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Life on Titan

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Title: Life on Titan  
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Life on Titan

Multi-spectral view of Titan

Whether there is life on Titan, the largest moon of Saturn, is at present an open question and a topic of scientific assessment and research. Titan is far colder than Earth, and its surface seems to lack liquid water; factors which have led some scientists to consider life there unlikely. On the other hand, some scientists speculate on Titan's suitability to sustain some form of non-water-based life.

In June 2010, scientists analysing data from the organic compounds.

Contents

  • Surface temperature 1
    • Past hypotheses about the temperature 1.1
    • Future temperature 1.2
  • Absence of surface liquid water 2
    • Possible subsurface liquid water 2.1
  • Formation of complex molecules 3
  • Hypotheses 4
    • Hydrocarbons as solvents 4.1
    • Comparative habitability 4.2
    • Titan as a test case 4.3
    • Panspermia or independent origin? 4.4
  • Proposed missions 5
  • See also 6
  • References 7

Surface temperature

Due to its distance from the Sun, Titan is much colder than Earth. Its surface temperature is about 90 K (−179 °C, or −290 °F). At these temperatures, water ice —if present— does not melt, evaporate or sublime, 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.[2] 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.[3]

Past hypotheses about the temperature

In the 1970s, astronomers found unexpectedly high levels of infrared emissions from Titan.[4] 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.[4]

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.[5]

Future temperature

Titan may become warmer in the future.[6] Six billion years from now, as the Sun becomes a red giant, surface temperatures could rise to ~200 K (−70 °C), high enough for stable oceans of 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.[6] 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.[6]

Absence of surface liquid water

Apparent lack of [7]

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.[8] If so, it has been suggested that life may exist in a sub-surface ocean consisting of water and ammonia.[9]

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.[10] It has also been proposed that liquid ammonia oceans could exist deep below the surface;[11][12] 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".[9] Heat transfer between the interior and upper layers would be critical in sustaining any sub-surface oceanic life.[11] 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.[9]

Formation of complex molecules

Titan is the only known natural satellite (moon) in the Solar System that is known to have a fully developed acetylene) to obtain energy.[13][14][15]

gases in Titan's atmosphereHNC (left) and HC3N (right).

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.[16]

In October 2010, Sarah Horst 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. Horst 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.[17]

In April 2013, NASA reported that complex [18] In June 2013, polycyclic aromatic hydrocarbons (PAHs) were detected in the upper atmosphere of Titan.[19]

Hypotheses

Hydrocarbons as solvents

Hydrocarbon lakes on Titan: Cassini radar image, 2006.

Although all living things on Earth (including methanogens) use liquid water as a solvent, it is conceivable that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane.[20] Water is a stronger solvent than hydrocarbons,[21] however, water is more chemically reactive, and can break down large organic molecules through hydrolysis.[20] A life-form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.[20]

Titan appears to have

  1. ^ a b c Mckay, Chris (2010). "Have We Discovered Evidence For Life On Titan".  
  2. ^ "Saturn's Moon Titan: Prebiotic Laboratory". Astrobiology Magazine. August 11, 2004. Retrieved 2004-08-11. 
  3. ^ Jonathan Lunine "Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity" (accepted for publication in Proceedings of the American Philosophical Society), July 21, 2009 (Revised November 7, 2009)
  4. ^ a b Sagan, Carl (1979). Broca's Brain - the Romance of Science. Hodder and Stoughton.   pp 185–187.
  5. ^ "The Pioneer Missions". Pioneer Project. NASA, Jet Propulsion Laboratory. March 26, 2007. Retrieved 2007-08-19. 
  6. ^ a b c Ralph D. Lorenz, Jonathan I. Lunine, Christopher P. McKay (1997). "Titan under a red giant sun: A new kind of "habitable" moon" (PDF). NASA Ames Research Center, Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona. Retrieved 2008-03-21. 
  7. ^ Pohorille, Andrew (2009-05-13). "Comment on Titan First". Retrieved 2013-09-02. 
  8. ^ Jia-Rui Cook and Dwayne Brown (2012-06-28). "Cassini Finds Likely Subsurface Ocean on Saturn Moon". NASA News release. 
  9. ^ a b c Fortes, A. D. (2000). "Exobiological implications of a possible ammonia-water ocean inside Titan".  
  10. ^ Artemivia N., Lunine J, (2003). "Cratering on Titan: impact melt, ejecta, and the fate of surface organics". Icarus 164: 471–480.  
  11. ^ a b Grasset, O.; Sotin, C.; Deschamps, F. (2000). "On the internal structure and dynamic of Titan".  
  12. ^ Richard A. Lovett Saturn Moon Titan May Have Underground Ocean, National Geographic, March 20, 2008
  13. ^ a b c Jia-Rui Cook and Cathy Weselby (2010-06-03). "What is Consuming Hydrogen and Acetylene on Titan?". NASA News release. 
  14. ^ a b c Hadhazy, Adam (July 30, 2008). "Scientists Confirm Liquid Lake, Beach on Saturn's Moon Titan".  
  15. ^ a b c Choi, Charles Q. (7 June 2010). "Strange Discovery on Titan Leads to Speculation of Alien Life".  
  16. ^ Raulin F., Owen T. (2002). "Organic chemistry and exobiology on Titan". Space Science Review 104 (1–2): 377–394.  
  17. ^ Staff (October 8, 2010). "Titan's haze may hold ingredients for life". Astronomy. Retrieved 2010-10-14. 
  18. ^ Staff (April 3, 2013). "NASA team investigates complex chemistry at Titan".  
  19. ^ López-Puertas, Manuel (June 6, 2013). "PAH's in Titan's Upper Atmosphere".  
  20. ^ a b c Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; The Limits of Organic Life in Planetary Systems; The National Academies Press, 2007; page 74.
  21. ^ a b c "What is Consuming Hydrogen and Acetylene on Titan?". NASA/JPL. 2010. Retrieved 2010-06-06. 
  22. ^ a b c d McKay, C. P.; Smith, H. D. (2005). "Possibilities for methanogenic life in liquid methane on the surface of Titan". Icarus 178 (1): 274–276.  
  23. ^ Darrell F. Strobel (2010). "Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions" (pdf). Icarus 208: 878–886.  
  24. ^ Could Alien Life Exist in the Methane Habitable Zone? Keith Cooper, Astrobiology Magazine16 November 2011
  25. ^ Andew Hough (June 5, 2010). "'"Titan: Nasa scientists discover evidence 'that alien life exists on Saturn's moon. Telegraph.co.uk. Retrieved 2010-10-26. 
  26. ^ a b Alan Boyle (2011-11-22). "Which alien worlds are most livable?". msnbc.com. Retrieved 2012-01-27. 
  27. ^ Raulin, F. (2005). "Exo-astrobiological aspects of Europa and Titan: From observations to speculations".  
  28. ^ Staff (October 4, 2010). "Lakes on Saturn's Moon Titan Filled With Liquid Hydrocarbons Like Ethane and Methane, Not Water". ScienceDaily. Retrieved 2010-10-05. 
  29. ^ Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; [1]; The National Academies Press, 2007; pages 74-75
  30. ^ Schulze-Makuch, D., and D.H. Grinspoon (2005). "Biologically enhanced energy and carbon cycling on Titan?".  
  31. ^ Leslie Mullen (September 22, 2005). "The Living Worlds Hypothesis". Astrobiology Magazine. Retrieved 2010-10-29. 
  32. ^ "Earth could seed Titan with life". BBC News. March 18, 2006. Retrieved 2007-03-10. 
  33. ^ Gladman, Brett; Dones, Luke; Levinson, Harold F.; Burns, Joseph A. (2005). "Impact Seeding and Reseeding in the Inner Solar System". Astrobiology 5 (4): 483–496.  
  34. ^ Jonathan Lunine "Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity" (accepted for publication in Proceedings of the American Philosophical Society), July 21, 2009 (Revised November 7, 2009), page 13
  35. ^ Jonathan Lunine "Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity" (accepted for publication in Proceedings of the American Philosophical Society), July 21, 2009 (Revised November 7, 2009), page 18
  36. ^ Chris Impey (Jan 31, 2011). "Jan 31st: Life on Titan". 365 Days of Astronomy. Retrieved 2011-06-23. 

References

See also

The proposed Titan Mare Explorer 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.[36]

Proposed missions

[35] An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it could have originated from Earth in a process called

Panspermia or independent origin?

[31]

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 Council. The committee, chaired by John 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..."[29]

Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution. [28] Scientists think 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.[27] While the Cassini–Huygens mission was not equipped to provide evidence for

Titan as a test case

[26] Using this index, based on data available in late 2011, the model suggests that Titan has the highest current habitability rating of any known world, other than Earth.[26] In order to assess the likelihood of finding any sort of life on various planets and moons,

Comparative habitability

The June 2010 findings gave rise to considerable media interest, including a report in the British newspaper, the Telegraph, which spoke of clues to the existence of "primitive aliens".[25]

Chris McKay agreed with Strobel 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, to a meteorological process, or to the presence of some mineral catalyst enabling hydrogen and acetylene to react chemically.[1][24] He noted that such a catalyst, one effective at −178 °C (95 K), is presently unknown and would in itself be a startling discovery, though less startling than discovery of an extraterrestrial life form.[1]

Evidence consistent with these predictions was reported in June 2010 by Darrell Strobel of Johns Hopkins University, who analysed measurements of hydrogen concentration in the upper and lower atmosphere. Strobel found that the hydrogen concentration in the upper atmosphere is so much larger than near the surface that the physics of diffusion leads to hydrogen flowing downwards at a rate of roughly 1025 molecules per second. Near the surface the downward-flowing hydrogen apparently disappears.[22][21][23] Another paper released the same month showed very low levels of acetylene on Titan's surface.[21]

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 ratio in the troposphere 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.[22]

on Earth obtain energy by reacting hydrogen with carbon dioxide, producing methane and water. methanogens By comparison, some [22], and produce methane instead of carbon dioxide.glucose instead of acetylene, react it with 2 in place of O2 Such hypothetical creatures would take in H[22]

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