Phobos has the look and shape of an asteroid 27 x 21.6 x 18.8 km high and wide, which corresponds to a mean radius of 11 km. Its surface is very dark and has craters of different sizes. The major crater, Stickney (diameter 10 km), has probably created a mass redistribution within Phobos. Deimos is slightly smaller than Phobos (dimensions of 12.2 x 15 x 11 km), but its look and shape also evoke those of an asteroid.
The Viking missions, and especially Phobos 2, were used to make measurements of the spectrum of sunlight reflected from the surface of these two moons. Scientists have found that these spectra showed similarities with the spectra of the surface of a few asteroids. However, these spectral measurements are not decisive enough to give the exact composition of the surface of Phobos and Deimos (and by extrapolation of their interiors). The material of these two moons may thus be closer to the rocks constituting the surface of Mars.
The two moons of Mars orbit in a circular path, in the equatorial plane of Mars, but at different distances : Phobos is located 5978 km from the surface while Deimos is further away (20,059 km). In the solar system, no planet possesses a natural satellite as close as Phobos is from Mars.
As Phobos is closer, it makes a complete revolution around Mars in only 7 hours and 39 minutes and Deimos in 30 hours and 18 minutes.
The trajectories of Phobos and Deimos depend on the gravitational field of the planet. This field may take different values depending on the location on the surface, and vary over time. These temporal variations are observed at different scales. The most important ones are seasonal because the values of the gravitational field depend on the proportion of CO2 in the atmosphere and captured in the polar caps. About a quarter of the atmosphere enters in this seasonal sublimation and condensation of CO2.
The trajectory of the moons of Mars is also affected by diurnal variations, semi-diurnal and long period variation caused by the tides related to the Sun : just like Earth, Mars is slightly distorted due to the gravitational pull of the Sun. These distortions cause a slight mass redistribution within Mars, which in turn slightly alters the gravitational field of the planet. This modification of the gravitational field is felt on the trajectory of the Martian moons. This controls the long-term evolution of the orbits of the Martian moons around their mother planet. Thus, it is anticipated that Phobos is approaching Mars at about 20 meters per century and that it should crash on the Martian surface in about 40 million years or break under the effect of these tidal forces before they reach the surface. Conversely, Deimos moves away from Mars at a rate of 2 millimeters per year, as the Moon moves away from Earth at a rate of 3.8 centimeters per year.
Why studying the moons of Mars?
The main purpose of the study of Phobos by the instrument of radio-science on board Mars Express is to precisely determine its mass and later the mass distribution.
The Viking and Phobos 2 orbiters were able to give a first estimation of the mass of Phobos, when measuring the deflection trajectory experienced by the probes during flybys near Phobos. Approximately 17 flybys close to Phobos were made, with a distance between the probes and Phobos between 100 and 300 km maintained during less than 5 minutes. A single flyby near Deimos has been achieved at a distance of 30 km. Mars Express is currently the only spacecraft in orbit around Mars capable of flybys near Phobos.
Here is an animation showing the deflection of the trajectory of a spacecraft as it flies by a moon of Mars.
Since the Viking and Phobos 2 missions, the technology has advanced and Doppler shift measurements provided by Mars Express are ten times more accurate than those by the old space probes. It is therefore possible to get the best measurements ever made for the determination of the mass of Phobos by the interpretation of the Doppler tracking of the Mars Express spacecraft during a close flyby of the Martian moon.
In addition to the mass of Phobos, the precise determination of its volume is also an objective of the Mars Express mission. For the calculation of the volume of this body, one needs to know the precise shape. The on-board stereoscopic camera (HRSC) has provided 3D images of the surface with a resolution never obtained before.
The association of radio-science data for the mass and the stereoscopic camera for the volume will accurately calculate the mean density of Phobos. This density is estimated at 1.85 gr/cm3 with an accuracy of 3.2%, which is much less dense than the rock surface of Mars (estimated between 2.7 and 3.3 gr/cm3). The average density of Deimos is estimated at 1.65 g/cm3 with an accuracy of only 17%. These low densities are close to those of some asteroids in the solar system and favor the hypothesis that Phobos and Deimos are asteroids captured by the gravitational pull of Mars and therefore do not come from Mars.
However, the precision with which the mean density of Phobos is known does not permit to identify precisely the origin of these asteroids. The low density of Phobos and Deimos may also be explained by the presence of light elements within the body such as water ice or empty spaces between the blocks of material. Like some asteroids, Phobos and Deimos could consist of an aggregate of material and not a monolith of rock and ice mixed. New data, more accurate, from Mars Express will provide further evidence to clarify this issue.
By compiling as much information on Phobos, scientists can try to determine its origin, and what was its evolution. If Phobos would actually have a certain kinship with the asteroids (from its density, its mineralogical composition ...), the scientists would be able to deepen the study of asteroids through the moons of Mars.
Indirectly, information about the interior of Mars could be gathered in parallel since the trajectory of Phobos depends on the gravitational field of Mars.
Future missions to Phobos and Deimos is currently under study, following the failure of the Phobos-Grunt mission.
Flybys of MarsExpress are still foreseen to study Phobos.