Exploring the Impact of Planetary Regolith Properties on X-ray Fluorescence: An Experimental Study

Among the payload of instruments on board the BepiColombo mission is MIXS – the Mercury Imaging X-ray Spectrometer. MIXS is designed to map the global elemental abundance of Mercury’s surface using X-ray fluorescence (XRF) measurements, revealing the origin and evolution of Mercury’s surface in unprecedented detail. However, it is understood that quantification of these observations will be affected by the roughness of the surface terrain, resulting in a decrease in the detected XRF intensity over theoretical predictions as shown across multiple studies. If unaccounted for, these ‘regolith effects’ could lead to a mischaracterisation of the surface elemental abundances due to the energy-dependent nature of these effects. This thesis explores the extensive experimental work undertaken to uncover the magnitude of these effects in preparation for the BepiColombo science results. A suite of granular samples with a wide range of compositions and grain size fractions have been created and characterised. The physical properties of these samples have been measured using a variety of instruments to characterise key properties such as composition, grain size distribution, and surface roughness. These results then go on to inform the data analysis of an experiment campaign to measure the XRF signal intensity of the samples across many viewing geometries. The XRF results are interpreted via a newly proposed correction to the fundamental parameters equation. The end results of the studies here provide a new correction methodology to account for the surface roughness of granular material in XRF observations. An empirically derived correction factor has been found to be consistent across a wide range of sample compositions and viewing geometries. Future planetary missions employing XRF can build upon the theoretical framework and physical interpretation presented here to develop a comprehensive and universally applicable correction technique that will lead to more accurate elemental abundance mapping of planetary bodies using XRF.