  Understanding the geological modification processes on asteroids can provide
information concerning their surface history. Images of small asteroids from
spacecraft show a depletion in terms of smaller craters. Seismic shaking was
considered to be responsible for crater erasure and the main driver modifying
the geology of asteroids via regolith convection or the Brazil nut effect.
However, a recent artificial impact experiment on the asteroid Ryugu by the
Japanese Hayabusa2 mission revealed minimal seismic activity. To investigate
whether a seismic shaking model can reproduce the observed crater record, the
crater distribution on Ryugu was analyzed using crater production functions
under cohesionless conditions. Crater retention ages were estimated as a
function of crater diameter for Ryugu, Itokawa, Eros, and Bennu using the
crater size-frequency distribution and crater production function estimated for
those asteroids. We found that the power-law indices "a" are inconsistent with
diffusion processes (e.g., seismic shaking, a=2). This result suggests that
seismic shaking models based on diffusion equations cannot explain the crater
distribution on small asteroids. Alternative processes include surface flows,
possibly at the origin of geomorphological and spectral features of Ryugu. We
demonstrate that the vertical mixing of material at depths shallower than 1 m
occurs over 10^3-10^5 yr by cratering and obliteration. The young surface age
of Ryugu is consistent with the slow space weathering that results from
cratering, as suggested in previous studies. The timescale (10^4-10^6 yr)
required for resurfacing at depths of 2-4 m can be compared with the cosmic-ray
exposure ages of returned samples to constrain the distribution of impactors
that collide with Ryugu.
