Re-evaluation of Lunar X-ray observations by Apollo 15 & 16

TL;DR

This study re-evaluates Apollo 15 & 16 lunar X-ray fluorescence data to produce updated elemental maps, revealing clearer distinctions between mare and highland regions and providing refined concentration ratios.

Contribution

It introduces improved data processing methods, including updated instrument response and trajectory reconstruction, leading to more accurate lunar surface elemental maps and ratios.

Findings

Clear distinction between mare and highland regions in Al/Si and Mg/Al maps.

New mean Al/Si and Mg/Al ratios for lunar regions with lower values than previous studies.

Refined concentration ratios accounting for solar activity and inter-orbit variability.

Abstract

The Apollo 15 & 16 missions were the first to explore the Lunar surface chemistry by investigating about 10% of the Lunar surface using a remote sensing X-ray fluorescence spectrometer experiment. The data obtained have been extensively used to study Lunar formation history and geological evolution. In this work, a re-evaluation of the Apollo 15 & 16 X-ray fluorescence experiment is conducted with the aim of obtaining up-to-date empirical values for aluminum (Al) and magnesium (Mg) concentrations relative to silicon (Si) of the upper Lunar surface. An updated instrument response, a newly reconstructed Lunar trajectory orbit, and improved intensity ratio calculations were used to obtain new intensity ratio maps. The resulting Lunar Al/Si and Mg/Al X-ray maps show a clear distinction in Lunar mare and highland regions. The mean Al/Si and Mg/Al intensity ratios for the mare regions obtained from the newly obtained maps are 0.54$\pm$0.07 and 0.54$\pm$0.17, respectively; for the highland regions, the values are 0.76$\pm$0.07 and 1.07$\pm$0.13, respectively. For the Mg/Si intensity ratio, no clear distinction between Lunar features is obtained and we derived a mean value of 0.47$\pm$0.13. Our determined intensity ratios are lower than previously published. These values can be used to infer concentration ratios when accounting for Solar activity, inter-orbit variability, and measurements from different instruments. We employed a correction to infer concentration ratios by comparing our intensity ratios directly to Lunar rock concentrations obtained from various Lunar missions.