Ancient Lunar Volcanism: Distribution and Mineralogy of Cryptomaria

Ancient volcanic deposits on terrestrial planetary bodies preserve the earliest record of planetary evolution, revealing important information about both the interior evolution and the surface modification history of planetary bodies. On the Moon, ancient volcanic deposits are referred to as cryptomaria and are basalt deposits that have been obscured by impact basin ejecta. As a result, cryptomaria appear as smooth high-albedo plains. This physical appearance can easily be confused with other formations such as the Cayley Plains, hypothesized to have formed from the ponding of impact basin ejecta into topographic lows. The difficulty in differentiating between cryptomaria and the Cayley Plains has led to the development of several recognition criteria. A high concentration of dark-halo impact craters (DHCs), a mafic enhancement in regolith, and an intermediate albedo are all different criteria used to positively identify cryptomaria. DHCs are small impact craters <10 km in diameter that excavate low-albedo basaltic material from beneath high-albedo surface materials. In this study we used multiple lunar datasets (M3, LOLA, LROC, Diviner) to map the distribution of cryptomaria and to determine the mineralogy of the identified cryptomare deposits. Of the 27 regions analyzed, 20 cryptomaria were positively identified. All identified cryptomaria cluster on the nearside, within large impact basins and along the periphery of younger mare basalts. The largest cryptomaria are confined to the interior of ancient impact basins (e.g., Schiller-Zucchius). Including cryptomaria, the total surface area of the Moon covered by mare basalts has increased by ~2%. After identification, M3 spectra collected from DHCs were modeled using the Modified Gaussian Model and indicate that the cryptomaria are basalts and have the same mineralogy as exposed maria in the same study region. Crater counts conducted on the largest cryptomaria produce minimum model age estimates between 3.8 and 4.0 Ga. This, combined with the close association between cryptomaria and exposed maria, suggests that the dated cryptomaria formed just prior to this period of time and imply that the process controlling the distribution of mare basalts was in place near this time in lunar history.