Based on a photo taken by the Mars Curiosity rover, scientists at the Jet Propulsion Laboratory say that water "once ran vigorously across the area on Mars where the rover is driving."
The photo shows a visible streambed, and rocks that have been smoothed into round shapes, just like river stones eroded here on Earth as they are carried through water.
The statement released by NASA explains the size of the rocks--from a grain of sand to a golfball, and many rounded--proves that they were moved by water.
"The shapes tell you they were transported and the sizes tell you they couldn't be transported by wind. They were transported by water flow," said Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Tucson, Ariz.
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."
NASA published the photo, with a side-by-side comparison of an earthly streambed, on Curiosity's Facebook page.