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Friday 1 July 2016

How were Mars’ moons formed?

The formation of Deimos and Phobos, the moons of Mars, is still somewhat of a mystery. They were discovered by Asaph Hall in 1877, and observed in 1971 by Mariner 9, a NASA spacecraft orbiting Mars[1]. Although not the smallest moons, they are much smaller in comparison to Mars than Earth’s moon is to Earth.

Color composite of Phobos and Deimos
Deimos and Phobos have mean diameters 280 and 154 times smaller than Earth’s moon respectively. Images courtesy NASA (Phobos) and NASA/JPL-Caltech/University of Arizona (Deimos), composite by TWDK

Possible Theories of Formation: Asteroid Capture

Despite being known for so long, there is no accepted theory regarding their creation. They appear to be made of “...carbon-rich rock mixed with ice”[1], and are oddly shaped, which led to the idea that they are captured asteroids. This would also explain their heavy cratering and small size.

An asteroid is captured when it passes a larger mass (in this case, a planet), and is “caught” by the planet’s gravitational field and is forced into orbit. This means that the orbits of captured asteroids are expected to be very eccentric ellipses, meaning that the asteroids pass close by before swinging out further away. The orbits of Phobos and Deimos, however, are almost circular. Because of this, we can’t consider asteroid capture to be the definitive theory of the formation of Mars’ moons.