NASA's newest Mars rover, Curiosity, has snapped photos of rocky outcroppings that jut out from the alien soil, and scientists say they look like the remnants of an ancient stream bed where water once flowed on the surface of the red planet.
The exposed rocks look like broken slabs of concrete sidewalk, about four inches thick, and are made of rounded bits of gravel in a sandy matrix. The rock has eroded a little bit, and some of the smooth pebbles — about the size of M&M candies — have fallen down into a little pile.
Scientists looked at all this and came to this conclusion: "This is a rock that was formed in the presence of water," says John Grotzinger, project scientist for the mission at the California Institute of Technology.
And not just any water, but a flowing stream.
Scientists have believed for a long time that Mars once had liquid water on its surface. Orbiting spacecraft can see canyons that must have been carved by water. In fact, researchers deliberately picked Curiosity's landing site because it looked like a place where a canyon stream had spilled water onto a plain — and now it looks like they were right.
Jim Bell, a planetary scientist at Arizona State University who is working on this mission, says the rounded pebbles and cemented sandstones look like what you would find if you were "walking up a stream in your favorite little canyon somewhere. These are all telling us that there was water really flowing across the surface there, and probably pretty deep water — ankle-deep, knee-deep water — like you'd have in an occasional desert flood on the Earth, in the Southwest, for example."
Other researchers agree that this is convincing evidence of flowing water. Peter Doran, at the University of Illinois, Chicago, says the rounded grains in that rock are big enough that they couldn't have been carried and smoothed by the wind.
"Before, we never really saw a rock on Mars where we could tell whether it was wind or water that was doing the transport," Doran says. "And now we have a clear sign of flowing water on Mars and we can get estimates of the size of the flow and so on. It's really fascinating."
Other Mars rovers have seen some evidence of water, but nothing like this. Andrew Knoll, a planetary sciences professor at Harvard University, says earlier rovers saw things that could be associated with groundwater that might occasionally bubble up — not the kind of flowing surface water that made these newly discovered rocks.
"Something happened on Mars that simply doesn't happen today," Knoll says. "And that is, there was water flowing at high rates over the Martian surface. That's really what they've found."
Water is important, of course, because it's needed for life. And Curiosity's main mission is to search for evidence that Mars was once capable of supporting life. Its mission is expected to last about two years.