Impact Dynamics of Moons Within a Planetary Potential

Current lunar origin scenarios suggest that Earth's Moon may have resulted from the merger of two (or more) smaller moonlets. Dynamical studies of multiple moons find that these satellite systems are not stable, resulting in moonlet collision or loss of one or more of the moonlets. We perform Smoothed Particle Hydrodynamic (SPH) impact simulations of two orbiting moonlets inside the planetary gravitational potential and find that the classical outcome of two bodies impacting in free space is altered as erosive mass loss is more significant with decreasing distance to the planet. Depending on the conditions of accretion, each moonlet could have a distinct isotopic signature; therefore, we assess the initial mixing during their merger, in order to estimate whether future measurements of surface variations could distinguish between lunar origin scenarios (single vs. multiple moonlets). We find that for comparable-size impacting bodies in the accretionary regime, surface mixing is efficient, but in the hit-and-run regime, only small amount of material is transferred between the bodies. However, sequences of hit-and-run impacts are expected, which will enhance the surface mixing. Overall, our results show that large-scale heterogeneities can arise only from the merger of drastically different component masses. Surfaces of moons resulting from merger of comparable-sized components have little material heterogeneities, and such impacts are preferred, as the relatively massive impactor generates more melt, extending the lunar magma ocean phase.