Kilometer-Scale Regions Of Primordial Flotation Crust May Be Accessible In Hertzsprung Basin
Small regions of nearly pure anorthosite are observed spectroscopically in outcrops on peak rings in several basins including Hertzsprung, Humorum, Nectaris and Orientale. These outcrops provide access to the primordial lunar crust thus enabling its characterization. A full analysis of the pristine anorthosite present at or near the surface of these basins is critical to the understanding of the magma ocean and the subsequent evolution of the Moon. Hertzsprung is a well-preserved impact basin on the Moon’s farside at 2°N, 128°W with an outer rim diameter of 570 km. The impact that created the basin excavated material from as deep as 40 km below the surface with most ejecta sourced from shallower than about 25 km. Previous work examining Clementine spectral data has shown that the ejecta from Hertzsprung is unusually feldspathic  and that the inner ring massifs contain almost completely iron free regions  as seen in the left panel of Figure 1. These Fe-free regions were interpreted as outcrops of nearly pure anorthosite. Lunar Prospector thermal neutron data are sensitive to the bulk concentration of neutron-absorbing elements including Fe, Ti, and the rare earth elements (REEs) Gd and Sm. Fe- and REE-poor materials are indicative of cumulate floatation during the cooling of a global magma ocean. Based on these data, Peplowski at al.  identified large regions of low neutron absorption whose properties are consistent with presence of high (>85 wt %) concentrations of plagioclase. However, the resolution of and noise in the LPNS data limit the error estimate on these inferences. We have applied a pixon image reconstruction technique  to improve the utility of the data by suppressing noise and improving its spatial resolution. This reconstruction confirms the presence of regions of plagioclase rich material of 10s of km diameter, including at Hertzsprung (Figure 1). The dynamic range of neutron counts is increased implying a greater concentration of plagioclase than previously inferred. Such high abundances imply that the anothositic material cannot be buried under more than a few cm of regolith and would be accessible to a lander or rover with basic excavating capabilities such as a scoop or dust removal tool. If landing can be achieved somewhere within the large plagioclase rich region, then mobility may not be necessary. Given the advances in in-situ composition and age-dating measurements, sample return might not be necessary to obtain key understanding of this region. However, samples returned from this location would nevertheless result in an invaluable resource for the lunar community. References:  Spudis, et al., (1996) LPSC XXVII.  Stockstill and Spudis, (1998) LPSC XXIX.  Peplowski, et al. (2016), JGR (Planets), 121, 388–401.  Eke, V. (2001), Monthly Notices of the Royal Astronomical Society, 324, 108-118. Figure 1: Clementine FeO, LOLA 1064 nm normal albedo and a pixon reconstruction of LP epithermal neutron data at Hertzsprung basin. The outer rim and peak ring are shown with dashed black lines.