  Spectral characterization of near-Earth asteroids (NEAs) has revealed a
continuum of space-weathered states for the surfaces of S-complex NEAs, with
Q-class NEAs, an S-complex subclass, most closely matching the un-weathered
surfaces of ordinary chondrite meteorites. Dynamical calculations of the
orbital evolution of S-complex NEAs revealed that Q-class NEAs tend to have
close encounters with terrestrial planets, suggesting that planetary tides may
play a role in refreshing NEA surfaces. However, the exact physical
mechanism(s) that drive resurfacing through tidal encounters and the encounter
distance at which these mechanisms are effective, has remained unclear. Through
the lens of the upcoming (99942) Apophis encounter with Earth in 2029, we
investigate the potential for surface mobilization through tidally-driven
seismic shaking over short-timescales during encounter and subsequent surface
slope evolution over longer-timescales driven by tumbling. We perform
multi-scale numerical modeling and find that the 2029 encounter will induce
short-term tidally-driven discrete seismic events that lead to high-frequency
(greater than 0.1 Hz) surface accelerations that reach magnitudes similar to
Apophis' gravity, and that may be detectable by modern seismometers. It is
still unclear if the shaking we model translates to widespread particle
mobilization and/or lofting. We also find there will be a significant change in
Apophis' tumbling spin state that could lead to longer-term surface refreshing
in response to tumbling-induced surface slope changes. We propose that through
these mechanisms, space-weathered S-class asteroid surfaces may become
refreshed through the exposure of unweathered underlying material. These
results will be tested by the future exploration of Apophis by NASA's
OSIRIS-APEX.
