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.
The term was coined by Isaac Asimov in his essay "The Thalassogens", later published in his 1972 collection The Left Hand of the Electron.
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:
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.