Telescopic observations of the Moon's hydration during lunar eclipse

Abstract

Hydration on the Moon, observed as a 3 um absorption feature in reflectance spectra, varies with surface temperature and illumination conditions. However, the physical and chemical mechanisms driving these variations remain uncertain. In this dissertation, I leverage the unique conditions of total lunar eclipses, which induce rapid surface cooling without changes in solar illumination, to isolate and investigate the temperature dependence of the 3 um hydration band. Using near-infrared spectra from NASA’s Infrared Telescope Facility's SpeX instrument, I observe a rapid strengthening of the 3 um band as the lunar surface cools during eclipse. To determine whether this effect results from observational artifacts or reflects real changes in hydration, I analyze spectra from the Chang’E 5 landing site and compare them to laboratory measurements of returned samples. The hydration band observed remotely is significantly deeper than what is measured in the samples, ruling out thermal infill,one specific thermal emission correction artifact, as the primary explanation for the observed band change. To test whether charged particle interactions influence hydration variability, I examine lunar swirls which are magnetic anomalies that deflect solar wind protons. Although these regions exhibit a persistent difference in hydration relative to their surroundings, the eclipse-induced change occurs similarly in both on swirls and off swirls, suggesting that solar wind flux is not responsible for the temperature-dependent behavior. I also investigate the subsolar point, the hottest location on the Moon, where the surface experiences the most rapid cooling during eclipse. The speed and magnitude of the observed band change here are inconsistent with mechanisms that require hydration to form or redistribute via ballistic transport or solar wind–derived hydrogen, as the required mass flux exceeds known sources by orders of magnitude. These findings challenge existing models of lunar hydration and indicate that the observed temperature-dependent behavior cannot be explained by current hypotheses. This work highlights the need for new frameworks to understand the dynamics of surface-bound water on the Moon and demonstrates the value of eclipse observations as a tool for studying volatile processes on airless bodies.