Reconciling Differences in Global Iron Estimates Using Gamma-ray/Neutron and Reflectance Spectroscopy

Iron abundance is a key petrologic discriminator for interpretation of airless body surfaces from both remote and field perspectives. Independent measurements of iron abundance from gamma-ray/neutron and reflectance spectroscopy are available for the Moon. However, there are significant discrepancies between these two data products for many locations (e.g., South Pole-Atken, Tycho, etc.) [Lawrence et al., 2002]. In some cases, these discrepancies are large enough to suggest different plausible conclusions regarding important lunar science problems. For instance, the divergence in estimated iron may be attributed to differences in mineralogy and/or other physical properties of surface materials, as each technique is sensitive to different aspects of these parameters and to differing depths. A first step in examining these divergent areas is to understand if the discrepancies are due to lateral spatial artifacts from data that have different spatial footprints (spectral data for Clementine have pixel resolutions up to 100 m and gamma-ray data have footprint sizes >50 km). In previous work, our analyses of data smoothing techniques allowed us to conclude that the majority of spatial discrepancies are not artifacts [Hagerty et al., 2010]. Instead, the discrepant iron abundances result from either inherent differences in how each technique estimates composition or lunar surface properties. In this work, we seek to determine if complexities affiliated with the spectral data could be a primary source of discrepancy. Several spectral reflectance algorithms have been created to estimate iron abundance, each trying to improve upon small deficiencies in previous algorithms due to regional geologic terrain, surface slopes, mineralogy, and maturity [Fischer and Pieters, 1995; Lucey et al., 1995; Gillis et al., 2000; Le Mouélic et al., 2000; Lucey et al., 2000; Lawrence et al., 2002; Wilcox et al., 2005]. Here we look at several lunar locations where discrepancies exist and examine their plausible causes and whether can be resolved.