Scientists have known for quite a while that at some point, there was flowing, liquid water on the surface of Mars. Here on Earth, liquid water almost always means life, but that's because we've got a lot of other life-friendly conditions along with all that water. Most of the actual evidence on Mars so far has made the case for life a tough one. Samples and measurements indicate that any surface water was likely so acidic and salty that no life in its right mind would want to go anywhere near the stuff.
Today, NASA's Jet Propulsion Laboratory has announced that samples drilled out of a rock in an ancient stream bed by the Curiosity rover show evidence of an environment that was much, much friendlier:
They indicate a habitable environment characterized by neutral pH, chemical gradients that would have created energy for microbes, and a distinctly low salinity, which would have helped metabolism if microorganisms had ever been present. "A fundamental question for this mission is whether Mars could have supported a habitable environment," said Michael Meyer, lead scientist for NASA's Mars Exploration Program at the agency's headquarters in Washington. "From what we know now, the answer is yes."
YES! This is damn exciting stuff, although we should be very careful to point out that just because conditions are favorable for life on Mars doesn't in any way imply that there ever was (or is) life on Mars. That said, if these rocks were on Earth, they'd probably be teeming with microbes of one sort or another, and there are plenty of types of Earth life that have managed to live in places far less hospitable.
We should also make clear that Curiosity isn't really set up for life detection. It's a roving chemistry lab, not a roving biology lab. And as such, it's better to think of it as being able to simply identify potentially habitable environments, as opposed to actually finding stuff inhabiting those environments. If Curiosity finds enough evidence for habitability, the next Mars rover will likely be outfitted with a different suite of tools designed to search for more direct evidence of past (or present) Martian life.