Remote determination of the mineralogy and optical alteration of lunar basalts using Clementine multispectral images: Global comparisons of mare volcanism

High spatial resolution Clementine images are examined to measure the reflectance properties of small mare craters that have sampled individual lunar volcanic deposits. Small and relatively young mare craters are important sites for lunar remote sensing because their reflectance properties provide a more direct measurement of emplaced basalt composition than multi-component soils which have been exposed to extensive space weathering and mixing processes. Multispectral data of mare craters are compared to associated mature soils to determine the effects of space weathering on specific basalt types. Space weathering is observed to alter the optical properties of mare basalts in a systematic manner allowing compositional differences to be recognized across maturity states. Compositional and mineralogic differences between basalt types are identified based on their ultraviolet/visible spectral ratios, albedo and the strength and shape of their 1 μm ferrous bands.; Results of this research provide new information about the diversity, distribution and stratigraphy of lunar basalts at a global scale. Farside deposits are observed to be highly contaminated by feldspathic mixing; a process which has altered their reflectance properties to a much greater degree than nearside deposits. Based on uncontaminated exposures of maria, a wider range of farside compositional types are identified than previously suggested. However, low to very-low titanium basalts with a range of iron contents are most prevalent on the farside. Extensive deposits of very-low titanium and low-iron basalts are observed in association with regions of thick crust on the near and farside, while regions of thinner crust demonstrate a greater diversity of basalt types. Associations between basalt types and crustal thickness suggest that magma density may have played an important role in their regional distribution. Craters and soils within the last extensive mare eruptions, the unsampled high-titanium western deposits, exhibit a stronger ferrous absorption than earlier basalts suggesting that these basalts may be most iron-rich on the Moon. The distinct long-wavelength shape of this ferrous absorption is found to be similar for surface soils and crater materials excavated from depth. The pervasive character of this absorption feature supports the interpretation of abundant olivine within these last extensive lunar eruptions.