Cratering Experiments on the Self Armoring of Coarse-Grained Granular Targets

TL;DR

This study experimentally investigates how coarse-grained surfaces on asteroids resist cratering, revealing that larger surface grains significantly reduce crater formation and suggesting armoring as a key factor in asteroid surface evolution.

Contribution

It provides the first experimental validation of the armoring effect on granular surfaces, linking grain size ratios to crater suppression on asteroid-like targets.

Findings

Crater brightness and shape depend on target grain size.

Crater formation is rare when projectile and target grains are similar in size.

A simple momentum transfer model explains the experimental results.

Abstract

Recently published crater statistics on the small asteroids 25143 Itokawa and 433 Eros show a significant depletion of craters below approx. 100 m in diameter. Possible mechanisms that were brought up to explain this lack of craters were seismic crater erasure and self armoring of a coarse, boulder covered asteroid surface. While seismic shaking has been studied in this context, the concept of armoring lacks a deeper inspection and an experimental ground truth. We therefore present cratering experiments of glass bead projectiles impacting into granular glass bead targets, where the grain sizes of projectile and target are in a similar range. The impact velocities are in the range of 200 to 300 m/s. We find that craters become fainter and irregular shaped as soon as the target grains are larger than the projectile sizes and that granular craters rarely form when the size ratio between projectile and target grain is around 1:10 or smaller. In that case, we observe a formation of a strength determined crater in the first struck target grain instead. We present a simple model based on the transfer of momentum from the projectile to this first target grain, which is capable to explain our results with only a single free parameter, which is moreover well determined by previous experiments. Based on estimates of typical projectile size and boulder size on Itokawa and Eros, given that our results are representative also for km/s impact velocities, armoring should play an important role for their evolution.