Whether there is life on Titan, the largest moon of Saturn, is currently an open question and a topic of scientific assessment and research. Titan is far colder than Earth, but of all the places in the Solar System, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface. Its thick atmosphere is chemically active and rich in carbon compounds. On the surface there are small and large bodies of both liquid methane and ethane, and it is likely that there is a layer of liquid water under its ice shell. Some scientists speculate that these liquid mixes may provide prebiotic chemistry for living cells different from those on Earth.
In June 2010, scientists analyzing data from the Cassini–Huygens mission reported anomalies in the atmosphere near the surface which could be 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 look directly for micro-organisms or to provide a thorough inventory of complex organic compounds.
Chemistry
Titan's consideration as an environment for the study of prebiotic chemistry or potentially exotic life stems in large part due to the diversity of the organic chemistry that occurs in its atmosphere, driven by photochemical reactions in its outer layers. The following chemicals have been detected in Titan's upper atmosphere by Cassinis mass spectrometer:
Study Magee, 1050 km Cui, 1050 km Cui, 1077 km Waite et al., 1000–1045 km Density (cm−3) (3.18±0.71) × 109 (4.84±0.01) × 109 (2.27±0.01) × 109 (3.19, 7.66) × 109Proportions of different species Nitrogen (96.3±0.44)% (97.8±0.2)% (97.4±0.5)% (95.5, 97.5)% 14N15N (1.08±0.06)% Methane (2.17±0.44)% (1.78±0.01)% (2.20±0.01)% (1.32, 2.42)% 13CH4 (2.52±0.46) × 10−4 Hydrogen (3.38±0.23) × 10−3 (3.72±0.01) × 10−3 (3.90±0.01) × 10−3 Acetylene (3.42±0.14) × 10−4 (1.68±0.01) × 10−4 (1.57±0.01) × 10−4 (1.02, 3.20) × 10−4 Ethylene (3.91±0.23) × 10−4 (5.04±0.04) × 10−4 (4.62±0.04) × 10−4 (0.72, 1.02) × 10−3 Ethane (4.57±0.74) × 10−5 (4.05±0.19) × 10−5 (2.68±0.19) × 10−5 (0.78, 1.50) × 10−5 Hydrogen cyanide (2.44±0.10) × 10−4 40Ar (1.26±0.05) × 10−5 (1.25±0.02) × 10−5 (1.10±0.03) × 10−5 Propyne (9.20±0.46) × 10−6 (9.02±0.22) × 10−6 (6.31±0.24) × 10−6 (0.55, 1.31) × 10−5 Propene (2.33±0.18) × 10−6 (0.69, 3.59) × 10−4 Propane (2.87±0.26) × 10−6 <1.84 × 10−6 <2.16e-6(3.90±0.01) × 10−6 Diacetylene (5.55±0.25) × 10−6 (4.92±0.10) × 10−6 (2.46±0.10) × 10−6 (1.90, 6.55) × 10−6 Cyanogen (2.14±0.12) × 10−6 (1.70±0.07) × 10−6 (1.45±0.09) × 10−6 (1.74, 6.07) × 10−6 Cyanoacetylene (1.54±0.09) × 10−6 (1.43±0.06) × 10−6 <8.27 × 10−7 Acrylonitrile (4.39±0.51) × 10−7 <4.00 × 10−7 <5.71 × 10−7 Propanenitrile (2.87±0.49) × 10−7 Benzene (2.50±0.12) × 10−6 (2.42±0.05) × 10−6 (3.90±0.01) × 10−7 (5.5, 7.5) × 10−3 Toluene (2.51±0.95) × 10−8 <8.73 × 10−8 (3.90±0.01) × 10−7 (0.83, 5.60) × 10−6
As mass spectrometry identifies the atomic mass of a compound but not its structure, additional research is required to identify the exact compound that has been detected. Where the compounds have been identified in the literature, their chemical formula has been replaced by their name above. The figures in Magee (2009) involve corrections for high pressure background. Other compounds believed to be indicated by the data and associated models include ammonia, polyynes, amines, ethylenimine, deuterium hydride, allene, 1,3 butadiene and any number of more complex chemicals in lower concentrations, as well as carbon dioxide and limited quantities of water vapour.
Surface temperature Due to its distance from the Sun, Titan is much colder than Earth. Its surface temperature is about 94 K (−179 °C, or −290 °F). At these temperatures, water ice—if present—does not melt, evaporate or sublimate, but remains solid. Because of the extreme cold and also because of lack of carbon dioxide (CO2) in the atmosphere, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for extraterrestrial life, than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth. Even though the usual surface temperature on Titan is not compatible with liquid water, calculations by Lunine and others suggest that meteor strikes could create occasional "impact oases"—craters in which liquid water might persist for hundreds of years or longer, which would enable water-based organic chemistry.Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; ; The National Academies Press, 2007; page 74
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 hypothesis about the temperature
In the 1970s, astronomers found unexpectedly high levels of infrared emissions from Titan. One possible explanation for this was the surface was warmer than expected, due to a greenhouse effect. Some estimates of the surface temperature even approached temperatures in the cooler regions of 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. pp 185–187.
In September 1979, Pioneer 11, 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.
Future temperature
Titan may become warmer in the future. Five to six billion years from now, as the Sun becomes a red giant, surface temperatures could rise to ~, high enough for stable oceans of a water–ammonia mixture to exist on its surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, lessening the anti-greenhouse effect on its surface and enabling the greenhouse effect 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 could cause the same chemical reactions to proceed more slowly.
Absence of surface liquid water
The lack of liquid water on Titan's surface was cited by NASA astrobiologist Andrew Pohorille in 2009 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 that would look for it.
Possible subsurface liquid water
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 hypotheses suggest that liquid water from an impact could be preserved under a frozen isolation layer. It has also been proposed that ammonia oceans could exist deep below the surface;Richard A. Lovett Saturn Moon Titan May Have Underground Ocean, National Geographic, March 20, 2008 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 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. For example, the atmospheric methane and nitrogen could be examined for biogenic origin.
Data published in 2012 obtained from NASA's Cassini spacecraft, have strengthened evidence that Titan likely harbors a layer of liquid water under its ice shell.
Formation of complex molecules
Titan is the only known natural satellite (moon) in the Solar System that has a fully developed atmosphere that consists of more than trace gases. Titan's atmosphere is thick, chemically active, and is known to be rich in organic compounds; this has led to speculation about whether chemical precursors of life may have been generated there. The atmosphere also contains hydrogen gas, which is cycling through the atmosphere and the surface environment, and which living things comparable to Earth methanogens could combine with some of the organic compounds (such as acetylene) to obtain energy.
The Miller–Urey experiment 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 tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.
In October 2010, Sarah Hörst of the University of Arizona reported finding the five nucleotide bases—building blocks of DNA and RNA—among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Hörst also found amino acids, the building blocks of protein. She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.
In April 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan. In June 2013, polycyclic aromatic hydrocarbons (PAHs) were detected in the upper atmosphere of Titan.
A team of researchers led by Martin Rahm suggested in 2016 that polyimine could readily function as a building block in Titan's conditions. Titan's atmosphere produces significant quantities of hydrogen cyanide, which readily polymerize into forms which can capture light energy in Titan's surface conditions. As of yet, the answer to what happens with Titan's cyanide is unknown; while it is rich in the upper atmosphere where it is created, it is depleted at the surface, suggesting that there is some sort of reaction consuming it.
In July 2017, Cassini scientists positively identified the presence of carbon chain anions in Titan's upper atmosphere which appeared to be involved in the production of large complex organics. These highly reactive molecules were previously known to contribute to building complex organics in the Interstellar Medium, therefore highlighting a possibly universal stepping stone to producing complex organic material.
In July 2017, scientists reported that acrylonitrile (C2H3CN), a chemical possibly essential for life by being related to cell membrane and vesicle structure formation, had been found on Titan.
In October 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex polycyclic aromatic hydrocarbon (PAH) chemicals. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it.
Hypotheses
Hydrocarbons as solvents