The Tiny Moons of Mars

February 8, 2010

You can see many things when you look at Mars through a small telescope, but you can’t see its two puny moons, Phobos and Deimos. Few have ever seen these moons directly. Even the largest scopes show them as faint points of light. But they’re there, and recent space probes have snapped close-up images of these potato-shaped satellites as they zip and wobble around the red planet.

In an odd way, the presence of Mars’s moons was predicted more than two centuries before they were discovered in 1877.

Johannes Kepler, always a believer in mathematics and symmetry, reasoned in the early 1600’s that if Earth had one moon and Jupiter had four moons, then Mars must have two moons. While Kepler was a good mathematician, his mystical beliefs led to some strange reasoning.

But it gets stranger.

In 1726, Jonathan Swift wrote of Mars’s moons in Gulliver’s travels and assigned orbital diameters and periods not far off the true values, though he presumably had absolutely no way of knowing these quantities. Voltaire also mentioned these moons a few years later in his story Micromegas, though he may have got the idea from Swift.

But the true discovery of the moons awaited the construction of the great 26-inch Clark refractor for the U.S. Naval Observatory in Washington. There, the sharped-eyed astronomer Asaph Hall spotted the moons in August 1877. (Hall had made other remarkable sightings at the observatory, including a visit by President Abraham Lincoln, who showed up alone one night to talk astronomy). Soon after their discovery, the moons were named after the sons of Ares, the Greek god of war. Phobos represented fear, and Deimos, dread.

Unlike our own moon, which formed at the same time and likely within the same gravitational well as Earth, Phobos and Deimos are likely just small captured asteroids. Phobos measures 22.2 km across and orbits Mars every 7.7 hours. Deimos is even smaller — just 12.6 km across– and orbits Mars every 30 hours. They’re a speedy sight in the Martian sky. Phobos, especially, moves quickly, rising and setting every 11 hours. The image at the top of the page shows how the moons looked to the Spirit Rover in 2005.

Here’s what the moons look like a little closer up:

Deimos (left), and Phobos, the moons of Mars

Over time, Deimos will experience tidal forces that will cause it to lose energy and move to a more distant orbit.

But Phobos will experience a different fate.

Tidal forces will cause Phobos to move closer to Mars until it undergoes a gravitational shearing force that rips it to pieces. The remains of the moon will scatter along the equatorial region of Mars. Other small Martian moons may have already experienced this fate: there are trails of craters strewn along the Martian surface that may have been caused by captured asteroids crumbling and spiraling into the planet.

Posted in Celestial Objects, Solar System Observing