  Asteroid (99942) Apophis' close approach in 2029 will be one of the most
significant small-body encounter events in the near future and offers a good
opportunity for in situ exploration to determine the asteroid's surface
properties and measure any tidal effects that might alter its regolith
configuration. Resurfacing mechanics has become a new focus for asteroid
researchers due to its important implications for interpreting surface
observations, including space weathering effects. This paper provides a
prediction for the tidal effects during the 2029 encounter, with an emphasis on
whether surface refreshing due to regolith movement will occur. The potential
shape modification of the object due to the tidal encounter is first confirmed
to be negligibly small with systematic simulations, thus only the external
perturbations are taken into account for this work (despite this, seismic
shaking induced by shifting blocks might still play a weak role and we will
look into this mechanism in future work). A two-stage approach is developed to
model the responses of asteroid surface particles (the regolith) based on the
soft-sphere implementation of the parallel N-body gravity tree code pkdgrav. A
full-body model of Apophis is sent past the Earth on the predicted trajectory
to generate the data of all forces acting at a target point on the surface. A
sandpile constructed in the local frame is then used to approximate the
regolith materials; all the forces the sandpile feels during the encounter are
imposed as external perturbations to mimic the regolith's behavior in the full
scenario. The local mechanical environment on the asteroid surface is
represented in detail, leading to an estimation of the change in global surface
environment due to the encounter. Typical patterns of perturbation are
presented that depend on the asteroid orientation and sense of rotation at
perigee.
