Mars itself is full of secrets, but its mysteries begin above its red surface on its strange moons and in its thin atmosphere.
Mars's moons, Phobos and Deimos, are tiny and dark, nothing at all like our own Moon. They seem to be just irregular chunks of rock trapped in orbits around the red planet. Phobos, the inner moon, is only 18 miles long and 12 miles wide. It circles the planet every 7.7 hours. Deimos, only 9 miles long by 7 miles wide, circles Mars in an orbit further out that takes 30.3 hours to complete.
The moons' orbits are strange because they are nearly circular and are on the inner and outer limits of possible orbits around Mars. If Phobos were any closer, it would be pulled into Mars, and if Deimos were any further, it would not be held by the planetÕs gravity. In fact, some astronomers estimate that in about 100 million years Phobos will crash down on Mars and Deimos might pull away.
The two moons are among the darkest bodies in the entire solar system; they only reflect 6% of the incoming sunlight. Because they are so dark and are only about half as dense as Mars itself, no one is sure what they are made of or where they come from. Many people think that Phobos and Deimos are asteroids that were captured long ago by Mars's gravitational pull. Another theory about the moons suggests that they are the last surviving remnants of a Saturn-like ring that once surrounded Mars. Until we can explore their surfaces, no one will know for certain.
