Where the stuff originates is still a mystery. Krasnopolsky leans toward a biological interpretation. He calculates that the methane concentration could be produced by a deeply buried bacterial ecosystem weighing 20 tons, which may sound like a lot of bacteria but actually is minuscule on a planetary scale. It amounts to a few oases of bugs toughing it out on an inhospitable world, which would match up with what scientists know about Mars.
Sushil Atreya of the University of Michigan, a member of the Mars Express team, says critters are possible but hardly definite. He argues that geothermal processes could do the trick. At a temperature of 100 degrees Celsius, which is naturally achieved underground on Mars, water can react with iron- or magnesium-rich rock to release hydrogen, which then combines with carbon dioxide to yield methane. Such a mechanism might also operate on the Saturnian satellite Titan, which also has an abundance of methane that has yet to be explained.
Mars Express is continuing to gather data. By the end of 2005, it should have completed a methane map for the whole planet. The probe will also peer into the subsurface with its radar. The beam can penetrate several kilometers down, more than enough to reach the depth at which hydrothermal activity should occur.
Ultimately, however, pinning down the source of the methane will probably require measurements of the isotopic variants of methane, which is hard to do from Earth or Mars orbit but should be possible with landers or special spaceborne telescopes now on the drawing board. Thoughts of a living Mars will tantalize planetary scientists for years to come.