Presentation #107.04 in the session “Asteroid Dynamics: Spinning, Tumbling, Running in Circles”.
In the taxonomic classification of asteroids, S- and Q-types are both ordinary chondrites but show slightly different absorption features and spectral slopes. While S-types have reddened, weathered surfaces, Q-types show bluer, fresher surface materials. This inconsistency is considered a result of interactions between space weathering and resurfacing. Tidally induced resurfacing driven by close encounters with terrestrial planets has been suggested as a critical contributor to reveal fresh materials beneath the weathered surfaces. Earlier studies support this, statistically indicating that Q-types have experienced close encounters with terrestrial planets. We believe (99942) Apophis – expected to have a close Earth flyby within 6 Earth radii on April 13, 2029 – represents a golden opportunity for detecting tidal resurfacing. Using a dynamic model and discrete element method (DEM), we numerically investigate the range of surface grain motions driven by the tidal forces from Earth on Apophis. The dynamic model simulates the orbital and spin evolution of Apophis 3 hours before and after the closest encounter and computes the surface slope evolution. The surface slope defines how a surface element normal is tilted with respect to the body center direction. The slope evolution indicates the change in slope in the direction of the net force (gravity, tidal, and rotational forces) acting on each facet. We then uniformly apply the accelerations computed by the dynamic model for selected surface elements to a periodic patch of polydisperse spherical grains in DEM simulations over the same encounter window. We alter the direction of the initial slope of each DEM patch to sample uncertainties in the spin state of Apophis. We then measure the scale of the regolith motion from the DEM simulations and associate this with the slope variations measured in the dynamic model to extrapolate statistics for the large-scale tidal resurfacing during the close approach. This investigation to better understand the surface dynamics during the 2029 encounter with Earth could indicate possible sites to detect granular alterations for potential missions to Apophis, like the OSIRIS-REx extended mission or KASI’s Apophis Rendezvous Mission.