  Like many airless planetary surfaces, the surface of the Moon is scattered by
populations of blocks and smaller boulders. These features decrease in
abundance with increasing exposure time due to comminution by impact
bombardment and produce regolith. Here we model the evolution of block
size-frequency distributions by updating the model of Hoerz et al. (1975) with
new input functions: the size-frequency distributions of cm-scale meteoroids
observed over the last few tens of years and a rock impact shattering function.
The impact shattering function is calibrated using measurements of a lunar
block size-frequency distribution of known age. We find that cumulative block
size-frequency distributions change with time from a power-law for young
populations (<~50 Myr) to an exponential distribution for older populations.
The new destruction rates are within the uncertainty of the original model,
although, for sizes >5 cm, two times faster than the original best estimate.
The faster rates are broadly consistent with observations reported by other
studies. Since the input functions are known for small rock sizes, the rock
abundance can be determined theoretically at sizes below the current image
spatial resolution (0.5 m). Surface exposure age of block fields can be
estimated together with the initial block abundance from the measurement of
block size-frequency distributions.
