A vast canyon called Valles Marineris—named in honor of the robotic spacecraft program that discovered it—runs 4,000 kilometers (2,500 miles) long and 2 to 7 km (1.2 to 4.4 miles) deep along the Martian equator.

Smaller channels and canyons run through many places on the planet's surface. And recent chemical analysis of some of the surface rocks by the Mars Exploration Rover spacecraft show that they were soaked with water for a long time. Where did all that water go? Is it now tied up in ice at the poles? Or under the planet's surface? Or did it escape to space?