Methane Hints at Life on Mars

NASA scientists have discovered regularly replenished supplies of methane gas on the Red Planet. That raises two possibilities: The gas could be formed by geologic activity or, as anyone who has spent time around cows can tell you, it could be a sign of life.

BBC:

Some scientists consider it possible that microbes could have survived for aeons below the Martian permafrost layer, where water changes from ice into liquid.

In deep canyons, or the walls of yawning craters, ice might plug fissures or pores connecting these sub-permafrost regions to the atmosphere.

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NASA