There's a camera orbiting Mars at the moment. The resolution isn't so good (1 pixel to 18m I think was the figure mentioned on the podcast) but it's found some large streaks that are changing rapidly. Rapidly in Mars terms. There's a number of possible options as to what is causing it, the most likely candidate though, is water. To warm for CO2, to regular for wind - most likely water. And with liquid water, the possibility of life increases significantly. Though funnily enough, the next rover they're sending can't land to close because it might be contaminated with microbes from earth. Which is something I thought they would have taken into account. I'm fairly sure that the Cassini-Huygens probe that went to Saturn and put a lander on Titan was sterilised before it left.