Solar Exploration

Mars is the most Earth-like of the rocky planets in terms of geography, with polar ice caps, seasons, canyons and dust storms that can occasionally engulf the entire planet. Its rust-red color comes from iron oxide — essentially rust — coating its dusty surface. Mars is home to Olympus Mons, the largest volcano in the solar system at roughly three times the height of Mount Everest, and Valles Marineris, a canyon system long enough to stretch across the continental United States.

The planet's two small moons, Phobos and Deimos, are irregularly shaped and thought to be captured asteroids rather than bodies that formed alongside Mars. Phobos orbits so close and fast that it rises and sets twice a day as seen from the Martian surface, and it's slowly spiraling inward — in tens of millions of years it will either crash into Mars or break apart into a ring.

Mars's atmosphere is a hundred times thinner than Earth's, too thin to support liquid water on the surface today — any exposed water instantly boils or freezes. But evidence from rovers and orbiters shows Mars once had rivers, lakes, and possibly an ocean billions of years ago, before it lost most of its atmosphere to space as its magnetic field faded. That ancient wet history is exactly why Mars remains the solar system's leading candidate in the search for evidence of past microbial life.