found unexpectedly high levels of infraredInfrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
emissions from Titan. One possible explanation for this was the surface was warmer than expected, due to a greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
. Some estimates of the surface temperature even approached temperatures in the cooler regions of the Earth. There was, however, another possible explanation for the infrared emissions: Titan's surface was very cold, but the upper atmosphere was heated due to absorption of ultraviolet light by molecules such as ethane, ethylene and acetylene.
In September 1979, Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...
, the first space probe to conduct fly-by observations of Saturn and its moons, sent data showing Titan's surface to be extremely cold by Earth standards, and much below the temperatures generally associated with planetary habitability
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...
.
Future temperature
Titan may become warmer in the future. Six billion years from now, as the Sun becomes a red giantRed giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
, surface temperatures could rise to ~200 K (-73.2 °C), high enough for stable oceans of water/ammonia mixture to exist on the surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, lessening the anti-greenhouse effect on the surface and enabling the greenhouse created by atmospheric methane to play a far greater role. These conditions together could create an environment agreeable to exotic forms of life, and will persist for several hundred million years. This was sufficient time for simple life to evolve on Earth, although the presence of ammonia on Titan will cause the same chemical reactions to proceed more slowly.
Absence of liquid water
Apparent lack of liquid water on Titan has been cited by NASA astrobiologistAstrobiology
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,...
Andrew Pohorille as an argument against life there. Pohorille considers that water is important not only as the solvent used by "the only life we know" but also because its chemical properties are "uniquely suited to promote self-organization of organic matter". He has questioned whether prospects for finding life on Titan's surface are sufficient to justify the expense of a mission which would look for it.
Formation of complex molecules
While the Cassini–Huygens mission was not equipped to provide evidence for biosignatureBiosignature
A biosignature is any substance -such as an element, isotope, or molecule - or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of...
s or complex organics, it showed an environment on Titan that is similar, in some ways, to ones theorized for the primordial Earth. Scientists believe that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan, with the important exception of a lack of water vapor on Titan. Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution.
The Miller-Urey experiment
Miller-Urey experiment
The Miller and Urey experiment was an experiment that simulated hypothetical conditions thought at the time to be present on the early Earth, and tested for the occurrence of chemical origins of life. Specifically, the experiment tested Alexander Oparin's and J. B. S...
and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like 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 can be generated. The reaction starts with dissociation
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
of 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...
and methane, forming hydrogen cyanide and acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
. Further reactions have been studied extensively.
In October 2010, Sarah Horst of the University of Arizona
University of Arizona
The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...
reported finding the five nucleotide bases -- 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...
and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
-- among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Horst also found amino acids, the building blocks of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
. She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.
Possible subsurface habitats
Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. While the analogy assumes the presence of liquid water for longer periods than is currently observable, several theories suggest that liquid water from an impact could be preserved under a frozen isolation layer. It has also been observed that liquid ammonia oceans could exist deep below the surface; one model suggests an ammonia–water solution as much as 200 km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive". Heat transferHeat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
between the interior and upper layers would be critical in sustaining any sub-surface oceanic life. Detection of microbial life on Titan would depend on its biogenic effects. That the atmospheric methane and nitrogen could be examined for biogenic origin, for example.
Liquid methane/ethane environment at the surface
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, and produce methane instead of carbon dioxide. By comparison, some methanogens on Earth obtain energy by reacting hydrogen with carbon dioxide, producing methane and water.
Solvents
There is debate about the effectiveness of methane as a solvent for life compared to water; while water is a more powerful 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.Findings consistent with surface life
In 2005, astrobiologists Chris McKay and Heather Smith predicted that if methanogenic life is consuming atmospheric hydrogen in sufficient volume, it will have a measurable effect on the mixing ratioMixing ratio
In chemistry and physics, the dimensionless mixing ratio is defined as the abundance of one component of a mixture relative to that of all other components...
in the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
of Titan. The effects predicted included a level of acetylene much lower than otherwise expected, as well as a reduction in the concentration of hydrogen itself.
Evidence consistent with the predictions was reported in June 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...
, who noted an overabundance of molecular hydrogen in the 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. Another paper released the same month showed very low levels of acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
on Titan's surface.
Chris McKay agreed with Stobel that presence of life, as suggested in McKay's 2005 article, is a possible explanation for the findings about hydrogen and acetylene; but also cautioned that other explanations are currently more likely: namely the possibility that the results are due to human error
Human Error
Human Error is the stage name of Rafał Kuczynski , a polish electronic musician, working mostly in the ambient music genre, produced only with a computer...
, to a meteorological process, or to the presence of some mineral catalyst. He noted that such a catalyst, effective at -178°C (95K), is presently unknown, and would in itself be a startling discovery, though less startling than discovery of an extraterrestrial life form.
Media coverage
The June 2010 findings gave rise to considerable media interest, including a report in the British newspaper, the TelegraphThe Daily Telegraph
The Daily Telegraph is a daily morning broadsheet newspaper distributed throughout the United Kingdom and internationally. The newspaper was founded by Arthur B...
, which spoke of clues to the existence of "primitive aliens".
Titan as a test case
Titan is presented as a test case for the relation between chemical reactivity and life, in a 2007 report on life's limiting conditions prepared by a committee of scientists under the United States National Research CouncilUnited 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, considered that "if life is an intrinsic property of chemical reactivity, life should exist on Titan. Indeed, for life not to exist on Titan, we would have to argue that life is not an intrinsic property of the reactivity of carbon-containing molecules under conditions where they are stable..."
Gaia hypothesis and Titan
David GrinspoonDavid Grinspoon
David H. Grinspoon is an American astrobiologist. He is the current curator of Astrobiology at the Denver Museum of Nature and Science. He has published numerous works, such as Lonely Planets: The Natural Philosophy of Alien Life, which won the 2004 PEN literary award for nonfiction.Currently, he...
, one of the scientists who in 2005 proposed that hypothetical organisms on Titan might use hydrogen and acetylene as an energy source, has mentioned the Gaia hypothesis
Gaia hypothesis
The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet.The scientific investigation of the...
in the context of discussion about Titan life. He suggests that, just as Earth's environment and its organisms have evolved together, the same thing is likely to have happened on other worlds with life on them. In Grinspoon's view, worlds which are "geologically and meteorologically alive are much more likely to be biologically alive as well".
Panspermia or independent origin?
An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it would be statistically more likely to have originated from Earth (the process called panspermia) than to have appeared independently (the process called abiogenesisAbiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...
). It is theorized that large asteroid and cometary impacts on Earth's surface have caused hundreds of millions of fragments of microbe-laden rock to escape Earth's gravity. Calculations indicate that a number of these would encounter many of the bodies in the solar system, including Titan. On the other hand, Jonathan Lunine has argued that any living things in Titan's cryogenic hydrocarbon lakes would need to be so different chemically from Earth life that it would not be possible for one to be the ancestor of the other. In Lunine's view, presence of organisms in Titan's lakes would mean a second, independent origin of life within the solar system, implying that life has a high probability of emerging on habitable worlds throughout the cosmos.
Proposed missions
The proposed Titan Mare ExplorerTitan Mare Explorer
Titan Mare Explorer is a proposed spacecraft lander that, if launched, would probe Titan, the largest moon of the planet Saturn, and would perform the first exploration of an extraterrestrial sea...
mission, a low-cost lander which would splash down in a lake, "would have the possibility of detecting life", according to cosmologist Chris Impey of the University of Arizona.