A3 - Ancient bombardment of the inner solar system – Reinvestigation of the “fingerprints” of different impactor populations

Research areas: Geology (planetary geology), remote sensing

Summary

The lunar cratering record provides valuable information about the late accretion history of the inner solar system. However, our understanding of the origin, rate, and timing of the impacting projectiles is far from complete. To learn more about these projectiles, we can examine the crater size–frequency distributions (CSFDs) on the Moon. For example, by comparing the shape of lunar CSFDs with the size–frequency distributions of potential projectile families like different groups of asteroids, comets, or even ejecta projectiles from the giant impact that formed the Moon we can potentially identify the origin and composition of the impacting projectiles in the inner solar system. In addition to helping identify these projectiles, the shapes of lunar CSFDs are particularly important for determining crater-based relative and absolute ages of surfaces, because CSFDs are used to define the so-called lunar “production function” (PF) from which these ages are calculated. Also, if any changes to the PF are observed over time, this indicates that more than one impactor population may have formed the lunar cratering record.

The potential existence of more than one impactor population has often been used to support a late heavy bombardment (LHB) around 3.9-4.1 Ga that would have significantly influenced the surface development of the terrestrial planets and the distribution of water and other volatiles. However, there is an ongoing debate about whether the shape of the PF has changed at all, and, if it has, at what time it transitioned.

Still, others question the interpretations of different CSFD shapes in general, because geologic processes can subsequently modify existing CSFDs. Thus, to address the questions of whether the PF has changed with time and when the potential transition occurred to produce differently shaped CSFDs, we propose to reinvestigate the key regions previously used for interpreting CSFDs and to improve upon past methods and techniques that were applied for analysing crater data.

Improvements in this area are crucial because currently, crater-based ages are determined using a constant PF; however, if the PF changed with time, current ages might be inaccurate. Thus, to address these issues, we propose to perform detailed geologic mapping and small-scale CSF measurements on high-resolution imagery and topography data from recent lunar missions to validate the suitability of the regions previously used to analyse CSFDs. Furthermore, using new approaches of GIS spatial analyses in combination with high-resolution digital terrain models, we will be able to test the statistical significance of previous CSFD measurements and to investigate more closely the processes modifying the original CSFDs. Overall, this subproject will help identify new constraints on the potential projectile families and their compositions (relevant for subprojects in areas B and C) and produce new constraints on a time-variable PF, which will improve the method for determining crater-based ages (relevant for subprojects A1, 2 and 4).