Mars has 2 small natural satellites in its orbit – Phobos (“fear”), the smaller, innermost, faster-orbiting one, and Deimos (“dread”), the larger, slower one, found on a less proximal orbit. Recent observations of the former indicate that it does not present the features of a compact space object, but rather those of a celestial body made of dust, with large cavities within. Its origins and genesis have eluded the researchers to this day.

 

The European Space Agency already has a spacecraft (Mars Express) in orbit of Mars, and they hope to get more accurate data from it. Based on the images received from the Mars Express during its passings by the small satellite, they were able to assemble a 3D model that aided them in calculating Phobos' volume. The varying radio signals emitted by the probe, which were influenced by the satellite's gravity, helped another group of scientists to determine its exact mass at 1.0721016 kilograms (2.36357944 pounds) – approximately 1/1,000,000,000 the weight of our planet.

 

These 2 measurements allow for the more precise calculation of Phobos' density, which is of 1.85 grams per cubic centimeter (0.067 pounds per cubic inch), as compared with the rocks on Mars' surface, which have a density of 2.7 to 3.3 grams per cubic centimeter (0.098 to 0.119 pounds per cubic inch). This proves that the small moon is not completely solid, but rather a conglomerate of gravitationally-bound rocks or, in scientific terms, “a pile of rubble.”

 

This low density is comparable to that of a regular asteroid, which may mean that Phobos and Deimos were not formed from Mars' body, but perhaps they were just asteroids that came to orbit the red planet after they were captured by its gravitational force. A future Russian mission, Phobos-Grunt (“Phobos-soil”), will rely on the data provided by the Mars Express in order to land on Phobos and collect samples of material that will help elucidate this mystery.