UA planetary scientist Alfred McEwen and his colleagues, using the UA-run HiRISE camera, have found seasonal streaks "like nothing else on Mars" that offer compelling evidence that liquid water is flowing on the Red Planet today.
A paper in the journal Science to be published tomorrow shows a sequence of images [right, one clip from the animated sequence] taken across the Martian year.
"Seasonal streaks act as if elevated temperatures around the melting point of water ice unleash liquid water. Dark streaks a few meters wide grew from rocky outcrops down steep, equator-facing slopes beginning in the martian spring and continuing until the early fall."
The animation sequence wouldn't copy to the blog, so you should go to Science and watch it directly. It's hard to argue with these images.
update, 8-5-11: here's CNN video of yesterday's press conference, with footage of the seasonal changes:
Ref: McEwen et al., Seasonal Flows on Warm Martian Slopes, Science, 333(6043) 740-743, 2011.