If microscopic Martian life is producing the methane,

it likely resides far below the surface, where it's still warm enough for liquid water to exist. Liquid water, as well as energy sources and a supply of carbon, are necessary for all known forms of life.

 

Dr. Michael Mumma of NASA's Goddard Space Flight Center

 

Read more:

http://www.nasa.gov/mission_pages/mars/news/marsmethane.html