  This paper studies the long-term migration of disturbed regolith materials on
the surface of Solar System small bodies from the viewpoint of nonlinear
dynamics. We propose an approximation model for secular mass movement, which
combines the complex topography and irregular gravitational field. Choosing
asteroid 101955 Bennu as a representative, the global change of the dynamical
environment is examined, which presents a division of the
creeping-sliding-shedding regions for a spun-up asteroid. In the creeping
region, the dynamical equation of disturbed regolith grains is established
based on the assumption of "trigger-slide" motion mode. The equilibrium points,
local manifolds and large-scale trajectories of the system are calculated to
clarify the dynamical characteristics of long-term regolith movement.
Generally, we find for a low spin rate, the surface regolith grains flow toward
the middle latitudes from the polar/equatorial regions, which is dominated by
the gradient of the geopotential. While spun up to a high rate, regolith grains
tend to migrate toward the equator, which happens in parallel with a
topological shift of the local equilibria at low latitudes. From a long-term
perspective, we find the equilibrium points dominate the global trends of
regolith movements. Using the methodology developed in this paper, we give a
prospect or retrospect to the secular motion of regolith materials during the
spin-up process, and the results reveal a significant regulatory role of the
equilibria. Through a detailed look at the dynamical scheme under different
spin rates, we achieve a macro forecast of the global trends of regolith motion
during the spin-up process, which explains the global geologic evolution driven
by the long-term movements of regolith materials.
