Here’s a pretty picture from JPL, based on data from NASA’s Mars Reconnaissance Orbiter. Click to see a larger version. (Note that the image is highly doctored, in the best NASA tradition; not just false-color, but they’ve “reprojected” so that a satellite image now looks like it was taken by a flying helicopter!)
“Water on Mars” is one of those things (like “black holes” or “the missing link”) that seems to be discovered over and over again. That’s because we’re not really finally discovering it once and for all; we’re slowly gathering new evidence, and also evidence for different manifestations. It seems clear that frozen water exists in the polar regions of Mars; also, there’s good reason to think that there used to be running water at some point. This new finding would be evidence for running water right now.
In this case, NASA scientists have noticed seasonal changes in hillside patterns such as this one. The dark streaks seen in the image appear in the spring and summer, then fade again in winter. (Kind of like the Los Angels River, but backwards.) The best idea we have for an explanation is running water. Not that the darkness is water itself, but some change in the underlying substance as a result of water. It’s a very good idea — likely true — but still not quite like we’ve filled up a cup and done a chemical analysis.
Anything with any tenuous connection to “life on other planets” runs the risk that everyone wants it to exist and is looking very hard; consequently, skepticism is always warranted. Still: awesome pictures!