Discrete Element Modelling of Pit Crater Formation on Mars

Stuart Hardy

doi:10.3390/geosciences11070268

Abstract

Pit craters are now recognised as being an important part of the surface morphology and structure of many planetary bodies, and are particularly remarkable on Mars. They are thought to arise from the drainage or collapse of a relatively weak surficial material into an open (or widening) void in a much stronger material below. These craters have a very distinctive expression, often presenting funnel-, cone-, or bowl-shaped geometries. Analogue models of pit crater formation produce pits that typically have steep, nearly conical cross sections, but only show the surface expression of their initiation and evolution. Numerical modelling studies of pit crater formation are limited and have produced some interesting, but nonetheless puzzling, results. Presented here is a high-resolution, 2D discrete element model of weak cover (regolith) collapse into either a static or a widening underlying void. Frictional and frictional-cohesive discrete elements are used to represent a range of probable cover rheologies. Under Martian gravitational conditions, frictional-cohesive and frictional materials both produce cone- and bowl-shaped pit craters. For a given cover thickness, the specific crater shape depends on the amount of underlying void space created for drainage. When the void space is small relative to the cover thickness, craters have bowl-shaped geometries. In contrast, when the void space is large relative to the cover thickness, craters have cone-shaped geometries with essentially planar (nearing the angle of repose) slope profiles. Frictional-cohesive materials exhibit more distinct rims than simple frictional materials and, thus, may reveal some stratigraphic layering on the pit crater walls. In an extreme case, when drainage from the overlying cover is insufficient to fill an underlying void, skylights into the deeper structure are created. This study demonstrated that pit crater walls can exhibit both angle of repose slopes and stable, gentler, collapse slopes. In addition, the simulations highlighted that pit crater depth only provides a very approximate estimate of regolith thickness. Cone-shaped pit craters gave a reasonable estimate (proxy) of regolith thickness, whereas bowl-shaped pit craters provided only a minimum estimate. Finally, it appears that fresh craters with distinct, sharp rims like those seen on Mars are only formed when the regolith had some cohesive strength. Such a weakly cohesive regolith also produced open fissures, cliffs, and faults, and exposed regolith “stratigraphy” in the uppermost part of the crater walls.

Highlights

Pit craters and pit crater chains are recognised as being an important part of the surficial morphology and structure of many planetary bodies (e.g., [1,2,3,4,5,6,7]; Figure 1a,b)
This study presents a high-resolution (1–2 m) discrete element model of cover collapse above either a simple static or a widening underlying void (Figure 1e,f)
The modelling of the deformation to high strain was an ideal candidate for the discrete element technique, as it is well-suited to studying problems in which discontinuities are important

Summary

Introduction

Pit craters and pit crater chains are recognised as being an important part of the surficial morphology and structure of many planetary bodies (e.g., [1,2,3,4,5,6,7]; Figure 1a,b) They have a very distinctive surface expression, often presenting funnel-, cone-, or bowl-shaped geometries, and do not possess the elevated rim, ejecta deposits, or other features that are typically associated with fresh, “young” impact craters unaffected by subsequent erosion (Figure 1a,b: see [3,4,8]). Implications of these results for the interpretation of pit craters on Earth, Mars, moons, and other planets are discussed

Methodology

Findings

Discussion and Conclusions