English: On January 3, Spirit, NASA's 400-pound rover, is scheduled to land on what may be a dried-up lake bed on Mars. "There's not much doubt: this site contained a body of liquid water, at least for some amount of time," says Jim Garvin, NASA's Lead Scientist for Mars Exploration.

The site is Gusev Crater, a 90-mile wide hole in the ground that probably formed three to four billion years ago when an asteroid crashed just south of Mars' equator. There's a channel system that drains into it, which probably carried liquid water, or water and ice, into the crater. "It's hard to imagine the landscape looking this way unless water was somehow involved," says Garvin.

This image was taken by NASA's Viking 1 orbiter.

Right now, inside the crater, researchers expect to find sediments, which may be nearly 3,000 feet thick. These sediments, which, researchers hope were deposited by water, may have been covered by dust and sand that's blown into the crater over the past two billion years. But if there was once water in Gusev, its signature should still be there.