When a moon orbits a planet, everything is fine as long as the gravity holding the moon together is greater than the planet’s gravitational pull. However, if the moon gets too close and the planet’s tidal forces exceed the moon’s gravity, the satellite will fall apart. This is known as the Roche limit. Fortunately for Earth’s moon, this limit is just under 10,000 km, and the satellite itself is almost 385,000 km away.
But this is not the case for the small Mars satellite system. Phobos, the larger of the two Martian moons with a diameter of 22 km, is slowly falling towards Mars and will reach the Roche limit in about 20 million years. It will then fall apart into a spectacular ring. That leaves only Deimos, which is smaller and more distant, as the last Mars satellite. It may be a lonely system, but a new simulation suggests Mars once had a very complex system of moons.
For a long time, scientists thought potato-shaped moons were likely captured asteroids, but their circular orbits around the equator argue for another possibility: their formation by a giant impact billions of years ago. A very gigantic impact. The new research, published in Natural Geosciencessuggests a massive 2,000 km protoplanet impacted Mars in the past, resurfacing much of the red planet and throwing a mass of debris more than 100 times the masses of Phobos and Deimos into orbit .
In an attempt to simulate the existence of Phobos and Deimos today, the new research finds that their existence may be explained by a dominant inner moon that forms after this collision, perhaps a few hundred kilometers across. This large moon would also have enabled a few other smaller moons, including the two that survive today. However, this large inner moon would have formed near or within the Roche limit and so probably fell into Mars within several million years due to tidal forces, and other small moons that also formed probably followed it. Today only Phobos and Deimos remain.
If this indeed happened, there should be deposits of these long-gone moons on the surface of Mars. Japan’s 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. Towards the end of their article, the authors note: “Our scenario provides further motivation for an example of a return mission to the Mars satellites.” Indeed.
Natural Geosciences2016. DOI: doi:10.1038/ngeo2742 (About DOIs).