Red Nova talks about a possible interpretation of the methane signatures seen in the Martian atmosphere. People are excited because life is one big source of earth methane, but Mars is different enough that it may come from a process not familiar to earth.
Dorothy Oehler (pronounced "oiler" I'm sure) of NASA thinks that the methane might be coming from kerogens, or residues of past life, which on earth release methane as they slowly degrade during geothermal heating. In the case of Mars there is not likely to be geothermal heat. But Oehler thinks heat might come from a meteorite impact (and there are reasons to believe it's only forming in some regions). Alternatively, as there has been some evidence for seasonal differences in the rate of methane production, maybe the sun has something to do with it. In any case, in the kerogen scenario, the methane would be the hangovers of a long ago party.
For me this feels like a glass half full. Readers of this blog might have realized that I'm pretty excited about the idea of life outside earth. But having it long gone isn't any fun. ( Don't look in that egg! ) Alternatively, the picture on earth is that once life gets a toehold, evolution can allow survival in unbelievably harsh conditions. Maybe Martian life vigorous enough to deposit kerogen could have left descendants.
Kerogens of course are also of interest with respect to petroleum formation . See also this fairly technical discussion about whether earth petroleum arises from biological processes or deep-earth processes (the latter is not plausible for tectonically dead Mars).
I don't even want to think about oil deposits on Mars. How did all our oil get under their sand?