When a moon orbits a planet, everything is fine as long as the gravity holding the moon together exceeds the pull of the planet. However, if the moon gets too close, and the tidal forces of the planet exceed the gravitational bind of the moon, the satellite will break apart. This is known as the Roche limit. Fortunately for the Earth's Moon, this limit is just under 10,000km, and the satellite itself is nearly 385,000km away.

But this is not the case for the tiny Martian system of satellites. Phobos, the larger of the two Martian moons at 22km in diameter, is slowly falling toward Mars and will reach the Roche limit in about 20 million years. It will break apart thereafter into a spectacular ring. That will leave just Deimos, which is smaller and further out, as the last Martian satellite. It may be a lonely system then, but a new simulation suggests that Mars once had a very complex system of moons.

For a long time, scientists thought potato-shaped moons were probably captured asteroids, however their circular orbits at the equator argued in favor of another possibility—their formation from a giant impact billions of years ago. A very giant impact. The new research, published in Nature Geoscience, suggests a massive 2,000km proto-planet struck Mars in the past, resurfacing much of the red planet and kicking a mass of debris more than 100 times the mass of Phobos and Deimos into orbit.

In trying to simulate the existence of Phobos and Deimos today, the new research finds that their existence can be explained by a dominant inner moon forming after this collision, perhaps a few hundred kilometers in diameter. This large moon would also have allowed a few other smaller moons, including the two that remain today, to form. However, this big inner moon would have formed near or within the Roche limit and thus likely fell into Mars due to tidal forces within several millions of years, and other small moons that also formed likely followed it. Today, only Phobos and Deimos remain.

If this indeed occurred, there should be deposits of these long-gone moons on the Martian surface. The Japanese Aerospace Exploration Agency is planning a sample return mission to Phobos and Deimos, the Martian Moons Explorer, and NASA has plans to eventually return samples from the surface of Mars, possibly in the 2020s. Near the end of their paper, the authors note, "Our scenario provides further motivation for a sample return mission to the Martian satellites." Indeed.

Nature Geoscience, 2016. DOI: doi:10.1038/ngeo2742 (About DOIs).