  Impact-cratering processes on small bodies are thought to be mainly
controlled by the local material strength because of their low surface gravity,
and craters that are as large as the parent bodies should be affected by the
target curvature. Although cratering processes on planar surfaces in the
strength-controlled regime have been studied extensively, the mechanism by
which target curvature affects the cratering processes remains unclear. Herein,
we report on a series of impact experiments that used spherical targets with
various diameters. The resultant craters consisted of a deep circular pit and
an irregular-shaped spall region around the pit, which is consistent with the
features reported in a number of previous cratering experiments on planar
surfaces. However, the volume and radius of the craters increased with the
normalized curvature. The results indicate that the increase in the
spall-region volume and radius mainly contributes to the increase in the whole
crater volume and radius, although the volume, depth, and radius of pits remain
constant with curvature. The results of our model indicate that the geometric
effect due to curvature (i.e., whereby the distance from the equivalent center
to the target free surface is shorter for higher curvature values) contributes
to increases in the cratering efficiency. Our results suggest that the
impactors that produce the largest craters (basins) on some asteroids are thus
smaller than what is estimated by current scaling laws, which do not take into
account the curvature effects.
