Heterogeneous impact transport on the Moon

Abstract

Impact cratering is the dominant process for transporting material on the Moon's surface. An impact transports both proximal material (continuous ejecta) locally and distal ejecta (crater rays) to much larger distances. Quantifying the relative importance of locally derived material versus distal material requires understandings of lunar regolith evolution and the mixing of materials across the lunar surface. The Moon has distinctive albedo units of darker mare basalt and brighter highland materials, and the contacts between these units are ideal settings to examine this question. Information on the amount of material transported across these contacts comes from both the sample collection and remote sensing data, though earlier interpretations of these observations are contradictory. The relatively narrow (~4–5 km wide) mixing zone at mare/highland contacts had been interpreted as consistent with most material having been locally derived from underneath mare plains. However, even far from these contacts where the mare is thick, highland material is abundant in some soil samples (>20%), requiring transport of highland material over great distances. Any model of impact transport on the Moon needs to be consistent with both the observed width of mare/highland contacts and the commonality of nonmare material in mare soil samples far from any contact. In this study, using a three‐dimensional regolith transport model, we match these constraints and demonstrate that both local and distal material transports are important at the lunar surface. Furthermore, the nature of the distal material transport mechanism in discrete crater rays can result in substantial heterogeneity of surface materials.

The equilibrium size-frequency distribution of small craters reveals the effects of distal ejecta on lunar landscape morphology

“…Unlike the relatively spatially uniform blanketing by proximal ejecta, the distal ejecta of an impact crater consists of a spatially heterogeneous population of energetic ejecta fragments that produces crater rays, secondary craters, and induces mass movement and mixing upon deposition in a process called ballistic sedimentation (Elliott et al, 2018;Huang et al, 2017;Oberbeck, 1975).The formation of secondaries in distal ejecta, whether in isolation, in clusters and chains, or as part of distal rays (Elliott et al, 2018;Pieters et al, 1985), should produce similar slopedependent mass transport of proximal ejecta that the Figure 4. A) A schematic diagram illustrating the spatially heterogeneous nature of impact-driven degradation.…”

“…Our analytical models are similar to the a 1-D models of Soderblom (1970), Marcus (1970), and in which we represent the observable craters as an time-evolving SFD. In Section 3 we will model the landscape using the CTEM code used in the study of the lunar highlands equilibrium in Richardson (2009), with added modifications introduced by Minton et al (2015), Huang et al (2017), and in this work. Because CTEM represents the full 2-D spatial distribution of craters on the landscape, as well as the 3-D morphology of each individual craters, we will use it to test the robustness of the assumptions inherent in the 1-D model analytical developed in Section 2.…”

“…It can also represent seismic shaking, which was modeled as a topographic diffusion process in Richardson (2009). The diffusive intensity function can also potentially have complex spatial heterogeneity if the high energy deposition of distal ejecta, such as secondary craters or ballistic sedimentation, are organized into ray-like features (Elliott et al, 2018;Huang et al, 2017;Oberbeck, 1975).…”

“…CTEM is a 3-D Monte Carlo landscape evolution code that models the topographic evolution of a surface that is subjected to impact cratering. It has previously been used to study the lunar highlands cratering record (Minton et al, 2015;Richardson, 2009) and impact transport of compositionally distinct surface materials (Huang et al, 2017). Because it is a three-dimensional code, it can readily model topographic diffusion.…”

“…To model the effect of crater rays, we apply our extra diffusive degradation in a ray pattern. We use ray geometry model modified from that described in Huang et al (2017). That work reported a polar function that defined the ray boundaries.…”