  We show that a model in which Mars grows near Earth and Venus but is then
scattered out of the terrestrial region yields a natural pathway to explain the
low masses of the Martian moons Phobos & Deimos. In this scenario, the last
giant impact experienced by Mars is followed by an extended period (tens to
hundreds of Myr) of close passages by other planetary embryos. These close
passages perturb and dynamically heat any system of forming satellites left
over by the giant impact and can substantially reduce the mass in the satellite
system (sometimes to zero). The close passage of massive perturbing bodies also
offers the opportunity to capture small objects by three-body scattering. Both
mechanisms lead to low mass moon systems with a substantially collisional
history.

Earth and Planetary Astrophysics