No. 82: Jul-Aug 1992
Mars may not be orbited by huge artificial satellites of alien provenance, but its geological history is looking more and more as if could have supported or perhaps still does support life.
"A large number of anomalous landforms on Mars can be attributed to glaciation, including the action of ice and meltwater. Glacial landscapes are concentrated south of lat -33° and in the Northern Plains suggesting vast Austral and Boreal ice sheets. Crater densities on the glaciated terrains indicate that the final glacial epoch occurred late in Martian history. Thus, Mars may have had a relatively warm, moist climate and dense atmosphere much later than previously believed."
(Kargel, Jeffrey S., and Strom, Robert G.; "Ancient Glaciation on Mars," Geology, 20:3, 1992.)
If Mars was warm and wet not too long ago, as implied above, perhaps life did gain a foothold there through either independent invention or, perhaps, through seeding by template-carrying comets or meteorites. P.J. Boston et al have investigated one possible Martian ecosystem:
"We have reexamined the question of extant microbial life on Mars in light of the most recent information about the planet and recently discovered nonphotosynthetic ecosystems on Earth -- deep sea hydrothermal vent communities and deep subsurface aquifer communities. On Mars, protected subsurface niches associated with hydrothermal activity could have continued to support life even after surface conditions became inhospitable. Geochemical evidence from the SNC meteorites and geomorphological evidence for recent volcanism suggest that such habitats could persist to the present time...We suggest a possible deep subsurface microbial ecology similar to those discovered to depths of several kilometers below the surface of the Earth."
(Boston, Penelope J., et al; "On the Possibility of Chemosynthetic Ecosystems in Subsurface Habitats on Mars," Icarus, 95:300, 1992.)
Comment. Although Boston et al speak in terms of microscopic Martian life, there is no reason why chemosynthetic life forms could not be large -- perhaps even large enough to leave traces on the Martian surface!
Reference. Our catalog The Moon and the Planets has an entire chapter (AM) on Martian geology and possible biology. Details here.