Radiative transfer model constraints on the shock state of remotely sensed lunar anorthosites

Abstract

Near-IR spectra of some lunar basin rings and crater central peaks exhibit no evidence of absorption due to ferrous iron, unlike all other spectral measurements of lunar soil in the laboratory or obtained remotely. These spectral anomalies have previously been interpreted to indicate the presence of a deposit of essentially pure plagioclase feldspar (anorthosite) which has been shocked above 250 kb. Plagioclase possesses a diagnostic absorption feature near 1250 nm caused by trace ferrous iron that is lost when shocked above 250 kb, whereas ferrous features of pyroxene are robust against shock. However, new methods for computing the optical effects of exposure of lunar materials to the space environment show that these optical effects can obscure the spectral features of plagioclase without resorting to shock modification of the plagioclase structure. Our modeling shows that shock is not necessary to explain the apparent lack of absorption, but is not excluded either. Thus, previous reports that some lunar surface features may have experienced high shock levels are not supported.