After finding clear evidence of water on Mars a couple of weeks ago, something even more exciting has been detected by Earth-based telescopes and spacecraft in Martian orbit: methane. What's so exciting about methane? The methane molecule is not stable in the Martian atmosphere, so unless it is replenished it would disappear within several hundred years. And methane, so far as we know, could be replenished in two ways. One is volcanic activity (for which there is no current evidence of there being any on Mars), and the other is microbial activity.
Now, if they find a beat-up road sign reading "Barsoom City -- 50 miles", that'll really be something.
(link via Warren Ellis)