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

Abstract

[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 hydroxyl formation at magnetic anomalies,

Formation of lunar swirls by magnetic field standoff of the solar wind

“…The Diviner data, therefore, suggest that a layer of fine particulate feldspar-enriched material is not responsible for the lunar swirl albedo anomalies and spectral properties. This interpretation supports visible and near-infrared measurements from the M 3 instrument 15 .…”

“…On the other hand, the solar wind standoff mechanism for swirl formation would lead to a slight shift of the CF to shorter wavelengths compared with the surrounding regolith due to a reduction in space weathering, but show no difference in the unique lunar regolith thermophysical properties because impact gardening is otherwise unaffected. These properties are exactly what are observed, and the Diviner observations, in addition to M 3 and Mini-RF 14,15,40 observations and laboratory experiments 42 , strongly support the solar wind standoff hypothesis for lunar swirl formation. Because they are clearly associated with magnetic field anomalies, lunar swirls present an opportunity to study and isolate the effects of solar wind sputtering/implantation and micrometeoroid bombardment processes on optical maturity.…”

“…If this is the case, micrometeoroid bombardment should be the major relevant space-weathering process 14 at swirl sites. A relative lack of solar wind interaction with the lunar surface at swirl sites is supported by recent observations from the Moon Mineralogy Mapper (M 3 ) instrument that confirm the Clementine observations of a substantial reduction in optical maturity and also show a reduction of the presence of surface hydroxyls at the swirl sites 15 . Recent observations in the ultraviolet by the Lunar Reconnaissance Orbiter Camera Wide Angle Camera subsystem also suggest that micrometeoroid bombardment, and not solar wind interaction, plays the dominant role in space weathering at the lunar swirls 16 .…”

Solar wind implantation into lunar regolith: Hydrogen retention in a surface with defects

“…Water transport from polar craters to mid-latitudes (liberated by impact vaporization and sputtering) was found to be plausible but could not account for the relatively large hydroxylation levels observed in the IR (Farrell et al, 2013). Kramer et al (2011) found that the local IR OH absorption feature was reduced (less OH content) in swirl regions associated with magnetic anomalies. The finding suggests that hydroxylation is locally reduced where the B-field also blocks solar wind propagation onto the surface (Kramer et al, 2011;Poppe et al, 2012).…”

Global distribution of lunar impact melt flows

“…The radar brightness indicates increased surface and/or near subsurface roughness close to the scale of the radar wavelength (in the case of Mini-RF, centimeter The radar image is displayed as colorized circular polarization ratio over total radar backscatter. An arrow indicates the edge of a newly discovered impact melt flow (Neish et al, 2011;Kramer et al, 2011;Carter et al, 2012). In this figure, north is up.…”