Phobos and Deimos: the Moons of Mars

Phobos and Deimos: The moons of Mars – The two small moons of Mars were discovered in 1877 by Astronomer Asaph Hall working at the Naval Observatory in Washington D.C.  Hall named the two natural satellites; Phobos (Greek for fear or panic) and Deimos (Greek for terror or dread) for the two children of Mars (Ares) in Greco-Roman mythology.

Phobos is 22 kilometers in diameter, with mass of 1.08 X 1016.

Deimos is 12.6 kilometers in diameter with a mass of 2X1012.

For comparison the Earth’s Moon is 3476 Kilometers in diameter with a mass of 7.36 X 1022.

Interestingly Johnathan Swift predicted the existence of the two moons of Mars in Part Three of “Gulliver’s Travels”: “A Voyage to Laputa”.  Voltaire following Swift also referred to two moons of Mars in “Micromegas” written in 1750.  In honor of these two literary “predictions” two craters on Deimos were named after the two writers.

There are two prevailing theories as to the origins of moons.  One theory is that Phobos and Deimos are captured asteroids. Both the moons have similar density and albedo to carbonaceous chondrite C-type asteroids common in the main belt of asteroids just out from Mars’ orbit.

The other theory is that Mars was once surrounded by many small moons, possible the result of an impact of a large object early in the formation of the solar system. The two moons than formed as the ejected planetary material came together in orbit.  The current theory is that a similar impact formed the Earth’s moon. The two moons’ low densities and nearly circular, equatorial orbits support this theory of their origins.

The two moons are tidally locked. They both always present the same face to the Martian surface.  Phobos orbits a little faster than Mars rotates, which means that tidal forces are slowing the orbit of Phobos and also bringing it closer to Mars.  Sometime in the future, Phobos will come too close to Mars and be broken into pieces by the tidal forces.  In Deimos’ case, the small moon is slowly being boosted away from Mars and in the future may well be flung away from the planet altogether.

Observed from the equator of Mars, Phobos at full phase would appear about one-third as large as a full moon from the Earth. It cannot be observed at all from the poles of Mars.  Tiny Deimos would appear as a bright point of light, a little larger than Venus as the evening star appears on earth.


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