Hansteen Alpha: A volcanic construct in the lunar highlands

[1] We have used data from the Clementine and Lunar Prospector spacecraft in conjunction with near-IR reflectance spectra collected with Earth-based telescopes to study the composition and origin of Hansteen Alpha, an arrowhead-shaped highlands feature located on the southern margin of Oceanus Procellarum. Hansteen Alpha is a member of a class of lunar spectral anomalies (Red Spots) that are characterized by a relatively high albedo and a strong absorption in the UV. It has been suggested that at least some of these spectral anomalies were produced by nonmare volcanism. The stratigraphic relationships among the geologic and compositional units in the region show that Hansteen Alpha was emplaced by extrusive igneous processes. The Imbrian-aged craters Hansteen and Billy emplaced relatively FeO- and TiO2-rich ejecta. Hansteen Alpha exhibits much lower FeO and TiO2 values. If Hansteen Alpha was present prior to the Billy and Hansteen impact events, it should have been covered with FeO- and TiO2-rich ejecta because it is located within one crater diameter of the rim crest of each crater. Since it is not, Hansteen Alpha was superposed on these ejecta units. Nonmare volcanism is the only viable process for the formation of Hansteen Alpha. The morphology and surface texture of Hansteen Alpha is similar to many terrestrial features of dacitic and rhyolitic composition formed by extrusions of relatively viscous lavas. Such highly evolved compositions should be very enriched in Th, but Hansteen Alpha exhibits Th abundances of similar to6 ppm. Hence Hansteen Alpha is not composed of a highly evolved highlands lithology.