¹Lingzhi Sun,¹Paul G. Lucey
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006691]
¹Department of Earth Sciences, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA
Published by arrangement with John Wiley & Sons

Mineral abundance and Mg# (100× molar Mg/(Mg + Fe)) are significant in understanding the crustal composition and thermal history of the Moon. In this study, we derive a new set of optical constants for olivine, orthopyroxene, and clinopyroxene using radiative transfer equations that include soil porosity and the opposition effect. Based on the new optical constants, we develop a mineral abundance and Mg# unmixing model, and build a spectral library composed of mineral mixtures of plagioclase, olivine, low‐Ca pyroxene (LCP) and high‐Ca pyroxene (HCP), and Mg# ranging within 40–90. The accuracy of this model in estimating mineral abundance and chemistry is better than 3 vol% for olivine, LCP and HCP, better than 6 vol% for plagioclase, and better than 10 for Mg#. This model is validated using forward and inverse modeling. For the forward modeling, we reproduce the spectra of powdered pure minerals and Lunar Sample Characterization Consortium (LSCC) lunar soils, and the modeled spectra are consistent with those measured in the laboratory. For the inverse modeling, we determined mineral abundances and Mg# of 19 LSCC soil spectra by searching the best match to the spectral library. The modeled mineral abundances of LSCC soils are consistent with those measured by X‐ray digital imaging. We derived a global Mg# map using our model and Moon Mineralogy Mapper images, and our Mg# map shows a peak concentration at 70, consistent with that measured by the Lunar Prospector gamma‐ray spectrometer.

¹Gabriel L. Eggers,¹James J. Wray,²Josef Dufek
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2020JE006383]
¹School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
²Department of Earth Sciences, University of Oregon, Eugene, OR, USA
Published by arrangement with John Wiley & Sons

Decades of study of the igneous martian crust concluded that it was primarily basaltic, but a range of new investigations find evidence of evolved compositions. Foremost of these is a highly feldspathic unit within the Nili Patera caldera of Syrtis Major, the only detection with preserved volcanic context but which nonetheless remains ambiguous in exact composition and formation. We conduct compositional mapping of this feldspathic unit via near‐infrared spectroscopy from the Compact Reconnaissance Imaging Spectrometer for Mars instrument and find that the unit occupies at minimum 104 km2 at high confidence and an additional 41 km2 at moderately high confidence, meaning the unit is locally significant. We compare our mapping with that inferred from geomorphology and find that while texture and albedo are useful proxies, they are not perfectly reliable as substitutes for thorough compositional investigation. Study of the boundary between the feldspathic unit and surrounding mafic rock indicates the former formed early and may extend locally in the subsurface. We consider what compositional mixtures could explain the conflicting interpretations derived from visible/near‐infrared and thermal infrared spectroscopy, concluding it is likely due to thermophysical differences between the light‐toned feldspathic unit and the infilling dark mafic sand. We discuss proposed plutonic and volcanic formation scenarios for the feldspathic unit, considering Earth analogs and implications for the parent magmatic system, and offer observations in rock texture and composition that would clarify.