Mars' natural satellites

Mars has two tiny natural moons, Phobos and Deimos, which are thought to be captured asteroids.

If viewed from the surface of Mars, the motions of Phobos and Deimos would appear very different from that of our own Moon. Speedy Phobos rises in the west, sets in the east, and rises again in just eleven hours, while Deimos, being only just outside synchronous orbit, rises as expected in the east but very slowly. Despite its 30 hour orbit, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars, and as long again to rise.

As seen from Mars, Phobos has an angular diameter of between 8' (rising) and 12' (overhead), while Deimos has an angular diameter of about 2'. The Sun's angular diameter, by contrast, is about 21'.

Both moons are tidally locked, always presenting the same face towards Mars. Since Phobos orbits Mars faster than the planet itself rotates, tidal forces are slowly but steadily decreasing its orbital radius. At some point in the future, when it approaches Mars closely enough (see Roche limit), Phobos will be broken up by these tidal forces. Several strings of craters on the Martian surface, inclined further from the equator the older they are, suggest that there may have been other small moons that suffered the fate expected of Phobos, and also that the Martian crust as a whole shifted between these events. Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead, as in the case of our own Moon.

Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Ares was known as Mars to the Romans.

Searches have been conducted for additional satellites. Most recently, Scott S. Sheppard and David C. Jewitt « [...] surveyed the Hill sphere of Mars for irregular satellites. [The] search covered nearly the entire Hill sphere, but scattered light from Mars excluded the inner few arcminutes where the satellites Phobos and Deimos reside. No new satellites were found to an apparent limiting red magnitude of 23.5, which corresponds to radii of about 0.09 km using an albedo of 0.07. (Astron. J., 128, 2542-2546 (2004))

Orbital details

Names and pronunciation			Diameter (km/mi)	Mass (kg)	Semi-major axis (km/mi)	Orbital period (h)	Average moonrise period (h/d)
Mars I	Phobos	foe'-bəs ˈfɔʊbəs	22.2 km (27×21.6×18.8) 13.79 mi (16.7×13.4×11.6)	1.08×1016	9377 km 5827 mi	7.66	11.12 h 0.463 d
Mars II	Deimos	dye'-məs ˈdaɪməs	12.6 km (10×12×16) 7.8 mi (6.2×7.4×9.9)	2×1015	23,460 km 14,540 mi	30.35	131 h 5.44 d

Trivia

The existence of two fictional Martian moons was described in Jonathan Swift's satirical novel Gulliver's Travels, published in 1726, 150 years before their discovery:

They [the Laputan astronomers] have likewise discovered two lesser stars, or 'satellites', which revolve about Mars, whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost five; the former revolves in the space of ten hours, and the latter in twenty-one and a half; so that the squares of their periodical times are very near in the same proportion with the cubes of their distance from the centre of Mars, which evidently shows them to be governed by the same law of gravitation, that influences the other heavenly bodies...

Phobos and Deimos are in fact about 1.4 and 3.5 diameters from Mars' centre, and their periods are 7.7 and 30.3 hours, respectively. A similar "discovery" was described by Voltaire in his interplanetary romance Micromegas, published in 1752.

In recognition of these "predictions", two craters on Deimos are named Swift and Voltaire.