Seems the greatest discovery/prize awaits the organization that can get a mass spectrometer onto the surface of Mars and see what is the isotopic ratio of the C in the methane. On that note , a recent paper by Encrenaz et al (Icarus 179 (2005) 43–54) reported measurement s of CO2 isotopic ratios in Mars. Compared to the terrestrial values, the values obtained of (18O/17O)[M/E] = 1.03 ± 0.09; (13C/12C)[M/E] = 1.00 ± 0.11 suggest that the martian isotopic ratios do not differ significantly from terrestrial values. (M/E = mars/earth). But the C in the CO2 doesn't show us if there's biological activity.