Mars may have once had lots of moons, but soon it will be down to just one

Mars may have once had a much larger moon that eventually fell into the red planet. (credit: Labex UnivEarths / Universiti(C) Paris Diderot)

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.

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