[chessplayer]

Where did you get the idea that methane is a positive indicator of living organisms? Some methane is produced by microbes, but the major portion is abiogenic. There are literally over a million tons of methane outgassing from the mantle of the earth every year.

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Because methane is a positive indicator of life. You just said it yourself. Yeah, there is a lot of methane outgassing from the earth’s mantle. The earth is a very active world. Lots of plate tectonics, earthquakes, and volcanic eruptions. Aside from wind, Mars is a geologically dead planet. The last tectonics, Marsquakes, or volcanic eruptions happened millions of years ago.

[chessplayer]

Even on Earth, only 10% of the methane produced is from non-biological processes.