Spin Evolution Of Ceres And Vesta Due To Impacts

All asteroid spins evolve due to collisions. Geophysical analysis (Mao and McKinnon 2018b) implies Ceres might have been modestly despun (by similar to 6.5%) by impacts. Vesta's postaccretion rotation rate, before the formation of the Veneneia and Rheasilvia basins, has also been proposed to have been higher (by similar to 6%) than today (Fu et al. 2014). We have designed Monte Carlo simulations to investigate Ceres' and Vesta's plausible spin evolution by impacts. We use the main belt asteroid size-frequency distribution, Ceres' and Vesta's cratering records, and their present-day impact velocity probability distributions to quantify possible spin histories. We consider dynamical effects from escaping ejecta, adopt mass-velocity scaling laws for two endmember surface materials (porous and nonporous), and track potential catastrophic disruptions. Results show that Ceres' spin period likely changed by a fraction of an hour (up or down) compared with its "initial," accretional spin, mostly due to large impacts. Different surface materials do not yield statistically distinguishable final spin distributions, but imply opposite crustal evolutions where Ceres loses/accretes mass, depending on surface porosity; as much as several 100 m worth of exogenous material, globally averaged, can be accreted if the surface remains porous. Vesta's final Monte Carlo spin distribution is more concentrated around its "initial" value, possibly implying a more relict spin state than Ceres. However, explicitly modeling the angular momentum changes wrought by Vesta's two existing planetary-scale impact basins shows that their formation alone can account for Vesta's proposed spindown, although spinup is a much more likely outcome because of the scale and location of the impacts.