Last year, the Mars Rover Spirit got stuck in the dirt and NASA couldn’t get it out. But today, it still manages to deliver. Under those spinning wheels is proof that there’s liquid water on Mars:
But the soil exposed by Spirit’s spinning wheels carries clues that Mars may still be wet. The newly exposed surface layers include minerals thought to be hematite, silica and gypsum, which don’t dissolve easily in water. But layers of iron sulfate minerals, which do dissolve easily, lie centimeters below the crust.
These layers suggest water, maybe in the form of frost or snow, seeped into the ground relatively recently and carried the soluble minerals deeper into the soil.