Continuing on the theme of silicic areas, the Lunar Volcanism session on Tuesday afternoon had quite a few talks on silicic volcanism! Most volcanism on the Moon is basaltic (the dark areas you see when you look up at the Moon are extensive basalt flows), but a few rare volcanic complexes on the Moon have silicic compositions. Silicic lavas produce lighter-colored rocks like granite and rhyolite. They have lower density, lower eruption temperatures, and higher viscosity than basaltic lavas, and are considered a "more evolved" form of volcanism that happened late in lunar history. The morphology, enhanced thorium concentration, high reflectance, and spectral signatures of these features indicate they are silicic in composition. Samples of granitic materials have been found in the Apollo sample suite, so we know that these types of rocks exist on the Moon.

Walter Kiefer, Staff Scientist at the Lunar and Planetary Institute, has been using GRAIL gravity data to determine the crustal density, and therefore constrain composition, at Hansteen Alpha and the Gruithuisen Domes. Both of these complexes have lower bulk densities than basaltic mare areas. The lower density could result from a different (silicic) composition, but it could also be caused by porosity from vesicles or pyroclastics, up to 10% porosity. Kiefer suggested that albite, orthoclase, and quartz are the most likely minerals at Gruithuisen Domes and Hansteen Alpha.

Silicic volcanics are near and dear to my own heart. I presented my work using high-resolution Lunar Reconnaissance Orbiter Camera Narrow Angle Camera (NAC) images to study how silicic regions reflect light differently when viewed from different angles. This is a powerful tool for understanding the mineralogy and compositional and physical properties of planetary surfaces. I created maps of the light-reflecting properties, also known as single-scattering albedo, of two silicic regions: the Compton Belkovich Volcanic Complex (CBVC) and Hansteen Alpha. The single scattering albedo is strongly related to composition and mineralogy, and these maps allow us to see variations in albedo properties, and therefore composition, on 1-meter scales. In the map for a portion of the CBVC, pictured here, higher albedo values (yellow and orange) correspond to areas with less iron and titanium than areas with lower albedo values (greens and blues).