Mars's moon mystery solved?

Were Mars's moons the result of a massive impact on the Red Planet, or the capture of two asteroids that got too close? A new study may have the answer.

Published: June 26, 2016 at 12:00 pm

A diagram showing the six phases of formation of Deimos and Phobos around the Red Planet. A. TRINH/ Observatoire Royal de Belgique


Mars’s moons formed out of Saturn-like rings around the Red Planet following a massive impact over four billion years ago, according to a new study.

The two martian moons, Phobos and Deimos, have long been a source of mystery for astronomers, as it is not yet clear exactly how they formed.

Two main theories currently exist: either Mars ‘captured’ two asteroids that flew too close to the planet’s orbit, or the moons formed out of the debris created by the impact of a protoplanet.

A new study using computer simulations has suggested that there was a huge impact on the Red Planet just over four billion years ago.

This collision created debris, which formed a long disc around Mars like one of Saturn’s rings.

Eventually, the debris accumulated into a moon a thousand times the mass of Phobos, in much the same way that our own Moon is thought to have formed.

The new study goes one step further and attempts to explain why Earth ended up with only one Moon, while the Red Planet acquired two.

The answer is related to the differing rotation speeds of the two planets.

Simulations suggest that at the time of their impacts, Earth took less than four hours to spin on its axis, while Mars took 24 hours to complete a full rotation.

This meant that over a space of a few thousand years the debris around Mars formed many smaller moons alongside one massive one, while Earth's collision left it with just a single, large Moon.

The slowness of Mars’s rotation led to tidal action that caused most of the moons, including the largest, to fall to the Martian surface a few million years later.

This left just Phobos and Deimos: the two moons we can currently see orbiting the planet.


The study is a collaboration between the Université Paris Diderot and Royal Observatory of Belgium, in collaboration with the CNRS, Université de Rennes 1 and the Japanese Institute ELSI.


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