Mars is a place of mysteries. Was it ever—or is it now—home to life? Why did it lose its magnetic field, allowing the solar wind to claw away its once-abundant atmosphere? And then there’s the riddle of the lost Martian water. As dry riverbeds, deltas, and ocean basins attest, the Red Planet once fairly sloshed with water—until three billion years ago, when it began turning into the desert world it is today.