The puzzle of methane on Mars

On Earth, most of the methane in the air is produced by living things. So when telescopes and the European Mars Express probe reported signs of the gas in the Martian atmosphere in 2004, the news raised a flag. Methane breaks down in sunlight within a few centuries, so something would have to be replenishing it.

The Curiosity rover, in Gale Crater, began measuring very small amounts that vary with the seasons, along with occasional much larger spikes. The problem is that the highly sensitive European Trace Gas Orbiter finds practically no methane high in the atmosphere. Explaining that contradiction is a challenge. The source could be biological, but it could also be geological, such as reactions between water and certain rocks underground. So far, no one knows.