European space scientists are getting closer to unravelling the origin of Mars’ larger moon, Phobos. Thanks to a series of close encounters by ESA’s Mars Express spacecraft, the moon looks almost certain to be a ‘rubble pile’, rather than a single solid object. However, mysteries remain about where the rubble came from. Unlike Earth, with its single large moon, Mars plays host to two small moons. The larger one is Phobos, an irregularly sized lump of space rock measuring just 27 km x 22 km x 19 km. During the Summer, Mars Express made a series of close passes to Phobos. It captured images at almost all fly-bys with the High Resolution Stereo Camera (HRSC). A team led by Gerhard Neukum, Freie Universität Berlin, also involving scientists from the German Aerospace Centre (DLR), is now using these and previously collected data to construct a more accurate 3D model of Phobos, so that its volume can be determined with more precision. In addition, during one of the nearest fly-bys, the Mars Express Radio Science (MaRS) Experiment team led by Martin Pätzold, Rheinisches Institut fuer Umweltforschung at the University of Cologne, carefully monitored the spacecraft’s radio signals. They recorded the changes in frequency brought about by Phobos’ gravity pulling Mars Express. This data is being used by Tom Andert, Universität der Bundeswehr Muenchen and Pascal Rosenblatt, Royal Observatory of Belgium, both members of the MaRS team, to calculate the precise mass of the martian moon. Putting the mass and volume data together, the teams will be able to calculate the density. Eventually, this will be a new important clue to how the moon formed.