The discovery by the Mars rovers of carbonate in sedimentary rock on the Red Planet has enabled planetary scientists to rewind the clock and tell the tale of how Mars' warmer, watery climate 3.5 billion years ago changed to the barren, dry and cold environment that it is today.
We know that, in the distant past, Mars was warmer than it is today and had liquid water on its surface. We can see evidence for this in the form of ancient river channels, deltas, lakes and even the eroded coastlines of a large sea in the north. Sometime in the past 3.5 billion years, Mars' atmosphere thinned and its water either froze or was lost to space. The question is, how did that happen?
