Impact crater populations provide key information about the overall dynamics of planet formation and populations of small bodies in the solar system. Small craters are sensitive to the mechanical properties of the crust. Large impacts can significantly alter the structure of the crust and perturb the thermal state of the mantle. We investigate the mechanics and thermodynamics of impact cratering both to understand this pervasive physical process but also to draw inferences about the subsurface structure of planets. In addition, we are interested in how collisions may modify the chemical composition of a planet through various processes, including delivery of material and removal of the atmosphere or solid planet.