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Mars' Moons
Photographs
Description When most people think of a moon, they think of the relatively large whitish globe that they see at night and sometimes during the day. While this is a moon, it is not typical of moons in the solar system. In fact, Earth has the largest moon in the solar system, relative to the planet's size, if you don't include Pluto. Most other moons are much smaller than their host planet.
Mars' moons also orbit very close to Mars. The illustration to the right shows the relative sizes of the orbits. The image of Mars is to scale, as are the orbital sizes of the moons. Thus, it can be plainly seen that these moons are extremely close to Mars, relative to their sizes. Since both of the moons lie closer to Mars than a synchronous orbit, they orbit faster than Mars spins. Thus, the - very small - tidal bulge that they cause on Mars lags slightly behind the moons, especially Phobos. This creates an effective drag on the moons. As a result, Phobos is slowly spiraling towards Mars at a current rate of 1.8 cm (0.7 inches) per year; it will hit Mars in about 50 million years. However, the tidal forces upon Phobos due to Mars are much larger. Edouard Roche (1820-1883) developed a formula called the "Roche Limit," which is the radial limit of how close a body can come to another before it will be destroyed due to the difference in the forces across it. The Roche Limit is given as:
For Mars-Phobos, the Roche Limit is estimated as 10,800 km. However, Phobos is about 1400 km inside of this. This is because the Roche Limit does not take into account the strength of the material of the object. With this in mind, the best estimate is that the Roche Limit is about 5000 km. Once Phobos comes within this, the moon will be destroyed by tidal forces. Discovery Unlike the planet itself, Mars' moons have not been known since antiquity. Rather, they were discovered in 1877 by a keen-eyed U.S. astronomer in Washington, D.C. In a hazy August evening, Professor of Mathematics in the U.S. Navy Corps of Mathematics, Asaph Hall, was studying Mars with the 26 inch "Great Equatorial" telescope at the U.S. Naval Observatory with his assistant, George Anderson. Prof. Hall was sweeping the telescope around the edge of Mars when he saw what looked like a star - except that he had seen it five nights earlier along with the planet. He noted that it appeared to be moving with Mars as opposed to staying fixed relative the the stellar background. Later that night, he saw a brighter "star" that was even closer to the planet's disk. On August 18, several astronomers came to the telescope with Hall and observed the "stars," recording their positions in the telescope's log book. Mars' two moons, Deimos and Phobos, had been discovered. Origin Mars' moons are probably not native to Mars. Most likely, they are asteroids that were captured by Mars' gravity, and could not escape. The largest reason behind this hypothesis is their small sizes. Data for Mars' Moons
*This semi-major axis is measured from the center of Mars. 
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Mars'
moons are no exception. With Phobos measuring 13.4x9.2 km (8.3x5.7 miles) and
Deimos at 7.5x5.2 km (4.7x3.2 miles), they are runts compared to Mars' size.
Spacecraft sent to Mars have photographed the moons. Phobos has craters of all
sized on it; it also has long grooves radiating from its largest crater. Deimos
has a few small craters. Its surface is covered by a fine dust, but it also has
boulders on it - some as large as a house.