The discovery of distinct plumes of methane that peak in Martian spring and Summer are to me strong evidence that something biological is going on:<p><a href="http://www.nasa.gov/mission_pages/mars/news/marsmethane.html" rel="nofollow">http://www.nasa.gov/mission_pages/mars/news/marsmethane.html</a><p>The more seasons we see this pattern happen the less likely it is some volcanic/geological source.