Mystery of why Mars has two moons is solved

Mars’ two tiny moons – Phobos and Deimos – are the sole survivors of a giant impact on the Red Planet, according to new research.

Potato-shaped Phobos and Deimos were initially thought to be asteroids caught by Martian gravitational pull. This scenario explains their small size, irregular shape, cratered surface and presumed non-Martian composition, but not their current orbits.

Mars' tidal rates aren't strong enough to alter their orbits in such a way that put them into their current formation. Instead, these orbits suggest they formed "in situ" around Mars, in particular in an extended disc of debris, likely from a giant collision. This impact could even be responsible for the spin rate of Mars.

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This impact likely produced many other now-extinct moons, too, but it is unclear why Mars ended up with two small moons from such an impact and not a single large satellite like ours.

To come to this conclusion, Pascal Rosenblatt and colleagues from the Royal Observatory of Belgium simulated a giant impact event on Mars. They also studied how the resulting disc of debris could have evolved.

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They found larger moons accrete, or "grow" 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 spread much thinner and, in theory, moons should not accrete as easily.

However, the authors showed the gravitational pull of a massive inner moon can be so powerful, it "stirs up the debris in the outer disc and allows small outer moons to form". They suggest a massive inner moon eventually succumbed to the tidal pull of Mars and fell back to the Red Planet, as did the majority of other outer moons that formed within the reaches of tidal forces.

This left behind Phobos and Deimos as the only survivors of the giant impact.

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

In an accompanying News & Views article, Erik Asphaug wrote: “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.”