Through the Looking Glass: Spectral Analysis of the Imbrium Basin

By excavating into the surface, basin-forming impacts should have exposed material from deep within the interior of the Moon. Given the distribution and size of lunar basins on the near side, one would expect to find samples of the lunar mantle in collections on Earth from Apollo, Luna, and meteorites, or detected remotely on the surface. Despite the abundance of basins, only a few mantle candidate samples have been identified. In order to locate sites potentially containing remnants of the mantle, we searched for early-crystallizing minerals using data from the Moon Mineralogy Mapper (M³) and the Diviner Lunar Radiometer (Diviner).

Previous work using the Spectral Profiler has identified olivine-bearing massifs around many of the larger basins. In this study we used M³ near infrared (NIR) spectral data to identify massifs around the basin which contained olivine, Mg-rich orthopyroxene, a second low-Ca pyroxene, and anorthite. In order to identify regional differences in composition, 10 sites were chosen around the basin for further analysis using Diviner.

The mantle should contain little to no plagioclase, making the high relative abundance of mafic minerals its primary identifying feature. Given that these minerals can produce very strong absorption bands in the NIR even if they comprise <50% of the sample, remotely differentiating between objects such as troctolites and those with more mafic compositions such as pyroxenite requires additional information. The composition information derived from the Christiansen Feature data from Diviner eliminated almost all of the potential candidates for mantle materials, with the exception of those found at Wolff Mons (in the south-by-southeast portion of the outer ring), where pyroxenite ( 90% pyroxene) was identified.

These results corroborate the impact analysis by Schultz and Crawford in 2016, which concluded that the impactor was travelling on a northwest-southeast trajectory. This also explains the lack of mafic materials in the inner ring of the basin; and the lower-pyroxene abundance in the norites identified in Montes Alpes (northeast, outer ring). Though some olivine was found in most of the sites, at most it was only 50% of the mixture, indicative of a troctolite which is not a candidate for mantle material. Mineralogical maps and spectral data will be presented.