The Swirls at SPA

We used Moon Mineralogy Mapper (M3) data to explore the lunar swirls in SPA. We characterized the major spectral differences of the surface regolith on the swirls (on-swirl) and adjacent regions (off-swirl). Spectra from small, fresh craters are used to approximate the mineral compositions of the sub-regolith bedrock units and demonstrate maturation trends. Results show that the on-swirl surface albedos correlate with strong mafic absorptions for both mare and highland swirls. Albedo and absorption bands are well preserved on-swirl; off-swirl absorption bands quickly flatten, albedos are low and exhibit a narrow range. In mare regions, the freshest off-swirl craters are consistently darker, but show the same spectral shapes as all fresh on-swirl craters. This suggests the on-swirl regolith may be compositionally different from the off- and inter-swirl regolith, but the differences do not derive from the bedrock lithology. One of the most striking discoveries is the variation of the 2.82 µm band depth between on- and off-swirl locations. The absorption feature at this wavelength is attributed to hydroxyl (OH), and the strength of the band is a function of OH abundance. Regardless of whether a swirl is on mare or highland, the 2.82 µm absorption is consistently weaker on-swirl than off-swirl, which indicates the swirls are depleted in OH relative to their surroundings. Interpretation: M3 results reflect a spatial control on nanophase iron (npFe0) production and support the solar wind deflection model. The spectral characteristics of space weathering are dominated by npFe0, which increases with increased exposure to solar wind ions and micrometeoroids. The deflection of protons by the magnetic anomalies slows the production of npFe0; so only micrometeoroid impacts to operate on-swirl, thus making on-swirl surfaces appear less mature. Off-swirl maturation is accelerated because these areas experience normal space weathering and receive an increased flux of protons deflected from on-swirl surfaces.