Exploring the Moon at the Microscale: Analysis of Apollo Samples with the Multispectral Microscopic Imager (MMI)

The Multispectral Microscopic Imager (MMI), similar to a geologist’s handlens, creates multispectral, microscale reflectance images of geological samples, in which each image pixel is comprised of a VNIR spectrum. This enables the discrimination of a wide variety of rock-forming minerals, especially Fe- and Mg-bearing phases, within a microtextural framework. The MMI composite images provide crucial geologic and contextual information: 1) for the in-situ analysis of rocks and soils to support hypothesis-driven, field-based exploration; 2) to guide sub-sampling of geologic materials for return to laboratories on Earth; and 3) in support of astronaut investigations during EVAs, or in a lunar base laboratory. To assess the value of the MMI as a tool for lunar exploration, we used a field-portable, tripod-mounted version of the MMI to image 18 lunar rocks and four soils, from a reference suite spanning the full compositional range found in the Apollo collection, housed in the Lunar Experiment Laboratory at NASA’s Johnson Space Center. The MMI composite images faithfully resolved the microtextural features of samples, while the application of ENVI-based spectral end-member mapping faithfully revealed the distribution of Fe-bearing mineral phases (olivine, pyroxene and magnetite), along with plagioclase feldspars within samples, over a broad range of lithologies and grain sizes (figure 1). The MMI composite images also revealed secondary mineral phases, glasses, and effects of space weathering in samples, where present. Our MMI-based petrogenetic interpretations compared favorably with thin section-based descriptions published in the literature, revealing the value of MMI images for astronaut and rover-mediated lunar exploration. We present our latest results from these analyses and their application to future lunar exploration. Figure 1. Multispectral images of Apollo sample 14321,88. Left: R = 635 nm; G = 525 nm; B = 470 nm. Right: R = 1450 nm; G = 975 nm; B = 525 nm. Field of view: 40 mm x 32 mm (62.5 μm/pixel). Images are 2% histogram stretched. The addition of near-infrared bands enabled the distinction of different rock-forming minerals on the basis of spectral differences.