Spectral Reflectance of Powder Coatings on Carbonaceous Chondrite Slabs: Implications for Asteroid Regolith Observations

Abstract

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‐ to 2,500‐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 1,000‐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 1,900‐nm regions by these Fe oxyhydroxides. Their effects can be mitigated through chemical treatment, but this may also affect preterrestrial 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, and dust on slabs). Other spectral characteristics (such as albedo and spectral slope) vary as a function of physical properties, indicating 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.

Spectral effects of varying texture and composition in two‐component “mudpie” simulations: Insights for asteroid (101955) Bennu

“…The pink arrow indicates increasing opaque abundance, and the green arrow indicates increasing particle size. dust onto slabs of the CM chondrite Murchison (Kiddell et al 2018) are similar to those observed on Bennu.…”

“…Further, the albedo and color differences resulting from depositing varying thicknesses (113–321 µm) of dust onto slabs of the CM chondrite Murchison (Kiddell et al. 2018) are similar to those observed on Bennu.…”

“…2019) and an arrow representing increasing thickness of layers of Murchison 45 µm particles on Murchison slabs (Kiddell et al. 2018). The dust thickness arrow runs almost parallel to the spread of the Bennu data, whereas the large spread of Tagish Lake sample color and albedo values (due to the lithologic variations within the meteorite) suggests a lithological distinction between bright and dark boulders on Bennu.…”

Weak spectral features on (101995) Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer

“…The red slope itself could be due to Fe-bearing oxide minerals, graphite, or both as these have been found together in some meteorites (Scott et al 1981). However, band depth and spectral slope can also be affected by factors other than abundance and composition, such as dust cover and space weathering, so the interpretation cannot be uniquely constrained (Cloutis et al 2018;Kiddell et al 2018).…”

A photometric study of regolith intimate mixing with ice-like impurity

“…[32]), asteroidal (e.g. [28]), or planetary surfaces (e.g. [4]), and also in the study of organic cells in biology (e.g.…”