Re-evaluation of Lunar X-ray observations by Apollo 15 and 16^★

¹ ESA/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: gloudemans@strw.leidenuniv.nl
⁴ INAF/OAS, Via Gobetti 101, 40129 Bologna, Italy
⁵ ESA/ESAC, Camino Bajo del Castillo, 28692 Villafranca del Castillo, Madrid, Spain
⁶ Department of Nuclear and Particle Physics, University of Geneva, 24 quai Ernest-Ansermet, 1205 Geneva, Switzerland
⁷ Bordeaux INP, Avenue des facultés, 33405 Talence, France

Received: 12 January 2021
Accepted: 16 April 2021

Abstract

The Apollo 15 and 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 and 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 ± 0.07 and 0.54 ± 0.17, respectively; for the highland regions, the values are 0.76 ± 0.07 and 1.07 ± 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 ± 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.