Spectral reflectance of powder coatings on carbonaceous chondrite slabs: implications for asteroid regolith observations

1C. B. Kiddell, 1E. A. Cloutis, 1B. R. Dagdick, 1J. M. Stromberg, 1D. M. Applin, 1J. P. Mann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005600]
1Dept. of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
Published by arrangement with John Wiley & Sons

Carbonaceous chondrite meteorites (CCs) are among the most primitive materials in the solar system and provide important insights into solar system history and evolution. A number of planetary spacecraft missions will visit asteroids that are thought to compositionally resemble these meteorites. To better assist sample acquisition in terms of how the physical properties of CCs affect their reflectance spectra, we investigated the spectral reflectance properties of solid and powdered CCs, and powder coatings on slabs of a number of CCs, including CB, CH, CK, CM, CO, CR, and CV classes. We found that decreasing grain size leads to increasing reflectance across the ~500‐2500 nm range and steeper spectral slope, regardless of CC type. Powdered CC reflectance spectra are brighter beyond ~500 nm and redder than bare roughened slabs. For powders sprinkled on slabs, as the powder coating gets thicker, spectral slopes get redder.

Optically thick fine‐grained powders are brighter beyond ~500 nm and are as red or redder, than slabs covered with airfall dust (for dust thicknesses up to a few hundred microns). Diagnostic absorption features of CC minerals, particularly those in the 1000 nm region attributable to Fe‐bearing silicates, are ubiquitous regardless of physical properties. Reflectance spectra of terrestrially weathered (i.e., “rusty”) CCs are strongly modified below ~700 nm and in the 900 and 1900 nm regions by these Fe oxyhydroxides. Their effects can be mitigated through chemical treatment, but this may also affect pre‐terrestrial ferric iron‐bearing phases. Some spectral characteristics, such as hydrous and anhydrous silicate absorption bands in CC spectra, are present regardless of physical properties (fine‐grained dust, powders, slabs, dust on slabs). Other spectral characteristics (such as albedo and spectral slope) vary as a function of physical properties, indicates that reflectance spectroscopy could be used to ascribe spectral variations across an asteroid’s surface to either physical or compositional causes. This information can, in turn, be used to inform site selection for asteroid sample return missions, where both composition and physical properties are important drivers. When searching for fine‐grained areas on an asteroid to sample, the best indication would be the brightest and reddest‐sloped spectra.

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