Nasa has now reported a seemingly simple discovery on Mars: its atmosphere contains methane.

Methane is fragile in air and easily absorbed when hit by ultraviolet light, so any Methane on Mars must have been put there recently.

The outburst of methane on mars follows the seasons, with strong outburst in the summer regions of the planet.

Theres only two plausible explanations to what could have been put methane on Mars.

One is vulcanic activity, but there are no signs that any volcanism has occurred there for millions of years.
The other explanation is life.

links:

Live Blogging The Mars Methane Mystery: Aliens At Last?

http://blog.wired.com/wiredscience/2009/01/marsmethane.html

http://www.nytimes.com/2004/11/23/science/space/23mars.html?_r=1