Captured Moons

Mars has two small moons, Phobos (top) and Deimos. Lumpy and heavily cratered, they may be captured asteroids. They were discovered in 1877 by Asaph Hall at the U.S. Naval Observatory. Both moons are tidally locked with Mars — that is, the same face of each moon always faces the planet, just as the same side of our own moon always faces Earth.

Deimos is farther away and moves slowly from east to west. Deimos would look like a small dot of light in the sky. Phobos is slowly moving closer to Mars. In another 50 to 100 million years, it will crash into Mars.

Phobos is small, dark, and airless. And it's one of the driest bodies in the solar system.

That doesn't mean that Phobos is dull, though. Quite the contrary. Phobos survived a powerful impact that may have fractured its interior. The impact gouged a large crater, and may be responsible for a series of long, deep grooves that appear to radiate away from the crater.

If the meteor that created the crater had been a bit larger, it might have destroyed Phobos. Instead, many scientists think the impact cracked the moon's interior. The cracks could contain water ice. If so, Phobos could serve as a refueling station for manned Mars-exploration missions. But observations by several spacecraft indicate that there's less water at the surface of Phobos than in almost any other body in the solar system.

Deimos is covered by a powdery dust that could be several hundred feet deep. The tips of giant boulders peek above the dust like icebergs floating in a dark sea. The dust formed as the result of billions of years of meteorite impacts, which pulverized the tiny moon's surface.

Many scientists believe that Phobos and Deimos are asteroids that wandered close to Mars billions of years ago. The Martian atmosphere may have been thick enough to act as a brake, slowing the small bodies enough to trap them in orbit.

But other scientists say that the moons formed at the same time as Mars, or that they're the remnants of a larger moon that was shattered by a collision with a large meteor.