We now know two things about the Martian atmosphere's carbon dioxide: volcanic eruptions created the gas quite recently, and it probably reacted with liquid water. Taken together, these discoveries confirm water is still a major player in Martian geology.
NASA's Mars Phoenix Lander has provided the most precise readings yet on the carbon dioxide gas that makes up a large part of Mars's atmosphere. Because carbon dioxide reacts so easily with water and silicon-based rocks, we can use their isotopes to figure out the mix of gases and liquids on the planet over time. With sufficient data, we can work up a rough outline of the planet's geological and climatic development over its four billion year history.
We're still working on the bigger picture, but we've learned something intriguing about Mars's recent geological history. The Mars Phoenix Lander has found that the vast majority of the atmospheric carbon dioxide comes from volcanic eruptions, which would have happened very recently in geological terms, perhaps only a few million years. And yet, the carbon dioxide has almost certainly reacted with liquid water. Previous data suggested there was surface water on Mars as recently as 20 million years ago, and this new evidence might push that date even closer to the present.
When combined with previous analysis of Martian meteorites, the Mars Phoenix Lander's data suggests that water-induced weathering of Martian rocks has mainly occurred in low temperature areas, not hydrothermal systems as previously believed. That means that liquid water and other weathering agents may have found the means to survive on the cold, dry, and inhospitable surface of Mars, and relatively recently too.
So what does all this mean? We don't really know, at least not yet. There's still a lot more data to consider, and carbon dioxide readings alone can't tell us the complete story of Martian geology. Even so, this is more convincing evidence that Mars was - and, perhaps, is - a far more geologically active planet than we thought a few decades, even a few years ago. And, with that news, there remains the very real possibility of life.