When water flows over stones, it smooths them out and carries them in its path. Even when the steam has long since dried up, the gravel it leaves behind provides distinct evidence of the water's former presence. And now the Curiosity rover has found tell-tale gravel embedded in the Martian bedrock, small stones rounded by water and too large for wind to have transported---rocky proof of water's presence on the Red Planet. Although previous photos suggested that water once flowed on Mars, the rocks in outcrops like the one pictured here, dubbed “Hottah” after Canada's Hottah Lake, are the most definitive evidence of water on Mars that we have ever found. A piece of the embedded gravel, called a clast, is circled in the close-up of Hottah below. Image courtesy of NASA / JPL-Caltech / MSSS
