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Like one, that on a lonesome road
Doth walk in fear and dread,
And having once turned round walks on,
And turns no more his head;
Because he knows, a frightful fiend
Doth close behind him tread.
– Samuel Taylor Coleridge - The Rime of the Ancient Mariner (1798)

The planet Mars has two moons. Both were discovered in 1877 by American astronomer Professor Asaph Hall Snr (1829 - 1907), who in 1879 was awarded the gold medal of the Royal Astronomical Society of London for his discoveries. The inner moon is called Phobos (ancient Greek for 'fear') and the outer one is Deimos ('dread'). Like the other bodies of our solar system the moons are named after mythological or literary characters; in this case, the twin sons of Ares, the Greek god of war, whose Roman name is Mars, and the Olympian goddess of the night, Nox.

Unlike other moons, Phobos and Deimos are not spherical. In fact they resemble the potatoes the Martian robotic creatures find so hilarious in that enduringly popular mashed potato TV advert. They are probably captured asteroids which ventured too close to the planet and got caught by its gravity. NASA created an animated gif of Phobos transiting the Sun from images recorded by the Mars Exploration Rover Opportunity on the surface of Mars. The animation clearly shows the dark potato shape of Phobos against the bright Sun in the background.

Phobos, the larger and nearer of the two moons, orbits Mars in 7.3 hours, rising in the west and setting in the east¹. Its features include grooves, the most striking of which are a pair approximately 20km long and 100m wide, which look just like a pair of giant ski tracks. Phobos has one very significant crater which covers about a third of its diameter. It is named 'Stickney' after the maiden name of Angeline, the mathematician wife of Phobos' discoverer, Asaph Hall. Another crater is named 'Hall' after Asaph himself. Although Phobos is much smaller than our moon, it orbits very close to Mars and would look quite large from the surface. At its closest approach to Mars, when directly overhead, it would look about half as big as our moon does in our sky. At the horizon, however, it would look only one third the size of our moon.

Unfortunately we know Phobos is doomed: gravitational tidal forces are dragging it down and its orbit will eventually decay, though this probably won't happen for another 100 million years. Phobos will either crash to the planet's surface, or the stress will tear it apart, leaving a ring of debris encircling Mars.

Deimos orbits Mars in 30.3 hours. German astronomer Johannes Kepler (1571 - 1630) did not know about Mars' moons but he was so convinced they were in attendance that, in around 1610, he predicted their future discovery. Jonathan Swift  (1667 - 1745) was influenced by Kepler to such an extent that he wrote about two Martian moons in Gulliver's Travels in 1726, over a century and a half before their actual discovery by Prof Hall.

Statistical Information

Phobos

• Mass: 1.063×10¹⁶ kg
• Size: 27×21×19 km
• Equatorial radius: 13.4×11.2×9.2 km
• Average distance from Mars: 9,378 km
• Orbit Period²: 0.32 Earth days

Deimos

• Mass: 2.38×10¹⁵ kg
• Size: 15×12×11 km
• Equatorial radius: 7.5×6.1×5.2 km
• Average distance from Mars: 23,459 km
• Orbit Period: 1.26 Earth days

Mythology

In ancient Greek stories Phobos and Deimos not only fought alongside their father-god Mars/Ares on the battlefield, but also had the honour of being his charioteers.

Phobos and Deimos are mentioned in:

• Homer's  Iliad
• Aeschylus' Seven Against Thebes
• Quintus Smyrnaeus' Fall of Troy
• Hesiod's Shield of Heracles and Theogony

Nonnus' Dionysiaca featured them as well, but the characters were referred to as Rout and Terror, the two grim sons of Enyalios (Ares/Mars). According to Nonnus, the fearsome god-monster Typhon was a frightening immortal creature from whom all the other gods fled when he stormed Mount Olympus during an attempted coup d'état:

The goddess of love, Aphrodite (Venus) and her son Cupid transformed themselves into fish to swim to safety up the River Nile. The god Apollo turned himself into a large black bird to fly away unnoticed.

Even the mighty Zeus was imprisoned by Typhon, although he did manage to pass along some of his powers to aid the battle. Zeus armed Rout/Phobos with thunderbolts and Terror/Deimos with lightning spikes in the battle with Typhon, who was eventually defeated when Zeus escaped and dropped Mount Etna on him.

Being immortal, Typhon could not die, so he remains trapped beneath the ground for eternity, occasionally erupting when disturbed. The disguised images of the fleeing gods were placed in the night sky as constellations as a reminder against complacency.