Robot Geologists

The rovers that explore Mars are often called robot geologists, and the nickname is earned. Like a field scientist, a rover studies layers of rock to read the planet's history. When one drilled into a dry lakebed and found clay that forms only in calm, fresh water, scientists concluded that Mars was once far friendlier to life than it is today. A rover's day is planned to the minute by engineers on Earth, yet the machine must handle surprises alone: a signal takes many minutes to cross the distance between the planets, so no one can steer a rover around a rock in real time. Each evening, the rover reports what it found; each night, humans write tomorrow's plan.