A General Formulation for the Distribution of Impacts and Ejecta from Small Planetary Satellites

Abstract

Small planetary satellites and ring moons are the source of dust rings around each of the giant planets, and presumably Mars, by cratering impacts from interplanetary material. The relative velocities of planetary satellites and interplanetary impactors lead to an anisotropic flux of impactors onto planetary satellites. The anistropy in the impactor flux seen from the frame of the satellite results in an anisotropic production of dust from the surface of the satellite. This anisotropy can have observable consequences for the distribution of material in dust rings. In this paper, an expression for the angular distribution of meteoroid ejecta from impacts onto a single moon or ring particle orbiting a planet with arbitrary obliquity is derived. I allow for any distribution of orbits of the impactors by using a Monte Carlo approach for the micrometeoroid orbits. Ejecta distributions are prograde due to the enhanced impact velocity on the leading face of the target. Ejecta orbits are therefore concentrated in the region exterior to the source satellite. Orbitally averaged ejecta distributions are weakly dependent on the planetary obliquity. Ejecta distributions vary with orbital season for planets with a significant obliquity. The orbitally averaged ejecta distribution is weakly dependent on the orbital distribution of the impactor population, largely due to the smearing effect of the assumed ejects "phase function." The impact velocity of micrometeoroids in the frame of the satellite is correlated with the orbital element distribution of the micrometeoroid population, allowing the orbital elements of interplanetary dust to be inferred from dust impact experiments, such as those on Galileo and Cassini. Dust rings derived from micrometeoroid ejecta from small planetary satellites may show seasonal variations in the vertical distribution of dust as the planet orbits the Sun.