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# 6.23 The Moons of Mars

How did Jonathan Swift know? In his famous satire "Gulliver& rsquos Travels" he refers to two moons of Mars discovered by astronomers from the mythical kingdom of Laupta. He gave their distances as 3 and 5 Martian diameters, and their orbits as 10 and 22 hours. But he wrote his book in 1726, 150 years before Phobos and Deimos were actually discovered by Asaph Hall at the U.S. Naval Observatory! The moons found by Hall matched Swift’s description fairly well, with distances of 1.4 and 3.5 Martian diameters, and orbits of 8 and 30 hours. Swift was not clarivoyant. He probably used a common argument at the time, that since Mercury and Venus had no moons, and Jupiter had four (known in the early 18th Century), that Mars by inference probably had two. Since they had not been discovered Swift know they would be small and close to the planet. Given that assumption, he had enough math background to estimate their orbits. A few decades later, Voltaire echoes Swift by alluding to the moons in one of his short stories.

Views on the Martian moon Deimos, as seen by HiRISE. Click here for original source URL.

The Martian moon Phobos as seen by the HiRISE instrument. Click here for original source URL

The two small satellites of Mars would be interesting places for a future expedition to visit on its way to the surface of the red planet. Phobos and Deimos are named after Fear (or panic) and Terror (or dread), respectively, the horses who drew the chariot of Mars, God of War.  They are both small, dark, potato-shaped chunks of rock, heavily cratered by meteorite impacts. Phobos, the inner satellite, is 27 by 19 kilometers (17 by 12 miles). Deimos, the outer satellite, is one of the smallest named satellites in the solar system, measuring only 15 by 11 kilometers (9 by 7 miles). Deep searches have ruled out any other moons down to a size limit of 100 meters.

Compared to the appearance of Earth’s Moon in our night sky, Phobos & Deimos would be somewhat disappointing to an observer on Mars, because of their small sizes. From the surface of Mars, Phobos would appear only about a third the size of our Moon, and Deimos would look even smaller. In about 100 million years, however, a Martian observer would have quite a show — Phobos’s orbit is quite close to the planet, and tidal forces are bringing it even closer. Eventually, it will either crash onto the Martian surface or get torn apart by those tidal forces, forming a ring of debris around Mars.

The origin of Deimos and Phobos is uncertain, but many astronomers suspect that they started out as asteroids in the nearby main belt, and were later captured into orbit around Mars. Their black surfaces are believed to be rich in carbonaceous material, similar to that found on some asteroids. This dark color is strikingly visible in a photo from a Russian spacecraft that caught Phobos silhouetted in front of the red planet. A mission to gather data on these satellites would help us figure out their chemical composition, and that of similar asteroids. This in turn might tell us whether mining asteroids would be worthwhile.