Effects of Viewing Geometry, Aggregation State, and Particle Size on Reflectance Spectra of the Murchison CM2 Chondrite

English Abstract:

Several current missions will investigate ‘dark’ asteroids, whose spectra have weak or no distinct spectral features. Many non-compositional effects can influence reflectance spectra, complicating the analysis of such data from remote surfaces. Using a sample of Murchison CM2 chondrite, a series of absolute reflectance (across a 0.3µm–2.5µm range) spectra was collected, varying a range of these non-compositional parameters. In situ spectra of three samples with both saw-cut and fusion-crusted faces were taken; sub-samples of these slabs were then powdered to <150µm to investigate spectral homogeneity within a sample. To explore the effects of grain size, a series of measurements was taken in which the same sample was measured at progressively smaller grain sizes (1000, 500, 250, 150, 90, and 45μm). Mixtures of coarse (0.5-1mm) particles combined with varying amounts of <45μm fine particles taken from the same sample as well as homogeneous samples prepared with different powder-packing (regular, fluffy, and packed) techniques were measured to examine surface texture effects. Lastly, spectra of one <90µm sample were taken at different phase angles to better account for the wide variations in viewing geometry that are possible from spacecraft encounters. Intra-sample heterogeneity, while spectrally detectable, is relatively limited and generally within the range of variations exhibited by duplicate measurements of the same sample after repacking. Decreasing grain size causes a decrease in spectral contrast and increased visible spectral slope. Fine-grained particles appear to exert a disproportionately strong influence on spectral properties relative to their volume. Phase angle effects include increased visible slope with increasing phase angle, a trend that may reverse at very high phase angles (>~100°). Overall, it should be possible to make reasonable compositional interpretations of carbonaceous chondrite spectra and constrain some physical properties (e.g., grain size). The full spectral resolution data collected may also inform the interpretation of hyperspectral data returned from Ceres, Bennu, and other dark carbonaceous chondrite-like surfaces.