Presentation #202.08 in the session “Main-Belt Asteroids 3: Very Large Asteroids”.
We have conducted Monte Carlo simulations of Vesta’s spin evolution due to impacts (Mao and McKinnon, MAPS, in press). Vesta’s current shape is nonhydrostatic, mostly due to its irregular topography caused by two planetary-scale impact basins, Rheasilvia and Veneneia, near its south pole (e.g., Fu et al., Icarus 240, 133-145, 2014). The formation of these two basins involves substantial angular momentum exchange sufficient to manifestly alter Vesta’s spin state. Fu et al. infer a faster spin (5.02 hr) prior to the formation of these two basins, by fitting Vesta’s northern hemisphere’s shape (degree-2 flattening). We model Vesta as a two-layer oblate spheroid with this “initial” spin, select other parameters pertaining to Vesta’s specific impact environment, and determine the general likelihood of Vesta’s despinning probability by impacts. Results indicate that a suite of impacts collectively likely affected Vesta’s “initial” spin by less than a few tenths of an hour in either direction, with only ~4% of the time Vesta despinning to its current value (5.34 hr) or more. The largest impacts have greatest effects, however, so we take advantage of the actual cratering history of Vesta to investigate the rotational response to the formation of the two known giant basins specifically. Their magnitude and positions at or near the south pole imply that spinup rather than spindown is far more likely, by a ratio of 4:1, but starting from a spin period of 5.02 hr, ~7% of the time our modeled Vesta obtains a final spin longer than its present-day value. Thus, the proposed despinning of Vesta can be explained by the formation of these two giant basins, though spindown is not guaranteed. Regardless, the effects on Vesta’s obliquity should have been substantial, with angular offsets of 20-30° or more possible. The preference for spinup raises the possibility that these two basin impacts actually did spin up Vesta, notwithstanding the arguments of Fu et al. Spinup and rotationally driven extension at the equator could have conceivably played a role in the prominent normal fault and graben formation of the Divalia and Saturnalia Fossae (Scully et al., Icarus 244, 23-40, 2014).