Modelling the Radiative Environment of the Lunar South Pole Aitken basin.

A unique virtual model of the interactions between galactic cosmic rays (GCRs) and the regolith in the Von Kármán crater in the South Pole Aitken basin (SPA basin) has been constructed using the BDSIM particle simulation environment, built on the Geant4 physics code. This project is the first use of BDSIM to model an extraterrestrial environment.

The model consists of a new synthetic composition for the regolith surrounding the Chang’e-4 probe, a series of GCR spectra and a simulation architecture. This measures the neutron flux, proton albedo and cosmogenic samarium and gadolinium populations within a 2-metre depth homogenous tranche of simulated lunar material. It was configured partly to produce secondary neutron production rates to be used as inputs to the LUCRES regolith gardening simulator.

Fig 1: Flowchart of the project, with references to internal sections.

The input composition is a synthesis of Kaguya orbiter, Yutu-2 and Chang’e-6 sample return data taken synthesised from various sources into an approximation of the elemental weight percent of the regolith in the Von Kármán crater, with major, minor and trace elements modelled. These data were analysed to give a mineral distribution of olivine, clinopyroxene, orthopyroxene and feldspar and then processes with numerical tools from Bütner and Putirka to produce elemental weight percentages for major element components. The minor element, Titanium, was sourced from Yutu-2 observations and converted into elemental weight percent using the previously described methods. The trace element distributions were taken from Chang’e-6 data and selected for this purpose by matching the bulk composition of Chang’e-6 regolith samples to that of the synthetic Von Kármán samples described previously.

The simulation input GCR spectrum that was fired at the lunar material was a recreation of the spectrum described by Li et al. While some of the authors for LUCRES provided a detailed GCR spectrum (mainly protons and alpha particles), their data and physics lists were difficult to integrate into BDSIM. Therefore, the project used the GCR model from Li et al. (2017), which was already compatible with Geant4. Though LUCRES collaborators noted that different solar modulation factors (a parametrisation of the sun’s effect on interstellar galactic cosmic rays) affect neutron flux, this study found that higher-energy GCRs (which are less affected by modulation) are primarily responsible for neutron production. In contrast, lower-energy GCRs (more affected by modulation) significantly influence proton albedo, although this was less apparent in the results of this project. As a result, different modulation factors were tested in simulations to assess their impact on albedo. The GCR spectrum was modelled in BDSIM as an external source of particles and generated a Monte-Carlo sampled particle distributions according to the chosen modulation factors.

Proton albedo was measured within the simulation with a scoring mesh at the surface that recorded retrograde movement of protons. Two different solar modulation factors (ϕ) were tested and showed that there is limited difference between the albedo distributions for different values of ϕ at this significance level. The outputs of this simulation are shown in fig 2 with proton albedo data from the Chang’e-4 rover (LND) and the Lunar Reconnaissance Orbiter (CRaTER). These data fall directly onto the curve of the proton albedo produced in this simulation, and the REDMoon simulation used by Xu et al (2022) falls within the bounds of the BDSIM simulation. The left side of the REDMoon simulation (between 60 and 10 MeV) diverges from the BDSIM simulation as the REDMoon simulation uses a different GCR input (CREME96) that has a solar particle contamination at the lower energies, as shown in the blue band in the figure. This contamination is not present in the BDSIM input and has been removed from the current version of the CRÈME GCR simulation.

Fig 2: Proton albedo from BDSIM shown with that of Xu et al (using REDMoon).

Neutron flux was simulated as a function of depth within the regolith column. Total neutron populations were measured, but the thermal neutron flux that causes cosmogenic Sm and Gd production was too low to be statistically significant. While cosmogenic Sm and Gd were produced in this simulation, future runs will require larger numbers of incident particles to produce significant thermal neutron induced Sm and Gd populations, which can be used in the gardening simulation LUCRES. Analysis of the meteorite NWA 2995 in conjunction with the BDSIM total neutron flux distribution at depth shows that the thermal neutron flux peak may be deeper into the regolith than the total neutron flux peak.

Fig 3: Neutron flux in regolith tranche with sample NWA 2995 shown at predicted depth and the point on curve (pink triangle). This demonstrates the difference in depth of peak all neutron and thermal neutron flux.

This project suggests an inquiry to settle the dispute between the BDSIM and REDMoon proton albedo predictions at the lower energy ranges may be of value to make sure use of different GCR models are appropriate in future predictions. The architecture of BDSIM can be used to produce static profiles of future drill cores that can be collected in the planned future exploration of the SPA basin, primarily by the Chinese space program. No drill cores have been collected from the Moon in the last half a century, and simulations like this (and the challenges with their configuration) show how crucial this kind of sample can be to the understanding of space weathering processes like gardening and GCR interactions. The SPA basin is extremely understudied compared to other lunar terranes, and the authors strongly suggest this region as an ideal candidate for human exploration, especially in the context of drill core return, as such samples have only ever been collected by humans.

This project therefore recommends the deployment of a segmented silicon solid state proton dosimetry device at the surface of the Moon to measure albedo within the 10-60 MeV range and the return of a regolith drill core for the analysis for cosmogenic samarium and gadolinium populations with comparative analysis with other material thought to be from the SPA region.