Martian Methane

For the first time, astronomers have detected methane on Mars and accumulated enough evidence to support their discovery. Methane is relatively rare in Mars's atmosphere, which is mostly made up of oxidized gases (an example being CO2). The researchers detected three major points in Mars's northern hemisphere that were releasing methane during the martian summer. The source of this methane is not known, but could be from fissures and holes in the martian ground unfreezing, allowing the gas to escape. Other possible causes include release from organic life forms (most methane on earth comes from organisms) or the hydrolysis of water due to radiation deep under the martian soil. Whatever the cause, scientists continue to discover more about Earth's mysterious neighbor.

http://blogs.physicstoday.org/update/2009/02/martian-methane.html