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Geological History:Moons of Mars
by Scott Ellis

Moons of Mars - "Fear" and "Panic"

The two moons are named Phobos and Deimos. In Greek mythology, Phobos and Deimos are the sons of Ares (Mars) and Aphrodite (Venus); "phobos" and "deimos" are Greek for "fear" and "panic"; fitting sons of the god of war.

We know from space probe observations that both Phobos and Deimos are natural objects and not the "artificial satellites...launched, by the Martians" that the journal Science once suggested [8]. Phobos, with dimensions of 13.5 x 10.8 x 9.4 km, is a chunk of blackish carbonaceous chondrite rock. Deimos, with dimensions of 7.5 x 6.1 x 5.5 km, is similar to Phobos in composition, but with a smoother surface. Both are very similar to the many asteroids found between the orbit of Mars and Jupiter.


Mission History:

Much of the information gathered about Phobos and Deimos comes from the NASA-Mariner, NASA-Viking and Soviet-Phobos missions. While the NASA-Mariner and Viking missions were more focused on exploring Mars, useful data and photographs of the Martian Moons were collected. Viking 1 was launched from Kennedy Space Center at Cape Canaveral on August 20, 1975, and arrived at Mars on June 19, 1976. On February 12, 1977, the orbit was changed to permit a flyby of Phobos, the larger, inner Martian moon. At closest approach, the orbiter flew within 90 km of the surface of Phobos. Viking 2 was launched September 9, 1975, and arrived at Mars on August 7, 1976. Later in its mission, Viking Orbiter 2 flew by Deimos, the smaller of the two Martian moons, at a distance of only 22 km. Spectacular pictures showing features as small as a compact car were taken.

The Soviet-Phobos Missions were probes intent on landing on Phobos (the larger of the two Martian Moons) and, in addition, doing experiments closer to the surface. While these missions successfully took photographs of Phobos, both probes were unsuccessful at landing on Phobos and completing their full missions.

All of the data collected from these missions has been quite conclusive in determining that Phobos and Deimos are composed of carbonaceous chondrite.

Scientific and Mathematical Theory

General Theory of Accretion:

The theory behind the creation of many of the planets in our solar system is accretion. When fragments circle the sun and impact each other, forming into planet-sized objects, the process is called "accretion." According to Newton’s laws of gravity, as the new planet-sized objects grew to a certain mass they would attract their own rings of debris and fragments (much like the rings of Saturn today). Theoretically, objects near the newly formed planets, outside of Roche’s Limit(see below), could impact each other, combine, and grow into larger objects, orbiting around the planets. According to this theory, the satellites coalesced with the gravitational effects and, assuming their orbits were not disrupted, are found in nearly circular, low inclination orbits [7, page 202]. This theory seems plausible and consistent with the laws of gravity, forces, and capture dynamics.

General Theory of Capture:

There are three basic methods of satellite capture. (1) Massive extended atmospheres of the early planets could have captured passing bodies and perturbed them into low-inclination orbits around the planet. (2) Close approach of passing large planetesimal to an existing satellite might have altered the satellite orbit and led to the capture of the planetesimal. (3) Collision with a satellite might have thrown out debris which could have reaggregated into a new satellite. This third type, also known as "Impact Disruption", requires a more in-depth look.

General Theory of Tidal Effects:

The force of gravity caused by an object gets weaker as you move farther away from that object. In this picture (see figure on right), the Earth is pulling on the Moon and the Moon is pulling on the Earth. The Moon pulls more strongly on the side of the Earth facing the Moon than on the side facing away from the Moon. Because the gravitational force on one side of the planet is different from that on the other side, it is called a tidal force[4]. If a body is not held together very strongly, the tidal forces can actually fragment the body(e.g. the comet Shoemaker-Levy, which fragmented over Jupiter).

Tidal forces depend on the radius. The force raising a tidal bulge is inversely proportional to the quantity of the radius raised to the third power and the entire tidal interaction is inversely proportional to the quantity of the radius raised to the sixth power. So, as the radius decreases, the stretching force gets very large[3, page 58].

General Theory of Roche's Limit (Law):

Roche's Limit is the distance from a planet inside which a satellite will be torn apart by tidal forces. That is, Roche's Limit is the distance from a planet at which the planet's tidal force on a satellite (force which stretches apart) is equal to the satellite's self-gravity (force which holds it together). As indicated above, as the satellite approaches the planet, the tidal force grows strong. The self-gravity is the same regardless of the distance from the planet. Eventually the tidal force will dominate and the planet will break apart[2]. For example, all of the rings of Saturn are inside Roche's Limit. Another way of looking at this is that being inside Roche's limit prevents scattered debris from accreting together.

The actual limits, of course, depend on the strength of the body. Bodies smaller than about 30 to 60 km in diameter can sometimes penetrate close to a planet if they are small and strong enough. Also note, this simple application of Roche's limit does not take into consideration any atmospheric affects [3].

Origin of the Moons:

Theory of Martian Moon Origin #1-Origin of Martian Moons via Atmospheric Capture and Tidal Evolution:

The popularity of the theory of origin via capture is due to the Viking Results which indicate (1) a low albedo, (2) low mean densities of 2 g/cc, (3) dark surfaces and spectrum suggestive of carbonaceous chondrite composition. Since asteroids, which resemble the Martian satellites, are commonly found in the outer part of the asteroid belt, this theory assumes that the Martian satellites originated from there [1, 6]. The Soviet Phobos-2 probe confirmed the density of Phobos to be 1,950 kg/m^3 and confirmed the composition of Phobos to be similar to a volatile-rich carbonaceous chondrite meteorite[3, page 120].

In the introduction, three types of capture were presented. According to Hartmann, the first one is most likely for Phobos and Deimos [3, page 120]. That is, asteroids originating away from Mars somehow approached Mars and were then captured by its massive extended atmosphere[3, 120]. While this seems to be the most likely method of capture, it is possible that, for example, instead of being captured by the atmosphere, Phobos or Deimos pushed out an already existing sattelite via collision and replaced it in orbit around Mars. This, however, is very unlikely.

For example, the process could have been:

1.Asteroid leaves asteroid belt (or some other distant origin)

2.It flies within a massive primitive atmosphere of Mars

3.Atmosphere/Gas drag slows it down and captures it into orbit causing an initially elongated and inclined orbit [7, page 203]

4.Tidal forces cause the now captured satellite to change orbit into a low inclination as observed today.

This capture theory has some flaws. For instance, step 4 is improbable but necessary to attain the current orbit of the moons. It is also possible that the tidal evolution would cause Phobos to collide with Deimos. However, depending on the assumptions made about past inclinations, this collision can be shown to happen or not happen (see graph above). Also, based on calculations, the forces on a planetoid during the deceleration (step 3) often fragment it. However, if one assumes that since Phobos and Deimos are relatively small, compact and strong, then there is the possibility of it surviving this fragmentation.

Theory of Martian Moon Origin #2 - Accretion and Regolith Formation: 

Accretion theory attempts to explain how a satellite could have formed around Mars and end up with a low inclination orbit. It is consistent with the laws of gravity, forces and capture dynamics. However, accretion theory is inconsistent with the composition of the moons. According to nearly all studies, from ground observation to the Mariner, the Viking and Phobos 1&2 missions, the composition of Phobos and Deimos are shown to be that of carbonaceous chondrite, much like a meteorite/asteroid from the outside asteroid belt. The theory of accretion is that the planets formed out of debris floating nearby and that some of this debris then circled around the planet and later formed satellites, thus insinuating that the composition of the satellites should be similar to that of the planet. This does not seem to be the case for Mars and its Moons: their compositions do not seem to match.

Regolith Formation and Composition:

To account for the fact that the surfaces of Phobos and Deimos are not similar to Mars in composition, accretion theory proposes that the carbonaceous chondrite is regolith, a layer of dust having accumulated outside a denser sattelite. It is possible for regolith to differ from the core of a sattelite in composition. Thus, according to this theory, the cores of Phobos and Deimos are similar to Mars in composition while the regolith, from a different source, has a different composition. However, this explanation is speculative. Even assuming that the regolith dust is giving misleading data about the density and composition of the core, one would then have to explain how this dust matches that of a carbonaceous chondrite meteorite. One possibility is that after formation of the core of Deimos or Phobos, an asteroid flew in and impacted one of the Martian moons (accounting for the Stickney crater). This impact collision would then spray fragments of the asteroid into orbit around Mars which would reaccumulate as regolith. This explanation is a variant of the Impact Disruption theory discussed earlier.

Meteorite observations show that low albedo and flat reflectance spectra are not unique to carbonaceous chondrite meteorites and that low-albedo solar system objects such as C-type asteroids may have other compositional analogs [6]. Based on these observations, two possibilities arise: (1)The regolith of Phobos and Deimos is heavily "contaminated" by recent asteroid fragments and various other material not of Mars composition. (2) A major impact on Phobos by a carbonaceous chondrite - a variant of the impact theory - caused the Stickney Crater and fragmented the asteroid which later accumulated as regolith.

Concerning Impact Disruption:

The present asteroid flux suggests that if Phobos was solid rock it could possibly have avoided disruption. However, if it has not been solid rock or if it has not been held together uniformly, then it could have easily been disrupted by asteroid flux. Another interesting point is that a disrupted satellite, when reaccumulated, returns to an orbit very close to that before disruption.

While this seems very speculative, there is evidence supporting the possibility of disruption. First, the presence of such large craters such as the Stickney Crater (see figure below) on Phobos indicates some sort of bombardment. This bombardment could have been "mild" or "severe." In a mild case an impact had enough force to create the Stickney Crater but not enough to cause disruption. In a severe case an impact hit a parent body and caused a disruption of which Phobos is a fragment. These are just two scenarios and many others could be derived. The important point that a serious impact did occur. It seems unlikely that the bombardment flux in the past was just enough to produce the catastrophic collision which created Stickney but not enough to cause disruption[1].


After looking at the discussion of capture theory and accretion theory, it is evident that each has significant flaws. The capture theory assumes such things as (1) asteroid capture of C-type composition, (2) high initial inclination during tidal evolution in order to avoid collision of Deimos and Phobos and(3) improbable capture and orbit dynamics. The accretion theory, consistent with current orbital-dynamics, is unable explain the current composition. To be consistent, accretion must combine with the idea of the formation of a regolith rich in carbonaceous-chondrite-like material. Thus accretion theory assumes that such material entered the Mars orbit from some source which accumulated on Phobos and Deimos.

It is important to consider the theory of impact disruption, which might describe events which may have occured after the initial capture or accretion and led to the current Martian moon configuration. Many possibilites for the origin of both Phobos and Deimos can be examined with these three theories. It becomes immedietly evident that Phobos and Deimos need not necessarily have formed in the same manner nor from the same source. In fact, it is entirely possible that one Moon was initially formed from capture while the other was initially formed from accretion. While this would seem to contradict the fact that the two moons have similar compositions, evidence has shown that debris from one moon could easily travel and adhere to the other moon - thus creating a similar composition[6, page 145]. This is just one of many possibilities. An explanation involving multiple theories seems most likely.


1] Burns, J.A., Matthews, M. S., 1986, Satellites, University of Arizona Press, pages 78-83,145-160

2] Drennon, Bill, 3/10/99 (accessed), The Roche Limit ""

3] Hartmann, W.K., 1999, Moons & Planets 4th Edition, Wadsworth Publishing, multiple pages throughout book.

4] author unknown, 3/10/99 (accessed), Tidal Forces ""

5] author unknown, 3/10/99 (accessed), "Phobos and Deimos" ""

6] Britt, D. T., Pieters, C. M., 1989 "The Origin of Phobos: Implications of Compositional Properties," Solar System Research, 22, 143-149.

7] Burns, J.A. 1978 "The Dynamical Evolution and Origin of the Martian Moons" Vistas in Astronomy, 22:193-210

8] author unknown, 3/10/99 (accessed), "The Mysterious Moons of Mars," ""

9] Selivanov, A.S., 1993, "The Mysterious end of Phobos 2," The Planetary Report, page 20.

Editing by Scott Ellis & Victoria Arrigoni, CalSpace Mars'99 team, UCSD


MARS SCIENCE - What We Know About Mars

Section Supervisors: Victoria Arrigoni, Kai Miller, Quinn Maughan
Content Creation: Victoria Arrigoni, Kai Miller, Quinn Maughan, Bill Baity, Scott Ellis

Copyright 1999-2000 Mars Now Team and the California Space Institute