The Moons of Mars: Phobos and Deimos

Phobos and Deimos
Phobos and Deimos photographed by NASA's Mars Reconnaissance Orbiter

Phobos is the larger and nearest of Mars’ two natural satellites, the other being Deimos. Both moons were discovered a few days apart in 1877 by American astronomer Asaph Hall from the U.S. Naval Observatory in Washington DC, who subsequently named them after the offspring of the deities Aphrodite (Venus) and Ares (Mars).

While many geological features on Phobos have been given names, the same can be said of just two of the many craters on Deimos, namely Swift and Voltaire, which were named after these famous writers, both of whom had first proposed the existence of Martian moons before they were actually discovered.


Phobos is named after the Greek god of horror and fear, while his twin brother Deimos is named after the the god of terror, both of whom accompanied their father the war god Ares into battle alongside his sisters the war goddess Enyo, and Eris, the goddess of discord.


While Deimos can sometimes be seen in large amateur telescopes, spotting Phobos, on the other hand, is another matter entirely, although the chances of doing so improve somewhat when Mars is closest to Earth. Nevertheless, Mars outshines its closest moon by at least 200,000 times, making it necessary to construct an occulting bar (to mask out Mars’ light) on a very large telescope under very dark skies in order to have even a small chance of spotting Phobos even at maximum elongation (maximum possible distance away) from Mars. Generally, though, Phobos cannot be spotted with modest or even medium-sized amateur equipment.


The origin of both of Mars’ moons is still unknown, although there are two controversial hypotheses that could help explain how they came about.

The first hypothesis holds that both moons were captured by Mars after their orbits in the main asteroid bet were disturbed by a mechanism that is yet to be explained. While the composition of Deimos (but not that of Phobos) shows remarkable similarities to C and D-type asteroids that inhabit the main asteroid belt, both moons nevertheless have masses that would have made it impossible for the thin atmosphere of Mars to slow the objects down sufficiently for the planet’s gravity to have captured them. Moreover, while atmospheric drag or tidal forces could have circularized the orbits of both moons, several studies suggest that the solar system is not old enough for this to have happened.

One other hypothesis holds that both moons were created when a body with about one third the mass of Mars collided with it, which seems probable given that the crust in parts of Mars’ northern hemisphere is markedly thinner than in the southern hemisphere. According to this proposed scenario, the collision could very well have created a ring of debris around Mars, which subsequently coalesced into the two moons we observe today. While the latter hypothesis appears to be the most likely, it does not explain why the putative ring of debris formed into two bodies, or why they have such disparate distances from Mars. Investigations are, however, continuing.


Physical Properties

Deimos closely resembles C or D-type asteroids in terms of its spectrum, albedo (reflectivity), and overall geometry in the sense that it is highly non-spherical having dimensions of 15 × 12.2 × 11 km. In terms of its composition, Deimos is made from rock that is rich in carbonaceous material, and while it is heavily cratered, most of its surface irregularities have been smoothed over by a thick layer of highly porous regolith.

Orbital Characteristics

As seen from the surface of Mars, Deimos rises in the east, and sets in the west. However, since the moon’s Sun-synodic orbital period of 30.4 hours is slightly longer than a Martian day (“sol”), which is 24.7 hours long, 2.7 Martian days would elapse between each successive rising and setting of the moon for an observer on Mars’ equator. However, since Deimos’ orbits is very close to Mars, and has a very small inclination with respect to Mars’ equatorial plane, an observer on Mars would not be able to see Deimos from Martian latitudes greater than 82.7 degrees.

Deimos’ orbit is progressively getting bigger, since at its distance from Mars, the planet’s gravitational influence is not strong enough to prevent tidal acceleration of the moon, and most investigators believe that Deimos will eventually escape Mars’ gravity altogether.


Physical Properties

Although Phobos is substantially bigger than Mar’s other moon, Deimos, it is not massive enough for its own gravity to form into a spherical shape, or for its gravity to hold onto an atmosphere of any sort. In fact, Infrared studies of Phobos have shown that the body is a carbon-poor rubble pile that is only loosely held together by a thin crust of mainly regolith. Moreover, studies have also shown that Phobos is extremely porous, and that large internal cavities (or voids), account for about 25-35% of the moon’s total volume, which argues against an asteroidal origin. These findings have prompted some investigators to suggest that the cavities in Phobos may contain large volumes of various ices, but this is difficult to demonstrate since no evidence of hydration of Phobos’ outer layers has been found.

While Phobos is also one of the least luminous objects in the solar system, with an albedo (reflectivity) of only 0.071, studies of its surface in thermal infrared frequencies have revealed the presence of phyllosilicates on its surface. This material comprises a large percentage of the surface of Mars, and the fact that Phobos’ spectrum is distinctly different from all other classes of chondritic meteorites appears to support the hypothesis that Phobos was created from ejected material as a second-generation solar system body after an impact had occurred on Mars.

The single biggest crater on Phobos is the 9 km (5.6 mile) wide crater Stickney, which covers a significant percentage of the moon’s surface. While the many prominent grooves and streaks that cover much of Phobos’ length was initially thought to be result of the impact that had created Stickney, later studies have shown that the features do not radiate from the crater, but are centred on the apex of Phobos’ leading apex. Some investigators are of the opinion that these grooves and streaks were caused by collisions with material that was blasted off the surface of Mars after violent impacts, which appears to be supported by the fact that all the resulting crater chains fade out towards the moon’s trailing apex.

However, a competing theory states that the grooves and streaks are in fact similar to “stretch marks” that are caused by tidal interactions with Mars, especially given the fact that Phobos is not a solid rock. Investigations are continuing.

Orbital characteristics

Phobos orbits Mars at a distance of only 5,989 km (3,721 miles), which is below the synchronous orbit radius. In practice, this means that Phobos travels around Mars at a speed that is higher than Mars’ equatorial rotation speed, which produces some interesting effects. For instance, an observer on the Martian surface would see Phobos rising in the west, and set in the east, as opposed to rising in the east and setting in the west. Moreover, a hypothetical observer on Mars would also see Phobos complete one orbit in 4 hours and 15 minutes (or less, depending on location) and he would see two such orbits in one Martian day.

In addition, since Phobos’ orbit is so low, a Martian observer would also see Phobos’ angular diameter change between rising and setting. For instance, at the horizon, Phobos will have an angular diameter of 0.14 degrees, and will grow visibly as it nears the zenith, where it will have an angular diameter of 0.20 degrees, before shrinking again to 0.14 degrees at its setting on the opposite horizon. By way of comparison, 0.20 degrees is about one third as wide as the full Moon when viewed from Earth. Note, however, that Phobos cannot be seen from the Martian surface at Martian latitudes greater than 70.4 degrees.

For an observer on Phobos, however, Mars would fill about 25% of the celestial hemisphere sphere, and appear at least 6,400 times bigger, and 2,500 times brighter than the full Moon when seen from Earth. Furthermore, these values are bound to increase, since the orbit of Phobos decays by about 2 meters per century; at this rate, Phobos is expected to collide with Mars sometime between 30 million and 50 million years from now. When this happens, it is likely that Phobos’ remains will form into a ring of dust around Mars that is expected to endure for up to one hundred million years.

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