The Moon is an excellent laboratory for studying the early history of our solar system, due to the preservation of surface features. However, there are still limitations in how far back we can study lunar surface features. Over the last 50 years we have been able to learn an incredible amount about the volcanic history of the Moon, but we have not been able to really understand the surface expression of volcanic activity pre- and syn-heavy bombardment. Knowing the composition and timing of the earliest volcanic eruptions can help us to understand the early thermal history of the Moon and also understand how volcanism changed over the course of lunar history. Cryptomare deposits are believed to be the most ancient volcanic deposits on the Moon, typically obscured with a layer of high albedo impact ejecta. Researchers have been able to identify several features indicative of buried volcanic deposits, including intermediate albedos, dark halo craters, and highly mafic soils. Here we expand on the characterization of the cryptomare deposits by using visible near-infrared (VNIR) spectral data to investigate the mineralogies of these ancient volcanic deposits.
The Mineralogy Mapper (M3) instrument aboard the Chandrayaan-1 spacecraft was an imaging spectrometer, collecting a global VNIR dataset in 85 bands with a spectral resolution of 20 to 40 nm. For this study M3 spectra were collected from both identified cryptomare deposits and the surrounding exposed mare deposits. Cryptomare deposits were previously mapped in seventeen different regions, including Mendel-Rydberg, Australe, and Balmer basins. There is significant overlap between two M3 optical periods for nine of the identified cryptomare regions. Average spectra were collected for each of the identified mare and cryptomare ponds in these nine study areas and then processed with the Modified Gaussian Model (MGM) to determine their mineralogies. This duplication in the spectral measurements allows us to compare calculated compositions for each optical period, and verify our derived compositions. Four Gaussians were used to model pyroxene absorptions using the MGM model, one Gaussian in the visible region, and three near 1 µm and 2 µm.
Current model results indicate the cryptomare deposit mineralogies fall within the range calculated for the mare basalt deposits, indicating that the ancient mare basalts did not have significantly different compositions compared to the exposed mare basalts. Most cryptomare deposits have a shorter 1 µm absorption feature. However, both mare and cryptomare deposits are occurring along the augite- pigeonite band center boundary. Comparisons of spectral band centers from two different M3 optical periods (OP1B, OP2C1 and OP2C2) suggest that the variation for both the 1 and 2 µm absorption, is typically <5%. Observed variations in the spectral band centers indicate that there is no significant mineralogic difference between the exposed mare basalts and the cryptomare deposits within a particular study region, but certain study regions do appear compositionally distinct from one another (e.g., Mendel-Rydberg and Australe).