The Marsmond Phobos is a rubble heap whose origin is not yet clear. Image: Gerhard Neukum, ESA / DLR / FU Berlin
The Marsmond Phobos is probably not a massive boulder, but a pile of debris held together by gravity. European Astronomers have come to this conclusion after evaluating data from the Mars Express spacecraft. Whether the rock material comes from the Martian surface itself or comes from the asteroid belt is so far unclear. This may be clarified when Russian space researchers bring rock samples from their Phobos mission, which is scheduled for next year. Gerhard Neukum of the Free University of Berlin and his colleagues used pictures of Phobos, which Mars Express had made on various fly-bys with a high-resolution stereo camera, to create the most accurate 3D model of the Martian moon. Using this model, they estimated the dimensions of the irregular rock lump to be 27 by 22 by 19 kilometers. Another team used radio signals to determine the exact mass of phobos, which is about one billionth of the Earth's mass.

With this hitherto most exact information about mass and volume of the moon, the density can now be calculated. According to initial estimates, it is only 1.85 grams per cubic centimeter. In comparison, the density of rock on the surface of Mars is almost twice as high. However, there is a certain kind of asteroid that has a density similar to phobos, explain the scientists. So-called D-asteroids probably consist of many small rocks and are therefore referred to by researchers as rubble pile ("rubble pile"). Older data also indicate that phobos and D-asteroids have a similar composition. This could mean that Phobos and possibly the second, smaller Marsmond Deimos are actually asteroids that eventually got into the gravity field of the red planet.

Phobos could also be made of mars rock that was thrown into space during a large meteor impact. These pieces may have accumulated and thus formed the rubble heap.
The truth about the origin of the Martian moon will probably come to light only through laboratory analysis of the moon's rock.

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