Correlating Swirls with Particle Tracking Simulations at Lunar Magnetic Anomalies in South Pole-aitken Basin and Mare Crisium

Introduction: Lunar swirls are high albedo curvilinear features that do not follow local topography, and appear optically immature compared to their surroundings. They are located at the lunar magnetic anomalies, however not all anomalies have identified swirls [1] [2]. One of the theories that explain the origin of the swirls suggests that swirls are areas of retarded surface weathering due to shielding from the solar wind ions by the anomalous magnetic fields [1] [3]. The area for the study presented here covered the South Pole-Aitken basin (SPA) and Mare Crisium. Swirls were mapped using several multispectral datasets. The maps were correlated with particle tracking simulations to test the solar wind deflection hypothesis on the SPA anomaly. For Crisium the particle tracking simulations were used to constrain the best locations where swirls might occur and explore possible reasons for the lack of apparent swirls in the Mare Crisium magnetic anomaly. Space weathering: The Moon is constantly bombarded by solar wind particles and micrometeorites due to the lack of an atmosphere and a global magnetic field. The process that causes physical and chemical changes in the regolith due to these influences is termed space weathering. Formation of nanophase iron (npFe) is one of the changes that results from space weathering. NpFe is responsible for the changes in the optical properties of the regolith that has been exposed to space weathering, which includes decrease in overall reflectance, increase in spectral slope and reduced absorption band depths [4]. Magnetic anomalies and solar wind deflection model: The crustal magnetic anomalies on the Moon were first detected by Apollo 15 and 16. Observations have shown that solar wind particles could be deflected by these magnetic anomalies [5]. The efficiency of the magnetic field to deflect the solar wind depends on its strength and coherence. The coherence of the field defines how fast the magnetic field directions change over distance. A more dipole-like field would change less often with distance, and would be stronger at deflecting incoming solar wind [5]. Methods: The datasets used in the study were the Lunar Reconnaissance Orbiter Wide Angle Camera (LRO WAC) 643 nm normalized (no shadows) reflectance map, slope map derived from the LRO Global Lunar digital terrain model (WAC_GLD100) [6], and a false color band ratio map derived from LRO WAC reflectance map (415 nm was displayed as red, 321/415 nm ratio as green, and 260/415 nm as blue) [7]. The slope map was inverted (so high slopes appear bright and flat regions dark) and overlain on the normalized reflectance WAC map with 30% transparent slope map. This muted high reflectance due to topography in order to distinguish from high reflectance due to swirls. The false color map displayed the swirls in bright magenta/red with a good contrast from the surroundings. Optical maturity (OMAT), were derived from Kaguya Multiband Imager (MI) mosaics [8], to improve the identification of the swirls. After testing on the SPA region, the same datasets were used to search for swirls in Mare Crisium anomaly in combination with the particle tracking simulation results.