Life on Titan

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Formation of tholins in Titan's upper atmosphere

For life on Titan , which is based on water, Saturn orbits with its moons in an orbit that is too far from the sun (outside the classic habitable zone ). The emergence of life on the surface is unlikely, since water cannot exist there in liquid form. However, preliminary stages are not excluded.

In 2010, after analyzing data from the Cassini spacecraft, evidence of methane-based life was found on Titan’s surface. However, it cannot be ruled out that these are unknown chemical processes .

General

Among the planets and moons of its size class, Titan is the only celestial body in the solar system with a dense and cloud-rich atmosphere . Although its surface temperature is much lower than that of Earth, it is considered to be the most Earth-like celestial body in our solar system in terms of the dense nitrogen-rich atmosphere and the presence of liquids in it . Its gas envelope is about five times denser on the surface and the pressure about 50% higher than on earth. The shell consists mainly of nitrogen and contains hydrocarbons and traces of other organic compounds . The surface and the top layer of the mantle are made of ice and methane hydrate . Underneath there is possibly an ocean with liquid water, even though the temperatures there are below 0 ° C.

Organic compounds in the atmosphere

The only known bodies in the solar system whose atmosphere consists mainly of nitrogen are Earth and Titan. In the latter case, it is 98.4% nitrogen and around 1.6% argon as well as methane, which is predominant in the upper atmosphere due to its low density (57% of nitrogen). There are also traces of at least a dozen other organic compounds , including ethane , propane , ethyne and hydrogen cyanide . Helium , carbon dioxide and water were also found, but practically no free oxygen .

Since titanium has no magnetic field worth mentioning , its atmosphere is directly exposed to the solar wind , especially at its outer edge . In addition, it is subject to the effects of cosmic radiation and solar radiation, of which the UV component mentioned above is of chemical importance. Nitrogen and methane molecules hit by such high-energy particles of matter or photons are split into ions or very reactive radicals . These fragments form new bonds with other molecules, forming complex organic nitrogen compounds, the carbon compounds mentioned above, and various polycyclic aromatic hydrocarbons . In this way, polyynes that contain triple bonds are also formed in the upper titanium atmosphere . The polycyclic aromatic hydrocarbons can also contain nitrogen and clump together to form aerosols.

Tholine

The resulting heavier molecules slowly sink into deeper layers of the atmosphere and form the orange-colored nebula that envelops Saturn's moon. The astrophysicist Carl Sagan coined the term “ tholin ” for this mixture of compounds with an as yet unknown composition . He also suspected a layer of such molecules on the surface of titanium, in which chemical reactions could take place when energized, which are similar to those in primeval times on earth and have contributed to the origin of life on our planet. With these assumptions, Titan became one of the most interesting places in the solar system.

During its descent to the titanium surface, instruments of the Huygens probe examined the atmosphere. With the ion neutral mass spectrometer (INMS) it could be shown that the orange-colored fog contains small and medium-sized molecules. More informative were the data from the Cassini plasma spectrometer (CAPS), which was carried along especially for investigating the orange color of the atmosphere and which for the first time provided an explanation for the formation of tholins. It detected large, positively and negatively charged ions. In particular, the negatively charged ions probably play an unexpected role in the formation of tholins from carbon and nitrogen-containing compounds.

Speculation about (precursors to) life

Since Saturn and its satellites orbit far outside the habitable zone , the emergence of life is unlikely, but preliminary stages are not excluded. Overall, despite the low temperatures for cosmochemistry, very interesting processes are to be assumed on this moon, perhaps also precursors for a kind of chemical evolution . Due to the dense atmosphere of nitrogen and organic compounds, it is an important research object in astrobiology , as these conditions could be similar to those on primeval earth . A prebiotic development in the direction of life, comparable to that on earth, would, however, prevent the surface temperatures.

Steven Benner of the University of Florida believes that life in lakes can form from liquid hydrocarbons such as methane or ethane, as these are also suitable solvents for chemical reactions such as those found in living things. The chemical aggressiveness of these hydrocarbons is also less than that of water. So giant molecules like DNA would be more stable.

A team of researchers from France thinks it is possible that microscopic organisms in the methane lakes could use the energy that is released when hydrogen reacts with ethine (acetylene).

Titan could hold a key to understanding the origin of life on earth, as it is believed that primordial earth had a similar atmosphere and thus similar conditions prevailed.

In 2010, researchers from the University of Arizona had simulated the conditions in the titanic gas envelope in the laboratory. In doing so, they mixed nitrogen, methane and carbon monoxide, the main constituents of Titan's atmosphere, together. In this environment without water and exposed to strong radio radiation, the amino acids glycine and alanine , the basic building blocks of earthly proteins, were created. In addition, all five basic components of the nucleic acids RNA and DNA were formed - cytosine , adenine , thymine , guanine and uracil . The reactions took place completely within a gaseous environment. Sarah Hörst and Roger Yelle from the University of Arizona think it is possible that the basic building blocks on earth could not necessarily form in a primordial soup, but also in the atmosphere and then rain down on the surface.

Methane based life

Since very low temperatures of −180 ° C prevail on Titan’s surface and life requires a liquid medium for various processes, life on Titan’s surface would have to use a different substance than water. This leaves only the option of using liquid methane or ethane.

In 2010, the Cassini space probe discovered that hydrogen sank to the ground disappears on the surface. Hydrocarbons were also detected on the surface , but not the expected acetylene . In 2005, astrobiologist Chris McKay designed a hypothetical model that postulated acetylene as the ideal energy supply for methane-based life. One possibility would therefore be that this life consumes hydrogen, similar to how earthly life needs oxygen .

Current models of Titan's atmosphere suggest an even distribution of hydrogen in the various layers of the atmosphere. Ultraviolet light breaks down acetylene and methane molecules in the upper atmosphere, creating hydrogen as a by-product. The observed disappearance of hydrogen on the surface corresponds approximately to the same amount that escapes via the outer atmosphere. It is unlikely that the hydrogen will be stored underground in any way.

According to Cassini's spectrometer data, there is also less water ice on the surface than expected, but benzene and another previously unknown organic material. It appears that this organic material is covering the water ice with a layer of hydrocarbons only a few millimeters to centimeters thick, which persists despite the liquid methane and ethane flowing over the surface. In the case of heavy methane rains, this layer is washed off, but quickly re-forms.

NASA scientist Mark Allen suggested an unknown process in the atmosphere in which sunlight or cosmic rays convert acetylene into icy aerosols, which then sink to the surface, explaining the lack of acetylene. Another non-biological possibility is a chemical process in which acetylene and hydrogen on the surface are converted back to methane; but since it is too cold for that, this would require a mineral catalyst , which chemists have not yet known. One must first exclude all non-biological processes and consider methane-based life as a last resort, said Allen.

Should there be methane-based life on Titan, however, this would already be the second independent development of life in our solar system, which means that there is a high probability that life would be present on many habitable worlds of the cosmos.

Panspermia or Independent Origin?

An alternative hypothesis for the existence of life on Titan was proposed in 2006: if life is found on Titan, it could originally have come from Earth. By panspermia hundreds of millions of fragments could participate in large comet hitting earth microbes have been hurled into space. According to calculations, after a few million years some of them would have hit other celestial bodies in the solar system, including Titan. However, it is considered unlikely that earthly carbon-based life could thrive on titanium.

See also

Web links

Individual evidence

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  3. ^ AD Fortes: Exobiological implications of a possible ammonia-water ocean inside Titan . In: Icarus . tape 146 , no. 2 , 2000, pp. 444-452 , doi : 10.1006 / icar.2000.6400 .
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  8. ↑ Chemical dynamics of triacetylene formation and implications to the synthesis of polyynes atmosphere in Titan's. National Academy of Science, August 12, 2009, accessed November 4, 2009 .
  9. Jump up ↑ JH Waite, Jr., DT Young, TE Cravens, AJ Coates, FJ Crary, B. Magee, J. Westlake: The Process of Tholin Formation in Titan's Upper Atmosphere . In: Science . tape 316 , 2007, pp. 870 , doi : 10.1126 / science.1139727 .
  10. ^ A b A. J. Coates, FJ Crary, GR Lewis, DT Young, JH Waite, EC Sittler: Discovery of heavy negative ions in Titan's ionosphere . In: Geophys. Res. Lett. tape 34 , 2007, p. L22103 , doi : 10.1029 / 2007GL030978 .
  11. ↑ jpl.nasa.gov JPL: Cassini Sees Precursors to Aerosol Haze on Titan , Date: June 5, 2013, Accessed: June 11, 2013
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  13. ^ JH Waite, Jr., DT Young, TE Cravens, AJ Coates, FJ Crary, B. Magee, and J. Westlake: The Process of Tholin Formation in Titan's Upper Atmosphere . In: Science . tape 316 , 2007, pp. 870 , doi : 10.1126 / science.1139727 .
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  16. ↑ Hiroshi Imanaka, Bishun N. Khare, Jamie E. Elsila, Emma LO Bakes, Christopher P. McKay, Dale P. Cruikshank, Seiji Sugita, Takafumi Matsui, Richard N. Zare: Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze . In: Icarus . tape 168 , no. 2 , April 1, 2004, p. 344-366 , doi : 10.1016 / j.icarus.2003.12.014 .
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  18. ↑ Helen Matsos: Great Alien Debates. Astrobiology magazine, December 11, 2006, accessed November 6, 2009 .
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  22. ↑ Brett Gladman, Luke Dones, Harold F. Levinson, Joseph A. Burns: Impact Seeding and Reseeding in the Inner Solar System . In: Astrobiology . tape 5 , 2005, p. 483-496 , doi : 10.1089 / ast.2005.5.483 .
  23. ↑ Ralph D. Lorenz, Jonathan I. Lunine, Christopher P. McKay: Titan under a red giant sun: A new kind of “habitable” moon. (PDF; 335 kB) In: NASA Ames Research Center, Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona. November 15, 1997, accessed November 5, 2009 .
  24. ↑ Ilka Lehnen-Beyel: Mysterious gas envelope. Wissenschaft.de, October 8, 2010, accessed on September 9, 2019 .
  25. ^ Rainer Kayser: Building blocks of life in a titanium atmosphere? astronews, November 1, 2010, accessed November 8, 2010 .
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  27. ↑ Jonathan Lunine : Saturn's Titan: A Strict Test for Life's Cosmic Ubiquity . July 21, 2009, arxiv : 0908.0762 (English).
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