Plagioclase Analysis Using Photometry
The Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC), which has been orbiting the Moon since 2009, takes high resolution images of the lunar surface, such as the images of Orientale Basin shown In Figure 1 below where long narrow strips (NAC images) are superposed on a context image from the Wide Angle Camera or WAC. We do photometric studies using the data in these images, adjusting for topography and illumination geometry using NAC-derived digital topographic models, to determine photometric properties of the surface such as the single-scattering albedo (SSA) using methods detailed in Watkins (2017). The SSA, which describes the percentage of the incident light that is scattered from the surface, is correlated with the composition of the material (see work by previous members of our group in this LPSC 51 abstract). In particular, we find a relationship (linear inverse correlation) between the SSA and the FeO content. This relationship allows us to investigate the compositions and rock types on the lunar surface at very high resolution (~ 3 to 5 meters per pixel).
Determining the lithologies of the lunar crust is integral to investigating the history and early differentiation of the Moon. Particularly, our group is interested in the composition and formation of the original or primary lunar crust. This primary crust is predominately composed of the mineral plagioclase, but also includes more mafic minerals crystallized from pockets of trapped melt and possibly rock types with higher contents of mafic minerals such as pyroxene and olivine. By determining the precise percentage of plagioclase in the primary crust, we can determine the efficiency with which plagioclase separated during lunar solidification, which helps to inform lunar differentiation models, for example, how efficient was the extraction of aluminous crust from the magma ocean?
Recent hyperspectral and multispectral studies (Ohtake et al. 2009; Cheek et al. 2013; Donaldson Hanna et al. 2014) have identified absorption features indicative of highly pure plagioclase across the surface, leading workers to propose the presence of a thick, global layer of “purest anorthosite” (“PAN” with plagioclase > 98%) within the crust (Ohtake et al. 2009). The existence of a highly pure plagioclase layer further suggests that plagioclase separation in the early Moon must have been extremely efficient.
Our group investigates the photometry of areas of the lunar surface where spectral studies find absorption spectra indicative of highly pure plagioclase. In these areas, we determine the composition of the regolith in areas of highly pure plagioclase at high resolution. Figure 2 shows the SSA distribution across two regions of the Inner Rook Ring of Orientale Basin. In this color scheme, orange pixels correspond to regions of regolith composed of highly pure anorthosite. In these areas, high concentrations of plagioclase, approaching 100% are not uncommon, supporting the presence of an exhumed extensive body of this material.
However, a broader analysis shows that the analysis sites in the Inner Rook Ring of Orientale have much more pure plagioclase than analysis areas in many other regions of the lunar surface. Thus, our findings support a more heterogeneous distribution of highly pure plagioclase within the crust, and not necessarily a global layer. For more information on these analyses, see our LPSC 52 abstract.
References:
Ohtake, M., et al. (2009) The global distribution of pure anorthosite on the moon. Nature 461, 236-241.
Cheek, L. C., et al. (2013) The distribution and purity of anorthosite across the Orientale basin: New perspectives from Moon Mineralogy Mapper data. J. Geophys. Res. 118, 1805-1820.
Donaldson Hanna, K. L., et al. (2014) Global assessment of pure crystalline plagioclase across the Moon and implications for evolution of the primary crust J. Geophys. Res. 119, 1516-1545.