Understanding Lunar Regolith Properties Through Thermal Modelling and Measurements

Understanding Lunar Regolith Properties Through Thermal Modelling and Measurements. C. Schuckart1, B. Aussel1, J. N. Cybulski1, T. Rückriemen-Bez1, J. Bürger2, and B. Gundlach1, 1Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany, 2Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany. Introduction: With a return to manned lunar missions on the horizon, a new wave of landers and the vision to establish long-term human presence on the moon, understanding the properties of lunar regolith has become more important than ever. A particular focus has been the investigation of water ice deposits on the lunar poles. Since the ice stability is directly linked to the temperature, the thermal and mechanical properties, hereafter referred to as material properties, of the lunar regolith must be well understood. Despite both in-situ measurements during the Apollo program and laboratory measurements from returned samples, questions and discrepancies regarding these material properties remain to be solved. We want to present a brief overview highlighting the new studies and developments carried out in Münster and Braunschweig to determine the material properties of lunar regolith. Thermophysical modelling: Instruments like the Diviner Lunar Radiometer Experiment (Diviner) on board of the Lunar Reconnaissance Orbiter (LRO) provide detailed brightness temperature maps across the lunar surface. Especially the nighttime temperatures are dependent on material properties like thermal conductivity and bulk density [1]. Thus, thermophysical models (TPMs) can be used to model the Diviner temperatures and interpret thermal measurements to derive material properties. 1DMTM: Bürger et al. [2] have developed a onedimensional microphysical thermal model (1DMTM) to investigate Diviner nighttime temperatures across different lunar latitudes. They have used microphysical descriptions for the thermal conductivity and stratification of the lunar regolith, by modelling them as a function of grain size and volume filling factor. The stratification model is based on [5] and lithostatic compression governs the increase of the volume filling factor as a function of depth. 1DMTM is limited by the assumption of a single grain size and a larger number of free parameters. In this study, the authors found that an incidence angle dependent albedo is needed to achieve good agreement between the model results and the measured temperatures, which agrees with previous studies [1,3,4]. Further they found that considering only the Diviner dataset results in nonunique solutions for the best-fitting material properties. Thus, a secondary dataset would be needed to discern these overlaps. MoCSI TPM: The Modular Computational Simulation Interface Thermophysical Model (MoCSI TPM) is currently being developed to allow investigation of material properties on a larger scale. It allows for calculations on shape models and provides a simple modular interface to investigate different physical descriptions of material properties. The interface allows for easy additions of additional parameters and complete control via input files. The MoCSI TPM framework will also be used to investigate lunar regolith properties on a larger scale. At the current stage, the MoCSI TPM can be used to simulate asteroids. However, for lunar applications the gravitational compaction of the uppermost meter must be considered [5]. We plan to conduct laboratory experiments, which are detailed in the next section, to improve the description of compaction behaviour of lunar regolith in MoCSI. In the current stage, benchmark tests have shown a very good agreement between the MoCSI TPM and analytical solutions. Laboratory Measurements: Previous works have shown that the grain size has a crucial influence on the compaction behaviour of granular materials [5] and hence on the thermal behaviour and feedback of the surface [2]. However, no quantitative analysis on this grain size effect has been conducted up until now. To improve our understanding of grain size on compaction we will compress samples with varying grain size distributions using a piston in combination with a force sensor to measure the compression curve of the granular matter. The experiments will be done in air and in vacuum and the results will then be used for the interpretation of Diviner data with the help of the mentioned thermal models. First results of these experiments will be presneted during the conference. References: [1] Vasavada A. R. (2012) JGR, 117, E00H18. [2] Bürger J. et al. (2024) JGR Planets, 129, e2023JE008152. [3] Feng J. et al. (2020) JGR Planets, 125, e2019JE006130. [4] Hayne P. O. et al. (2017) JGR Planets, 122, 2371–2400. [5] Schräpler R. et al. (2015) Icarus, 257, p. 33-46.