Regional and Temporal Variations in the Western Mare Basalts: New Observations from the Moon Mineralogy Mapper

Introduction and Background: The history of volcanism on the western near side of the Moon is unique in both its duration and the basaltic compositions it left behind on the lunar surface. Together, Oceanus Procellarum and Mare Imbrium compose the largest expanse of the lunar maria with deposits ranging from the early history of lunar volcanism through its last major phases. Various techniques have dated many of these flows as younger than 3.0 Ga [1] with some areas having erupted as recently as ~1.2 Ga [2, 3], or almost two billion years after the youngest basalts collected by sample return missions. These last major phases of lunar volcanism produced spectrally unique, high-titanium basalts that cover large areas of Procellarum and Imbrium as well as smaller regions within several surrounding maria. The reflectance properties of soils and craters within these deposits display a relatively strong 1 μm feature and a weaker 2 μm absorption, suggesting the presence of abundant olivine [4, 5, 6]. However, previous telescopic and orbital measurements lack the combined spectral and spatial resolutions necessary for detailed mineralogical characterizations of these basalts. In the current study, visible to near-infrared reflectance data acquired by the Moon Mineralogy Mapper (M) on Chandrayaan-1 are used to investigate the mineralogy of these late-stage basalts and implications for the heterogeneity and evolution of their source regions. M Data Analysis: The M imaging spectrometer was a guest instrument on India’s Chandrayaan-1 mission which launched on October 22, 2008 and mapped the lunar surface from a polar orbit through August of 2009. M acquired visible to infrared reflectance data at spatial and spectral resolutions capable of measuring discrete basaltic flows within the lunar maria. This global data covers the wavelength range of ~430 to 3000 nm in 85 spectral bands at 140 to 280 m/pixel. Small amounts of data were also acquired of targeted regions in 259 spectral bands and at higher spatial resolutions. To perform preliminary compositional assessments, global mosaics were created by reducing the spatial resolution of the M data by a factor of 10 [7]. After calibration of the data to apparent reflectance and the application of thermal corrections [8, 9], a range of spectral parameters were calculated to explore broad mineralogical differences across the Moon. Figure 1 provides an example of a hemispheric color composite image that distinguishes several near side mare regions based on their 1 and 2 μm integrated band depths (IBD) and near infrared albedo. The latestage western maria are unique within this color composite due to their relatively weak 2 μm absorptions and strong 1 μm ferrous bands (distinguished by red hues in this color composite image).