Mars's thin atmosphere is now about 95% carbon dioxide, but when the planet's atmosphere is only 1/200th the size of Earth's, that doesn't mean very much. Now deeply buried minerals reveal Mars once had nearly Earth-like levels of carbon dioxide.
One of the best ways to reconstruct Mars's ancient atmosphere is to examine the mineral composition of the planet's interior miles below the surface. The types of minerals buried down below can reveal the gases that were abundant on the surface when they first formed eons ago, and lots of carbonate minerals are evidence of extensive carbon dioxide.
Obviously, we can't yet mine the Martian interior to bring up mineral samples, but impact craters bring up these deeply buried minerals, and we're able to inspect what's there using probes and satellites. The Leighton crater, near Mars's Syrtis Major volcano, has turned up the biggest deposit yet of carbonate minerals. All previous findings had been relatively small, but this is the best find yet to demonstrate that Mars was once rich in carbon dioxide on a scale far greater than what the planet is today. For more, you can check out the original paper over at Nature.