Lunar swirls: Far-UV characteristics

Abstract

Lunar swirls – the enigmatic, magnetically-anomalous regions – are observed for the first time at far-UV (FUV) wavelengths using LRO/LAMP. Swirls in both highlands and mare regions are spectrally relatively red (or less blue) than surrounding terrains, indicating a difference in weathering and/or composition in the swirls vs. non-swirl regions.We compare spectra of the highlands swirl Gerasimovich with mature and immature low-Fe highlands regions as measured by LAMP and show that the swirl itself does not have the spectral characteristics of either the mature or the immature regions. Mature, weathered highlands regions are spectrally blue in the FUV; immature highlands are less blue, especially at wavelengths >∼160nm. In contrast, the Gerasimovich swirl is spectrally red at wavelengths >∼160nm. We also compare Reiner Gamma, a mare swirl, with mature and immature high-Ti mare regions as measured by LAMP. We find that the mature and immature high-Ti mare regions are spectrally indistinguishable while the Reiner Gamma spectra are less blue at wavelengths >∼160nm. We conclude that both swirls (Reiner Gamma and Gerasimovich) are consistent with less mature spectra than the immature terrains studied here, in accordance with the lower amounts of weathering expected in a solar wind standoff scenario. However, the swirl spectra are also consistent with greater abundances of feldspathic material, as we show that anorthite exhibits a characteristic red spectrum at wavelengths >∼160nm. Thus, the LAMP data are also consistent with a model wherein compositional sorting occurs at swirls.