If it's volcanoes or similar sources, though, we have to wonder where they actually are. Nothing that looks like an active volcano has been seen. And even if the activity were more subtle and less splashy -- the injection of lava below the surface somewhere, say -- it's hard to see how it would avoid giving off heat that TES on Mars Global Surveyor or Themis on Mars Odyssey would have picked up. A small and not very warm spot of geothermal energy might escape TES, which divides the surface up into fairly big parcels. But with a resolution of 100 metres in the infrared Themis should be able to pick up such things, and so far it hasn't, even though it's taken infrared data on large parts of the planet.

The other obvious possibility is life. On earth, almost all the methane in the atmosphere is produced by bacteria, specifically methanogenic archae. These are anaerobic organisms, which would suit them to Mars. Their metabolisms depend on making methane from carbon dioxide and hydrogen (the hydrogen itself sometimes a product of other bacterial life). These have long been seen as the most likely forms of life to be found on Mars.