From National Geographic:
Even while snared in a sand trap, NASA’s Mars rover Spirit has hit “wet” pay dirt: evidence of relatively recent groundwater activity on the red planet. For almost six months the rover has been precariously perched on the edge of a shallow crater in an equatorial region of Mars. The area is filled with cooled lava flows pitted by meteorite impacts.
While on a routine drive, Spirit broke through a thin crust of hard soil that capped a filled-in impact crater, and its wheels became half buried in the soft sand.
Since early November the rover team has been remotely spinning Spirit’s wheels to try and maneuver the rover out of its trap.
During one of these rescue attempts, Spirit churned up the soil and uncovered an intriguing layer of bright, fluffy soil. Mission managers had the rover take a closer look, and they discovered that the layer is in sulfates, minerals known to form on Earth only in the presence of liquid water.
[Read more at National Geographic]