NASA's Curiosity rover has found definitive proof that water once ran across the surface of Mars, the agency announced today. NASA scientists say new photos from the rover show rocks that were smoothed and rounded by water. The rocks are in a large canyon and nearby channels that were cut by flowing water, making up an alluvial fan.

"You had water transporting these gravels to the downslope of the fan," NASA researchers say. The gravel then formed into a conglomerate rock, which was in turn likely covered before being exposed again.

This photograph brings some perspective to the amazing feat of landing a small vehicle on Mars:

The picture was taken by NASA's Mars Reconnaissance Orbiter just as the spacecraft carrying Curiosity deployed its parachute. The High Resolution Imaging Science Experiment at The University of Arizona, which released the image, explains:

(Image courtesy of the NASA Jet Propulsion Laboratory)

Not long after midnight central time tonight, the rover known as Curiosity will land on Mars.

It will take the rover seven minutes to get from the Mars atmosphere to the planet's surface. But because it takes about twice that long for signals to travel from Mars to Earth, scientists won't know anything about the landing until after it's already over.