Catastrophic rupture of lunar rocks: Implications for lunar rock size-frequency distributions

TL;DR

This study models lunar rock size distributions over time, revealing how impact shattering transforms distributions from power-law to exponential and enabling estimates of surface age and initial block abundance.

Contribution

The paper updates a lunar rock evolution model with new impact data and shattering functions, providing improved estimates of rock size distributions and surface ages.

Findings

Block size distributions evolve from power-law to exponential with age.

Impact shattering rates are roughly twice as fast as previous estimates.

The model allows estimation of surface exposure ages from size-frequency data.

Abstract

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.