Formation of Phobos and Deimos via a giant impact

Although the two moons of Mars, Phobos and Deimos, have long been thought to be captured asteroids, recent observations of their compositions and orbits suggest that they may have formed from debris generated by one or more giant impacts of bodies with $0.01Â target mass. Recent studies have both analytically estimated debris produced by giant impacts on Mars and numerically examined the evolution of circum-Mars debris disks. We perform a numerical study (Smoothed Particle Hydrodynamics simulation) of debris retention from giant impacts onto Mars, particularly in relation to a Borealis-scale giant impact (E $ 3 Â 10 29 J) capable of producing the Borealis basin. We find that a Borealis-scale impact is capable of producing a disk of mass $5 Â 10 20 kg ($1–4% of the impactor mass), sufficient debris to form at least one of the martian moons according to recent numerical studies of martian debris disk evolution. While a Borealis-scale impact may generate sufficient debris to form both Phobos and Deimos, further studies of the debris disk evolution are necessary. Our results can serve as inputs for future studies of martian debris disk evolution. The origin of the two moons of Mars, Phobos and Deimos, is widely debated. One prevailing theory is that the two moons are captured asteroids, as indicated by their size, shape, and prior observations of their composition (Murchie et al. and Deimos may have formed from debris generated from re-accretion after one or more giant impacts of bodies with $0.01Â target mass, a process that could more readily explain the composition, density, and orbits of the martian moons (Rosenblatt, 2011). Compositionally, observations from both the European Space Agency's Mars Express mission and NASA's Mars Global Surveyor mission show that Phobos and Deimos have thermal infrared spectra in poor agreement with the primitive carbon-rich materials normally associated with carbonaceous chondrites (Giuranna et al., 2011; Witasse et al., 2014). Additionally, minerals were detected on Phobos that are also present on the martian surface (Giuranna et al., 2011; Witasse et al., 2014), suggesting that the material that formed the martian satellites could have been ejected from Mars. Furthermore, the Mars Radio Science Experiment on board Mars Express mission measured the density of Phobos to be 1.87 g cm À3 with a porosity of 25–35% (Andert et al., 2010; Pätzold et al., 2014; Rosenblatt et al., 2010) – such low density and high porosity are uncommon for …