Methane detected on Mars could be a sign of extraterrestrial life, scientists said yesterday. But don’t get ready for E.T. just yet. There are many possible explanations for the methane, and tiny Martian critters are only one. Still, the detection of methane had scientists buzzing in Louisville at a meeting of the American Astronomical Society’s Division of Planetary Sciences. “I stand before you and tell you, quite honestly, I’m shocked by these results,” said Michael Mumma, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Md.