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In astronomy, a thalassogen denotes a substance capable of forming a planetary ocean. Thalassogens are not necessarily life sustaining, although most interest has been in the context of extraterrestrial life.
Elements making up thalassogens have to be relatively abundant, the substance must be chemically stable in its environment, and must remain liquid under the conditions found on some planets. Freitas gives the following table, noting that the liquid range typically increases with increasing pressure:
|Possible Thalassogen||Melting Point (K)||Boiling Point (K)||Liquidity Range (K)||Critical temperature (K)||Critical pressure (atm)|
|Helium||0.95 (26 atm)||4.55||3.6||5.3||2.26|
|Carbon Dioxide||(216.6) (5.2 atm)||(304.3) (72.8 atm)||(< 87.7)||304.3||72.8|
The critical temperature and pressure represents the point where the distinction between gas and liquid vanishes, a possible upper limit for life (although life in supercritical fluids has been discussed both in science  and fiction, such as in Close to Critical by Hal Clement).
Later authors have also suggested sulfuric acid, ethane, and water/ammonia mixtures as possible thalassogens. The discovery of possible subsurface oceans on moons such as Europa also extends the range of possible environments.
- Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm
- 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. National Academies Press (2007)
- Isaac Asimov, The Thalassogens, The Magazine of Fantasy and Science Fiction, December 1970, ed. Edward L. Ferman, publ. Mercury Press, Inc., $0.60,
- Isaac Asimov, The Left Hand of the Electron, Doubleday, 0-385-04345-7, 225pp, 1972
- Bains, William (June 2004). "Many chemistries could be used to build living systems". Astrobiology 4 (2): 137–67. doi:10.1089/153110704323175124. PMID 15253836.