Follow the Water

More than 20 years after the Vikings landed in 1996, the Mars Pathfinder spacecraft took off for the Red Planet. Pathfinder was a low cost mission, intended initially just to demonstrate new ways to land on Mars. But a tiny robot geologist, named "Sojourner" after the 19^th Century abolitionist Sojourner Truth, was added. Though the rover was only about the size of a laser printer, it returned some amazing new insights into the climate history of the Red Planet.

Pathfinder project scientist Matt Golombek is a devoted rock enthusiast. He calls Mars’ rocks a book in which you can read the history of the planet. So when he helped select a landing site for Pathfinder, he wanted a place with as many different kinds of rocks as possible. He and his colleagues chose an area known as Ares Vallis, which looked like places on Earth where sudden floods sweep down lots of rocks and deposit them as the water dries up. Matt had chosen well: that’s just what Pathfinder and Sojourner found.

Continuing to operate on Mars far longer than initially intended the mission imaged and analyzed some rocks that geologists call "conglomerates" with lots of little rounded pebbles bound up in larger rocks like this one, nicknamed "Shark." On Earth, it takes running water to make round pebbles like these, and a relatively long period for them to grow into large boulders.

Venturing farther from the landing site, Sojourner also found sand dunes: to a geologist, once again, that’s a sign of liquid water continuing to exist long enough to grind down larger rocks into tiny grains. While Pathfinder was looking at rocks, it was in fact on the trail of liquid water.