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Mars' Moons - Phobos and Deimos

Orbiting Mars are two small moons. They are Phobos and Deimos. They are both irregularly shaped and are possibly asteroids that have been captured by Mars' gravity.

Phobos and Deimos were discovered in 1877 by the American astronomer Asaph Hall. Their names were suggested by the English science teacher Henry Madan and chosen by Asaph Hall. In Greek mythology, Phobos is the god of fear and panic and Deimos is the god of terror and dread. They are the twin sons of Ares and Aphrodite. Ares and Aphrodite are the Greek equivalents of Mars and Venus, so Phobos and Deimos are their children.

Phobos and Deimos taken from Mars Reconnaissance Orbiter. Phobos is the larger of the two moons. Image credit: NASA/JPL-Caltech/University of Arizona

Phobos is the larger of the two moons but it is still very small, having a diameter of about 23 kilometres (14 miles). It looks a lot like a baked potato. Unfortunately its insides aren't very appetising and it is thought that Phobos is actually a ball of rubble loosely held together by magic, sorry, gravity. Phobos has the closest orbit to its planet of any moon in the solar system. It is situated about 9,400 kilometres (5,800 miles) from Mars. Each year, it gets 2 centimetres closer to Mars and it's thought that in about 40 million years, it will be torn apart by Mars' gravity. Its remnants may go onto form a ring around Mars while other parts will fall to the planet's surface.

Much further out is Deimos, Phobos' smaller twin. Deimos is only 12 kilometres wide (8 miles). While Phobos has quite a lot craters on its surface, Deimos appears to have very few. This is largely because most have been filled by debris and dust. Only two of the craters on Deimos have names. One is called Swift and one is called Voltaire. Swift and Voltaire were writers who separately wrote about Mars having two moons. They did this over 100 years before they were actually discovered! Deimos is getting further away from Mars each year and could possibly, sometime in the very very distant future, escape Mars' gravity and be free to wander off elsewhere into the solar system.

If your a fan of stats, here's a tiny table of statistical stuff about Phobos and Deimos. If you want to find out more about either of them, click on their names.

Name of Moon Diameter Orbital Distance Length of Orbit Discovered Discoverer
Phobos 23 km (27 x 22 x 18 km)
14 miles
9,376 km
5,826 miles
7 hours, 39 mins 1877 Asaph Hall
Deimos 12 km (15 x 12.2 x 11 km)
8 miles
23,463 km
14,579 miles
30 hours 1877 Asaph Hall

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