  Small craters of the lunar maria are observed to be in a state of
equilibrium, in which the rate of production of new craters is, on average,
equal to the rate of destruction of old craters. Crater counts of multiple
lunar terrains over decades consistently show that the equilibrium cumulative
size-frequency distribution (SFD) per unit area of small craters of radius >r
is proportional r^(-2), and that the total crater density is a few percent of
so-called geometric saturation, which is the maximum theoretical packing
density of circular features. While it has long been known that the primary
crater destruction mechanism for these small craters is steady diffusive
degradation, there are few quantitative constraints on the processes that
determine the degradation rate of meter to kilometer scale lunar surface
features. Here we combine analytical modeling with a Monte Carlo landscape
evolution code known as the Cratered Terrain Evolution Model to place
constraints on which processes control the observed equilibrium size-frequency
distribution for small craters. We find that the impacts by small distal ejecta
fragments, distributed in spatially heterogeneous rays, is the largest
contributor to the diffusive degradation which controls the equilibrium SFD of
small craters. Other degradation or crater removal mechanisms, such cookie
cutting, ejecta burial, seismic shaking, and micrometeoroid bombardment, likely
contribute very little to the diffusive topographic degradation of the lunar
maria at the meter scale and larger.
