Processes governing the VIS/NIR spectral reflectance behavior of lunar swirls

¹ Image Analysis Group, TU Dortmund University, Otto-Hahn-Str. 4, 44227 Dortmund, Germany
e-mail: marcel.hess@tu-dortmund.de, christian.woehler@tu-dortmund.de
² Physical Research Laboratory, Ahmedabad 380009, India
³ Sternberg Astronomical Institute Universitetskij pr., 13, Moscow State University, 119234 Moscow, Russia
⁴ Kazan Federal University, Institute of Physics, Kazan 420008, Russia

Received: 11 December 2019
Accepted: 27 April 2020

Abstract

We investigated six bright swirls associated with magnetic anomalies of variable strength using Chandrayaan-1 Moon Mineralogy Mapper (M³) hyperspectral image data. We examined the 3 μm absorption band generally ascribed to solar wind-induced OH/H₂O and spectral trends in the near-infrared wavelength range at on-swirl and off-swirl locations. We found that the 3 μm absorption band is weaker at on-swirl than at off-swirl locations and shows only weak variations with time-of-day. This result is consistent with magnetic anomaly shielding that reduces solar wind interaction with the surface. For a small swirl structure in Mare Moscoviense, we found the 3 μm absorption band to be similar to that of its surroundings due to the absence of strong magnetic shielding. Our spectral analysis results at on-swirl and off-swirl locations suggest that the spectral trends at on-swirl and off-swirl locations cannot always be explained by reduced space-weathering alone. We propose that a combination of soil compaction possibly resulting from the interaction between the surface and cometary gas and subsequent magnetic shielding is able to explain all observed on-swirl vs. off-swirl spectral trends including the absorption band depth near 3 μm. Our results suggest that an external mechanism of interaction between a comet and the uppermost regolith layer might play a significant role in lunar swirl formation.