Mars’ two tiny moons, Phobos and Deimos, are the sole survivors of a giant impact that produced many other now-extinct moons, suggests a study published online this week in Nature Geoscience.

Although the potato-shaped Phobos and Deimos were initially thought to be captured asteroids, it has been proposed that they may have formed from a giant impact, similar to the one that formed Earth’s Moon. However, it is unclear why Mars ended up with two small moons from such a giant impact scenario and not, for example, with a single large moon like ours.

Pascal Rosenblatt and colleagues numerically simulated a giant impact event on Mars, and the evolution of the resulting disc of impact debris. They find that larger moons accrete in the inner part of the disc, where the debris is most densely packed. In the outer part of the disc, where Phobos and Deimos are thought to have formed, the debris is thinly dispersed and moons should not easily accrete. However, the authors show that the gravitational tug of a massive inner moon stirs up the debris in the outer disc and allows small outer moons to form. They suggest that the massive inner moon eventually succumbed to the tidal pull of Mars and fell back to the Red Planet, as did most of the other outer moons that formed within the reaches of tidal forces, leaving behind Phobos and Deimos as the only survivors of the giant impact.

The authors conclude that the scenario explains why Mars has two moons today, but also why Mars will have only one moon in the future: although the orbit of Deimos is stable, Phobos too is being gradually pulled towards Mars.

In an accompanying News & Views article, Erik Asphaug writes that, “There could have once been many moons around Mars, the most massive sculpting the system and the smallest being the last to come down. Phobos could be the last straggler in a series of crashing moonlets, readying its final approach.”