Quantitative Mapping of Hydration in Lunar Pyroclastic Deposits: Insights into Water from the Lunar Interior

Introduction: Pyroclastic deposits on the Moon represent early lunar volcanic processes [1], and models of explosive volcanism indicate that the range of pyroclastic deposits is proportional to volatile content [2]. Therefore, the distribution of lunar pyroclastic deposits and their estimated water content can provide indirect information on the distribution of volatiles in magma source regions [3]. In return, knowledge of interior volatile content, any spatial heterogeneity, and how it may vary with depth or through time can improve our understanding of lunar magma ocean models and provide constraints for the loss or retention of volatiles during the Moon formation event. Moreover, these pyroclastic deposits are potential sources for water and other in situ resources for future exploration [4]. Here we discuss the mapping of ‘water’ (H2O/OH) content of lunar pyroclastic deposits using Moon Mineralogy Mapper (M) visible-near infrared surface reflectance spectra. Our previous quantitative mapping results of global hydration with M data showed that surface hydration is dominantly controlled by latitude and local surface temperature [5]. There was almost no detectable hydration between 30° N and 30° S latitude on the lunar surface, which suggests any hydration bonds (i.e., OH) formed superficially by solar wind implantation are likely broken by high surface temperatures during local lunar daytime. This indicates hydration formed by solar wind implantation is not stable over long periods at these latitudes and will likely not accumulate to significant degrees for typical surface materials. In contrast, pyroclastic deposits in these latitudes exhibit increased hydration signals even at local noon [5]. In this study we focus on pyroclastic deposits between 30° N and 30° S and, to avoid ambiguity in identifying small pyroclastic deposits (e.g., <1000 km), analyze those classified as ‘large’ deposits by [6]. We analyze the relationship between water content and other characteristics including mineralogical, chemical, spectral, topographic, and morphological characteristics for eleven large pyroclastic deposits in this study region.