Climatic change on Mars.

It is pointed out that Mars is the only known planet with a major atmospheric constituent condensable at typical surface temperatures. The temperatures range from 290 K at equatorial noon to a temperature at the cold pole of 145 K in polar winter. There may be three different periods of climatic variation on Mars. Aspects of reversible climatic instability might possibly explain the channels and other features suggestive of the extensive occurrence of liquid water on Mars. An aqueous epoch on Mars would have important biological and other geological implications. Putative Martian organisms which flourish in the aqueous epoch may now be in cryptobiotic repose.