Investigations of the Moon Phobos by Mars Express and Implications Towards its Origin

Kurzfassung

Mars Express has opened a new chapter in the exploration of Phobos, thanks to its elliptical orbit that allows for regular flybys, sometimes as close as 50 km. Initially designed for studying Mars’ surface and atmosphere, its instruments have provided a wealth of data, including precise geophysical bulk parameters, insights into Phobos’ interior, high-resolution images, remote sensing data of its surface, and observations of interactions between Phobos and the space environment. This resurgence of interest in the Martian moons revolves around the fundamental question of how they formed. Despite the abundance of data gathered by Mars Express, Phobos’ surface composition remains uncertain with many unresolved questions. This ambiguity prevents a definitive confirmation or refutation of the asteroid capture scenario, which likens Phobos to a D-type asteroid. This scenario contradicts the current orbits of the moons and poses other dynamical challenges. However, an alternative hypothesis suggests that Phobos and Deimos formed through the accretion of a rocky debris disk in Mars’ orbit. Their refined shapes, low mass, and bulk density support this idea. The non-detection of MARSIS echoes implies a porous interior, aligning with the low bulk density, indicative of high porosity. The next phase of Phobos exploration will be led by JAXA’s Mars Moon eXplorer (MMX) mission, set to quasi-orbit Phobos, land on its surface, and return a sample to Earth in 2029.