Mars’s two moons, Phobos and Deimos, have puzzled researchers since their discovery in 1877. They are very small: Phobos’s diameter of 22 kilometers is 160 times smaller than that of our Moon, and Deimos is even smaller, with a diameter of only 12 kilometers. Earth’s moon is essentially spherical, while the moons of Mars are very irregularly shaped. They look more like asteroids than natural moons.
This led people to suspect that they might in fact be asteroids that were captured in Mars’s gravity field. Captured objects would be expected to follow an eccentric orbit around the planet, and that orbit would be at a random inclination. However, the orbits of the Martian moons are almost circular and move in the equatorial plane of Mars. So, what is the explanation for the current orbits of Phobos and Deimos? To solve this dynamic problem, researchers relied on computer simulations. They came to the conclusion that Phobos and Deimos are the remains of a larger Martian moon that was disrupted between 1 and 2.7 billion years ago.