How many moons does Mars have?

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Tim Cook answered

Mars has two small moons, Phobos and Deimos, named after the Greek gods of horror and terror. Both were discovered in 1877 by the American astronomer Asaph Hall (1829 - 1907). As so often with astronomy, the more these moons are studied, the more questions are raised.

Phobos (pictured above) is around 6 miles in radius and of all the moons in the solar system, Phobos orbits its parent planet at the closest distance, just 3700 miles above the Martian surface (compared to the 250,000 miles between the Earth and our own Moon). 

Due to the speed at which Phobos orbits Mars, it appears to rise, journey across the sky and set twice during a Martian day. The large crater is named 'Stickney' and the impact that formed this crater must have nearly destroyed Phobos. 
The moon is doomed in any case, its orbit taking it closer to Mars each year, until it is pulled apart by its gravity. Don't worry though, as scientists predict this won't happen for around 43,000,000 years!

Deimos (above) is much smaller (around 3.75 miles in radius) and orbits at a greater distance (14570 miles) from Mars, making Deimos barely visible from the surface of Mars. It is just as cratered as Phobos, but appears smoother due to a thicker coating of dust and sand known as 'regolith'.

How Mars acquired its two moons is a controversial subject in astronomy. We know that did not form 'naturally' from the same material as Mars during the creation of the solar system, nor from a collision that took place shortly after the planet's formation, as may be the case with our Moon. 

Both Phobos and Deimos could be captured asteroids who strayed too close to the Martian gravitational field, or are the remains of an impact event upon Mars in the distant past.

In this YouTube film, a planetary geologist discusses this debate:

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