An Investigation into the Effects of the Magnetic Anomaly on Regional Space Weathering at Mare Ingenii and Its Influence on the Spectra of the Basalts and Lunar Swirls

AT MARE INGENII AND ITS INFLUENCE ON THE SPECTRA OF THE BASALTS AND LUNAR SWIRLS G. Kramer1†, J.-P. Combe, T. McCord, E. Harnett, B. R. Hawke, D. Blewett Bear Fight Center, Winthrop, WA, University of Washington, Seattle, WA, Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, 4 Applied Physics Laboratory, John Hopkins University, Laurel, MD †[email protected] Introduction Mare Ingenii (Fig. 1a) is one of several isolated basalt ”ponds” in South Pole-Aitken (SPA) basin. Ingenii’s most striking features are its collection of lunar swirls, which spectrally dominate the southwestern portion of the mare. Lunar swirls are high albedo and optically immature curvilinear surface features that are found in discrete locations across the Moon’s surface [1, 2, 3]. Earthbased telescopic and Clementine orbital multispectral data show spectral characteristics suggesting the swirls are optically immature. In addition, each swirl is coincident with a region of remnant magnetism on a planetary body that does not, and may never have had, an active core dynamo with which to generate its own magnetic field. These characteristics led to two models to explain the lunar swirls: (1) The swirls are only apparently fresh. The swirls’ optical characteristics reflects regions where crustal magnetic fields have been selectively preserving silicate surfaces over time from the effects of space weathering by solar wind ions [2]. Several lunar magnetic anomalies are antipodal to large impact basins formed between 3.8-3.9 Ga [4]. Magnetization of these antipodal regions could result from a plasma cloud generated by the basin forming impact interacting with a weak magnetic field present at the Moon at the time of basin formation [5]. The magnetized surface could be also due to impact-induced currents and seismic waves ringing the planet [6]. An unusually thick and/or strongly magnetized deposit of basin ejecta can be invoked to explain swirls, such as the Reiner Gamma Formation, that are not antipodal to a basin. (2) The swirls are actually fresh. The swirls depict a relatively recent impact of disrupted meteoroid debris [7] or unconsolidated comet material [3, 8], which scoured the surface and exposed fresh material. The sinuous form of the swirls is the remnant manifestation of the coma’s turbulent flow of gas and dust across the lunar surface. The comet impact could create a magnetic anomaly from magnetization of near-surface materials heated above the Curie temperature through hyper-velocity gas collisions and micro-impacts.