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[23.03] The Origin of Lunar Crater Rays

B.R. Hawke, D.T. Blewett, P.G. Lucey, C.A. Peterson (U. of Hawaii/HIGP), J.F. Bell III (Cornell U./CRSR), B.A. Campbell (CEPS/NASM), M.S. Robinson (Northwestern U.)

The nature and origin of lunar crater rays have long been the source of major controversy. Some lunar scientists have proposed that rays are dominated by primary crater ejecta, while others have emphasized the role of secondary cratering or even more exotic processes in producing rays. In an effort to better understand the processes responsible for the formation of rays, we have utilized a variety of remote sensing data to study selected rays associated with Olbers A, Lichtenberg, the Messier crater complex, and Tycho. These data include near-IR reflectance spectra, radar images, and FeO, TiO₂, and maturity maps produced from Clementine UV-VIS images. Messier (14 km) and Messier A (11 km) are located in Mare Fecunditatis. Spectra were obtained for portions of the rays west and south of the crater complex, as well as for Messier A and nearby mature mare regions. The spectrum of Messier A exhibits an extremely deep ferrous iron absorption; a fresh mare composition is indicated. Near-IR spectra as well as the FeO, TiO₂, and maturity maps indicate that the rays west and south of the Messier complex are also dominated by immature mare material. Lichtenberg is a Copernican-aged impact structure that displays a relatively high-albedo ray system to the north and northwest but is embayed by basalt in the south. The FeO map indicates that ejecta and rays exhibit low FeO abundances and are dominated by highlands debris. The maturity image demonstrates that these highlands-rich ejecta and rays are fully mature. Hence, the Lichtenberg rays exhibit a high albedo because of their composition. Remote sensing data suggest that the Tycho ray in Mare Nectaris is dominated by fresh mare material excavated and emplaced by Tycho secondaries.