|The Moon has been affected by volcanism during the first half of its history. Most of these volcanic deposits are concentrated on the nearside and have crater retention ages that cluster around ~3.6 Ga. The age distribution of volcanic deposits suggests there was a sharp increase in the number of volcanic deposits beginning around 3.9 Ga, with few older deposits. Is this observation due to the lack of ancient volcanic deposits? Or is this gap between the formation of the anorthosite crust and the onset of observed mare volcanism due to limited preservation? In this study we strive to address this fundamental question and investigate what the distribution and mineralogy of ancient volcanic deposits reveal about the early thermal history and evolution of the Moon.
Light plains are smooth high albedo surfaces that can be produced from basin impact ejecta ponding in topographic lows and by similar processes covering ancient mare deposits, creating cryptomaria. Cryptomaria are lunar volcanic deposits that have been covered with a layer of high albedo ejecta and have a similar morphology to impact-produced pre-mare Cayley Plains. In this study, we use a variety of remote data sets from the Lunar Reconnaissance Orbiter (e.g., Lunar Orbiter Laser Altimeter (LOLA), Diviner, Lunar Reconnaissance Orbiter Camera (LROC)) and Chandrayaan-1 (Moon Mineralogy Mapper (M3)) in order to assess the distribution and mineralogy of ancient cryptomaria as well as to identify criteria to distinguish cryptomaria from Cayley Plains produced solely by impact processes.
M3 VNIR spectroscopic data were used to identify high concentrations of dark-halo craters (DHC) superposed on light plains. This type of occurrence of DHC, small impacts ~5-10 km in diameter that excavate low albedo material from beneath a high albedo surface, indicate the presence of a buried volcanic deposit beneath a high albedo surface. Mosaics of optical period 2c1 were produced with a resolution of 140 m/pixel. Approximately 30 different suspected regions across the Moon were analyzed for the presence of cryptomaria. From these 30 regions, only 18 were positively identified to contain cryptomaria on the basis of the presence of DHC. Once the cryptomare were mapped, other datasets such as topography, surface roughness, and rock abundance were used to characterize the surfaces of cryptomaria and the global distribution of Cayley Plains. M3 3x3 average spectra were collected from DHC and then processed using the Modified Gaussian Model to determine the pyroxene compositions of cryptomaria.
Identified cryptomaria are concentrated around the nearside maria, especially in the eastern hemisphere. The most useful criterion for distinguishing between cryptomaria and end-member impact-produced Cayley Plains is a high concentration of DHC with a basaltic mineralogy. Analysis of the M3 spectra indicates that the mineralogy of all identified cryptomaria are consistent with mare basalts. These findings suggest that mare basalt volcanism was occurring prior to 3.9 Ga and that mantle dynamics and crustal thickness variations controlling the eruption of magmas onto the surface was in place during the emplacement of cryptomaria.