Photos: New evidence of water on Mars

Based on photographic evidence from the Mars rover Spirit, NASA scientists conclude that water must have flowed over an area on Mars. The ground exposed by Spirit's tire tracks shows an area so rich in the brightly-colored silica that "water must have been involved in concentrating it," according to NASA.

One of Spirit's six wheels no longer works so it drags along exposing the soil.

The Mars Express spacecraft has also taken high-resolution photos of an area that was probably formed by glaciers. Its instruments also show ice on the south polar region that could be up to two miles thick.