The Satellites of Mars

Mars has two satellites, Phobos and Deimos. They are named after creatures summoned by Ares (the Greek equivalent of the Roman war god Mars) in the Iliad and their names mean Fear (Phobos - where the word Phobia comes from) and Fright (Deimos). Unlike the Moon, these satellites did not form in the same location in the solar nebula as Mars but rather formed elsewhere (likely, the asteroid belt between Mars and Jupiter) and wandered too close to Mars and were captured by its gravity.

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Phobos is an 11-km diameter, irregularly shaped object that only orbits 6000 km from the surface of Mars. Compare that to the Moon, which orbits 384,400 km from the surface of the Earth. If the Moon orbited only 6000 km from the Earth, not only would our tides be much higher (see post on tides) but the Moon would appear 64 times bigger in the sky making it about 32° across. At that apparent diameter, it would fill up a sixth of the sky!  Because Phobos is so close to Mars, it only takes about seven and a half hours to orbit Mars, which means that it crosses the Martian sky twice in one Martian day, taking only about four and a quarter hours to cross the sky.  It also orbits retrograde around Mars, meaning that it rises in the west and sets in the east.

 

Deimos is smaller than Phobos, being only 6.2 km in diameter, but orbits much farther away, at 23,500 km from the Martian surface.  At that distance, Deimos takes about 30.3 hours to complete one orbit around Mars, or about a Martian day and a quarter.

 

Both Phobos and Deimos were discovered in August of 1877 by Asaph Hall at the United States Naval Observatory in Washington, D.C. (Quick fact - the USNO is home to the official Master Clock for the US and is also the official residence of the Vice President.)  Despite being smaller, Deimos was actually discovered first on August 12th and Phobos was discovered on August 18th.  The names were suggested by Henry Madan from the Iliad.

 

The reason why Phobos and Deimos are believed to be captured asteroids is because they are similar in composition, albedo, densities of C- or D-type asteroids.

• C-type asteroids (carbonaceous asteroids)
• The most common type of asteroids (make up about 75% of all asteroids
• They have a low albedo which means they do not reflect a lot of light, almost appearing black
• Their compositions are similar to the early solar nebula except for the lack of volatile elements (gases, water, etc) but do contain hydrated minerals (water-containing minerals)
• D-type asteroids
• They have a lower albedo than C-type asteroids
• Their spectra are the strongest toward the red end of the electromagnetic spectrum
• They contain organic, carbon, and anhydrous (lacking-water) silicates
• However, they may have water ice cores