Radar mapping of lunar cryptomaria east of Orientale basin

Abstract

Lunar cryptomare deposits represent early basaltic volcanic material that has been mantled by or incorporated into highland-rich ejecta from basin- or crater-forming impacts. Mapping these buried basalts is important for understanding regional stratigraphy and the history and extent of lunar volcanism. We use new 70 cm wavelength radar images, collected using Arecibo Observatory and the Green Bank Telescope, and Clementine data to study cryptomaria east of Orientale basin. Earlier multispectral analyses showed that mare material is mixed with the highland terrain along the western margin of Oceanus Procellarum, leading to a detectable increase in the FeO and TiO2 abundance of the regolith surface. The highland margin of western Procellarum is also characterized by low 70 cm radar returns, consistent with an increased regolith loss tangent due to ilmenite in the mare-derived material. The low 70 cm radar echo, however, persists well to the west of the mixed zone evident in multispectral data and includes the region surrounding Cruger crater. It is likely that mare basalt, or a mixed zone of mare and highland material, exists at depth across the region and only reaches the visible surface near the western Procellarum margin. The plausible depth to this mixed zone is dependent upon the loss tangent of the overlying ``pure'' highland ejecta. If Orientale ejecta is primarily low-loss anorthosite, then the mixed zone could lie at depths of up to several tens of meters. We propose that pre-Orientale mare deposits flooded the region between Cruger, Grimaldi, and Oceanus Procellarum and also patches west and northwest of Humorum basin. The total area of these deposits represents 178 ? 103 km2, or an additional ~0.5% of the lunar surface, covered by mare basalts.