Spin evolution of Ceres and Vesta due to impacts, and the outsize role of Veneneia and Rheasilvia for Vesta
Abstract
All asteroid spins evolve due to collisions. We have designed Monte Carlo simulations to investigate how Ceres' and Vesta's spin could have evolved under the influence of impacts. For Ceres, geophysical analysis in earlier work (Mao and McKinnon 2018, Icarus 299, 430-442) implied Ceres might have had its spin altered --- specifically, modestly despun (by $\sim$6.5%) --- by impacts. We use the crater record on Ceres' surface, the main belt asteroid size-frequency distribution, and Ceres' present impact velocity probability distribution to constrain and quantify the impact events, and model Ceres' possible impact history. A total of 3500 impactors, with a velocity range from 520 m/s to 13 km/s and a size range from 1-to-200 km, are chosen to collide with Ceres in the simulations. We consider dynamical effects from escaping ejecta and adopt mass-velocity scaling laws for two end-member surface materials (porous and non-porous). Results show that Ceres' spin evolution is mostly dominated by large impacts, with minimal cumulative effect from smaller ones. While not an unexpected result, throughout Ceres' impact history its spin period likely changed by some fraction of an hour, and possibly by an hour or more, and in either direction (up or down), compared with its "initial" accretional spin. Evolution of spin axis displacement due to impacts is likely limited to a couple of degrees or so. The final spin period distributions from the Monte Carlo simulations with different surface materials are statistically indistinguishable, but Ceres should have undergone erosional evolution with an originally non-porous surface; conversely, Ceres accretes (gains mass) with a porous (i.e., regolith dominated) surface, possibly by as much as several 100 m worth of exogeneous material, globally averaged. Regarding our original motivation, we find that it is possible for Ceres to be modestly despun by impacts alone, ending in its present-day spin. For Vesta, its post-accretion rotation rate, before the formation of the Veneneia and Rheasilvia basins, has also been proposed to be higher (by $\sim$6%) than today, based on a hydrostatic interpretation of the shape of the oldest northern hemisphere terrain (Fu et al. 2014, Icarus 240, 133-145). We carry out a similar Monte Carlo simulation of Vesta's possible spin evolution, based on a suite of 1000 simulated impacts with the same range of impactor sizes for Ceres, impact velocities from ~360 m/s to 13 km/s, and Vesta's equivalent oblate "paleo" equilibrium shape, and track potential catastrophic disruptions. While the spin evolution due to impacts resembles that of Ceres in many ways, Vesta's final Monte Carlo spin distribution is more concentrated around its "initial" value, implying a more relict spin state throughout its impact history, than Ceres. Disruptions rarely occur under the assumed impact environment for Vesta (<0.5% of the modeled simulations, dependent on disruption criteria). These results are based on the main-belt size-frequency distribution, however, and do not explicitly incorporate the formation of Veneneia and Rheasilvia --- Vesta's two, known planetary-scale impact basins. A Monte Carlo study of the formation of these two basins alone shows that they can alter Vesta's presumed post-accretion spin to its present-day value, but from a statistical point-of-view, spin up of Vesta is strongly favored over spin down because of the scale of the impacts and the polar position of Rheasilvia. The size of the impactors involved is not well constrained, but if they were as large as 65-km across (Jutzi et al. 2013, Nature 494, 207-210), there is the intriguing possibility that these impacts actually did spin Vesta up, notwithstanding the arguments of Fu et al. (2014). Spinup and rotationally driven extension at the equator could conceivably have played a role in forming the prominent normal fault and graben of the Divalia and Saturnalia Fossae on Vesta.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.281M