D. Dhingra

M3 spectral analysis of lunar swirls and the link between optical maturation and surface hydroxyl formation at magnetic anomalies

[1] We examined the lunar swirls using data from the Moon Mineralogy Mapper (M 3 ). The improved spectral and spatial resolution of M 3 over previous spectral imaging data facilitates distinction of subtle spectral differences, and provides new information about the nature of these enigmatic features. We characterized spectral features of the swirls, interswirl regions (dark lanes), and surrounding terrain for each of three focus regions: Reiner Gamma, Gerasimovich, and Mare Ingenii. We used Principle Component Analysis to identify spectrally distinct surfaces at each focus region, and characterize the spectral features that distinguish them. We compared spectra from small, recent impact craters with the mature soils into which they penetrated to examine differences in maturation trends on‐ and off‐swirl. Fresh, on‐swirl crater spectra are higher albedo, exhibit a wider range in albedos and have well‐preserved mafic absorption features compared with fresh off‐swirl craters. Albedoand mafic absorptions are still evident in undisturbed, on‐swirl surface soils, suggesting the maturation process is retarded. The spectral continuum is more concave compared with off‐swirl spectra; a result of the limited spectral reddening being mostly constrained to wavelengths less than ∼1500 nm. Off‐swirl spectra show very little reddening or change in continuum shape across the entire M 3 spectral range. Off‐swirl spectra are dark, have attenuated absorption features, and the narrow range in off‐swirl albedos suggests off‐swirl regions mature rapidly. Spectral parameter maps depicting the relative OH surface abundance for each of our three swirl focus regions were created using the depth of the hydroxyl absorption feature at 2.82 mm. For each of the studied regions, the 2.82 mm absorption feature is significantly weaker on‐swirl than off‐swirl, indicating the swirls are depleted in OH relative to their surroundings. The spectral characteristics of the swirls and adjacent terrains from all three focus regions support the hypothesis that the magnetic anomalies deflect solar wind ions away from the swirls and onto off‐swirl surfaces. Nanophase iron (npFe 0 ) is largely responsible for the spectral characteristics we attribute to space weathering and maturation, and is created by vaporization/deposition by micrometeorite impacts and sputtering/reduction by solar wind ions. On the swirls, the decreased proton flux slows the spectral effects of space weathering (relative to nonswirl regions) by limiting the npFe 0 production mechanism almost exclusively to micrometeoroid impact vaporization/deposition. Immediately adjacent to the swirls, maturation is accelerated by the increased flux of protons deflected from the swirls. Citation: Kramer, G. Y., et al. (2011), M 3 spectral analysis of lunar swirls and the link between optical maturation and surface

The distribution of Mg-spinel across the Moon and constraints on crustal origin

Abstract A robust assessment is made of the distribution and (spatially resolved) geologic context for the newly identified rock type on the Moon, a Mg-spinel-bearing anorthosite (pink-spinel anorthosite, PSA). Essential criteria for confirmed detection of Mg-spinel using spectroscopic techniques are presented and these criteria are applied to recent data from the Moon Mineralogy Mapper. Altogether, 23 regions containing confirmed exposures of the new Mg-spinel rock type are identified. All exposures are in highly feldspathic terrain and are small-a few hundred meters-but distinct and verifiable, most resulting from multiple measurements. Each confirmed detection is classified according to geologic context along with other lithologies identified in the same locale. Confirmed locations include areas along the inner rings of four mascon basins, knobs within central peaks of a few craters, and dispersed exposures within the terraced walls of several large craters. Unexpected detections of Mg-spinel are also found at a few areas of hypothesized non-mare volcanism. The small Mg-spinel exposures are shown to be global in distribution, but generally associated with areas of thin crust. Confirmation of Mg-spinel exposures as part of the inner ring of four mascon basins indicates this PSA rock type is principally of lower crust origin and predates the basin-forming era.