Mapping the Moon with LROC: Global ultraviolet through visible color observations

The multispectral wide-angle camera (WAC), one of three instruments that comprise the Lunar Reconnaissance Orbiter Camera (LROC), is a push-frame imager with seven narrow-band filters. Two ultraviolet (UV) filters at 321 and 360 nm are imaged with a lens with a cross-track field of view of 60°, and are 4×4 summed on-chip for an effective resolution of 400 m/pixel from a 50-km orbit. Five visible filters at 415, 566, 604, 643 and 689 nm are imaged with separate optics and an effective resolution of 100 m/pixel from a 50-km orbit. All filters are exposed simultaneously and 14 lines by 704 samples of each visible filter and 16 lines by 512 samples (summed to 4 lines by 128 samples) of each UV filter are read out. Continuous coverage is obtained by imaging at regular intervals consistent with the spacecraft’s motion. The wavelengths of the WAC filters were selected to provide information about common lunar minerals and to complement the multispectral images available from the Clementine UV-VIS camera (415, 750, 900, 950, and 1000 nm). Imaging at shorter wavelengths than previously available provides an opportunity to better characterize color differences in terms of mineralogic and compositional variations. The WAC bandpasses are particularly well suited for characterizing the distribution of ilmenite, which has a distinctive reflectance peak near 300 nm and a minimum near 500 nm. The WAC is also of particular use for discriminating minerals of the olivine solid solution, whose reflectance maximum varies between 550 and 700 nm depending on magnesium number (fayalite to forsterite). This paper will present the first analysis of LROC WAC kilometer-scale ultraviolet to visible color variations across the whole surface of the Moon.