Gale crater, the bowl on Mars that NASA’s Curiosity rover has been exploring for 2.5 years, was once filled with water over the course of millions of years, and maybe even longer, scientists reported today in a press conference. That’s good news for astrobiologists who hope that the ancient environment on the Red Planet was habitable for a sustained period of time. Mars scientists had always suspected that Gale crater once contained water and had already found evidence for lakebed mudstones close to the spot where the rover landed. But now there is evidence that, about 3.8 billion years ago, there were repeated bouts of deposition in river deltas and lakebeds that may have lasted a million years or more. The scientists described seeing cycles of deposition from sediment carried in from the rim of Gale crater by rivers. Some rocks contained the angled bedding associated with the slope of a river delta. Other rocks, like the mudstone shown here, display the flat bedding of sediments deposited at the bottom of a lakebed. As Curiosity begins its drive up a mountain of sediments in the middle of the crater, it may discover that this cycle repeats itself. That could boost estimates for the duration of the wet period to tens of millions of years. What remains unclear is whether Mars was wet in a sustained way throughout this period of time or whether all the deposition occurred in short, episodic bursts. The view that the planet was warm and wet enough to support large and long-standing bodies of water is being challenged by theories that propose that Mars was only occasionally wet, during short-lived periods after volcanic activity or asteroid impacts.