Characterizing Lunar Materials: Spectral Analysis of Apollo 16 and Lunar Meteorite

Interpreting the surface composition of the Moon and other airless bodies relies heavily on spectroscopic techniques spanning visible and near-infrared wavelengths. This study employed an integrated spectroscopy approach to measure and characterize a suite of lunar rock samples, a lunar meteorite, and lunar minerals. The primary objective was to demonstrate the methodology for building a robust spectral library that can support the interpretation of data from future lunar missions focused on understanding surface compositions and resource potential.

The analog samples characterized included 15 lunar mineral endmembers (pigeonite, augite, ilmenite, orthopyroxenes, olivine, anorthite, and bytownite), an Apollo 16 regolith sample (62231.44), and the Tisserlitine 001 lunar meteorite. These materials were prepared as fine powders sieved to grain sizes ranging from 25 to 125 micrometers. Spectroscopic measurements were performed under vacuum conditions using the state-of-the-art Bruker Vertex 80V FTIR instruments at the Planetary Spectroscopy Laboratory. Bidirectional reflectance spectroscopy was employed to acquire spectra at illumination angles of 30° and 45° relative to the sample surface normal, utilizing attached visible and near-infrared detectors. Hemispherical reflectance measurements were conducted to collect visible and near-infrared spectra separately, using integrating spheres.  The calibrated hemispherical reflectance spectrum for the Apollo 16 sample exhibits a clear hydroxyl group feature near 3 micrometers, consistent with previous observations. Despite originating from different locations, the Apollo 16 lunar sample and the Tisserlitine 001 lunar meteorite displayed striking similarities in their visible and near-infrared bidirectional reflectance spectra, exhibiting absorption features characteristic of lunar highland compositions, including broad 1 and 2 micrometer bands indicating the presence of pyroxenes. However, the Tisserlitine 001 sample had slightly more pronounced pyroxene bands, although the overall spectral shape and absorption features were comparable between the two samples.

Further spectral analysis revealed notable observations. Ilmenite exhibited the highest overall reflectance, followed by orthopyroxene and pigeonite, while anorthite and bytownite displayed relatively low reflectance across most wavelengths. Olivine exhibited a distinct 10 micrometer absorption feature. Interestingly, the Tisserlitine 001 lunar meteorite displayed a unique spectral signature distinct from the other minerals studied. The study's findings contribute to developing a robust spectral library to aid in identifying lunar mineralogy and chemistry from orbital or landed measurements, enabling assessments of resource potential and improving understanding of lunar formation processes.