Pale Blue Dot III.4 - Alien Farts Stinking Up Mars

A couple of years ago, some research groups reported preliminary indications of Methane of Mars.
At very low concentration - about 10 parts per million - but in the highly oxidised Martian atmosphere, methane has a short lifetime, and any amount implies either an unusual recent event, or ongoing production. There aren't that many ways to make methane...

Allen from JPL discussed this issue.

I think the suggestion that it was suit ventilation from the secret alien base was meant jokingly; but methanogenic subsurface bacteria would do to produce the flow of methane necessary to sustain that level. They'd have to be in wet warm rock, below the iceline, the energetics are plausible, and if they're the adapted residual survivors of an earlier wet and warm biosphere, it is not implausible.
Future observations (coming real soon now in a journal near you) and possibly missions with relevant instrumentation, would help elucidate; but ultimately the only way to check is to go dig a hole.

So, what are the alternatives? One is a recent ~ 1 km³ cometary impact, and we're seeing the tail end of the outgassing. That'd have to have occured in the last few thousand years, and the odds of an impact that big over that time are ~ 1/1000. So not inconceivable.
But, where is the crater? That'd leave a decent sized fresh crater. Don't know of a suitable candidate.

Another possibility is a subsurface magma injection ~ 10 km³ of basaltic lava comes up to the surface (but not through, no major fresh lava on the surface) and melts subsurface ice, the water-rock reaction generates methane through serpentization.
Or, there is slow widespread bottom melt in a global ice layer, dripping onto reduced subsurface warm rock, giving a thin diffuse serpentization zone.

Sounds kinda boring, but - that implies warm, wet subsurface layers with a persistent chemical redox gradient right there. Which is well suited as a survival zone for anaerobic bacteria persisting from a previous wet, warm phase. Just not methanogens. I hadn't thought of that.

Another point, which I had not appreciated, is that if there is steady CH₄ flux over long times, some wee beastie will evolve to lie above the methanogens and eat as much of the methane flux as it can - that's good high energy gas going into an oxidised environment!
Which would imply the true flux may be significantly higher, with much bigger and higher throughput subsurface biosphere.

The methane detection may go away on closer inspection; or it may be some boring inorganic chemistry, on closer inspection; but it is difficult to avoid the conclusion that it involves warmth and water, whether it is abiotic or biotic, and either way that is very interesting.

An additional point I had not appreciated is that if it is wet and warm down there, and there is NO life, that also tells us something: namely that maybe getting life kickstarted, and ongoing, is not so easy after all, which is also interesting.