New hints on Phobos collisional capture origin from Rosetta-OSIRIS observation

On 2007 February 24 and 25, the ESA Rosetta mission flew by the planet Mars during its complex interplanetary trajectory towards its main target: comet 67P/Churyumov-Gerasimenko. The geometry of this gravitational assist gave the chance to image Phobos before and after Rosetta-Mars closest approach (CA) from a distance range between 115 000 to 21 000 km. Different surface areas of Phobos were observed belonging to the leading and trailing hemisphere of the anti-Mars hemisphere of the satellite, and also a section of its sub-Mars hemisphere. We compared our spectra, obtained during the pre- and the post-CA, with the reflectance spectra of D-type asteroids, showing that Phobos near-ultraviolet, visible and near-infrared (263.5-992.0 nm) reflectivity is within the spectral dispersion of the D-type asteroids. We investigated then the possibility of a dynamical collisional capture of Phobos similar to the origin of the irregular satellites of the giants planets. The coupled observational and dynamical ones suggest an early capture of Phobos in the first 10-100 Ma of the lifetime of the Solar System, consistently with the results of previous studies of the orbital evolution of Phobos.