Wow, that's like three puns in one title.
Anyway, scientists have revealed they have found large amounts of carbonates (minerals containing CO3 in them) in rocks on Mars. That's kind of a big deal: it's been expected that a lot of rocks would have this compound in them, because there's lots of carbon dioxide afoot there, and plenty of evidence that Mars was once wet. Those two ingredients lead to carbonates. Yet the rocks looked at closely by the rovers have been strangely devoid of them.
For the rover Opportunity it's not all that strange; the water on that part of Mars was acidic, and that makes carbonates tough to form. But Spirit is on the other side of the planet, and it was expected it would find carbonates all over the place. Well, turns out it finally has. Some rocks it examined back in 2005 are loaded with carbonates, but it took this long to figure that out because dust that got in the instrument on the rover screwed things up. The scientists had to do some heroic work to tease the data out.
At this point we've pretty much exhausted my knowledge of this, but happily we have access to Emily Lakdawalla and her blog, where she goes into detail about the rocks, talking to a scientist involved in all this, too. So go over there and get the rest of this interesting story.
And when you're over there, don't forget: we're talking about a whole planet here. A world. And it was once warmer, wetter, with a thicker atmosphere. Sure, it was over a billion years ago, but it's always a good idea to keep an eye on Mars when thinking about Earth. There but for the grace of random chance go us.