When a narrow stream, flowing downhill, meets a wide, significantly-flatter valley, you get an alluvial fan — a place where the flow of water spreads out, slows down, and leaves behind all the rocks and sediment it's no longer moving fast enough to carry. At least, that's how it works on Earth.
Once upon a time, it may have worked that way on Mars, too. Yesterday, NASA announced that the Curiosity rover had documented geology that looks very much like an alluvial fan and rocky deposits that also look very much like what would be left in an alluvial fan on Earth. You can see the comparison of some of those in the image above. In these Martian geological features — as in an Earth-bound stream bed — you find smooth, rounded pebbles and conglomerates, masses of pebbles cemented together over time. The rocks photographed by Curiosity are also too large to have been blown into this sort of arrangement by the wind.
All of this adds to the long string of evidence that Mars once had flowing water on its surface. In fact, reading up for this post, I was surprised to see how much evidence there actually is for this, some direct and some indirect, stretching all the way back to the Mariner 9 orbiter mission in the early 1970s. And, of course, there is water on Mars right now. It's just not flowing water. Previous probes have measured a small amount of water in the Martian atmosphere, and the planet's polar regions contain both frozen carbon dioxide and frozen water. Viking 2 took pictures of frost on the ground in the late 1970s, and in 2008, the Phoenix lander literally dropped out of the sky onto a patch of ice.
The new photos from Curiosity add to this growing body of evidence. Taken all together, it's pretty safe to assume that Mars was once a wetter place. Here's NPR's Bill Chappell:
"There is earlier evidence for the presence of water on Mars," the agency said in a press release, "but this evidence — images of rocks containing ancient streambed gravels — is the first of its kind."
Scientists have not yet estimated the age of the rocks, which may have been buried beneath the surface. Their age could be several billion years.
The next step will be to find a good spot to drill into the rock, NASA says. And they'll be looking for possible carbon deposits to determine whether the water on Mars once supported life.
Check out this 2007 report prepared by the National Academies of Science, which discusses a strategy for looking for evidence of life on Mars. It includes a summary of evidence for water on the planet.