Spectral reflectance properties of carbonaceous chondrites: 1. CI chondrites

• Published in: Icarus (New York, N.Y. 1962) Vol. 212; no. 1; pp. 180 - 209
• Main Authors: Cloutis, E.A., Hiroi, T., Gaffey, M.J., Alexander, C.M.O’D., Mann, P.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.03.2011; Elsevier
• Subjects: Angle of incidence; Asteroids, Composition; Asteroids, Surfaces; Astronomy; Chondrites; Earth, ocean, space; Exact sciences and technology; Magnetite; Meteorites; Phases; Reflectance; Reflectivity; Slopes; Solar system; Spectra; Spectroscopy; Asteroids, Surfaces; Spectroscopy; Meteorites; Asteroids, Composition; Grain size; Particle size; Organic matter; Carbonaceous chondrite; Spectral properties; Variability; Abundance; Spectral reflectance; Reflectivity; Asteroids; Absorption band; Incidence angle; Absorption spectra; Solar system
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2010.12.009

► Most CI reflectance spectra exhibit mineral-associated absorption bands. ► Magnetite seems the likeliest explanation for blue-sloped CI spectra. ► Larger grain size samples generally have the bluest and darkest spectra. ► Phase angle can cause CI spectra to become redder or bluer. ► Underdense CI samples have lower reflectance than regularly packed CI samples. Existing reflectance spectra of CI chondrites (18 spectra of 3 CIs) have been augmented with new (18 spectra of 2 CIs) reflectance spectra to ascertain the spectral variability of this meteorite class and provide insights into their spectral properties as a function of grain size, composition, particle packing, and viewing geometry. Particle packing and viewing geometry effects have not previously been examined for CI chondrites. The current analysis is focused on the 0.3–2.5μm interval, as this region is available for the largest number of CI spectra. Reflectance spectra of powdered CI1 chondrites are uniformly dark (<10% maximum reflectance) but otherwise exhibit a high degree of spectral variability. Overall spectral slopes range from red (increasing reflectance with increasing wavelength) to blue (decreasing reflectance with increasing wavelength). A number of the CI spectra exhibit weak (<5% deep) absorption bands that can be attributed to both phyllosilicates and magnetite. Very weak absorption bands attributable to other CI phases, such as carbonates, sulfates, and organic matter may be present in one or a few spectra, but their identification is not robust. We found that darker spectra are generally correlated with bluer spectral slopes: a behavior most consistent with an increasing abundance of fine-grained magnetite and/or insoluble organic material (IOM), as no other CI opaque phase appears able to produce concurrent darkening and bluing. Magnetite can also explain the presence of an absorption feature near 1μm in some CI spectra. The most blue-sloped spectra are generally associated with the larger grain size samples. For incidence and emission angles <60°, increasing phase angle results in darker and redder spectra, particularly below ∼1μm. At high incidence angles (60°), increasing emission angle results in brighter and redder spectra. More densely packed samples and underdense (fluffed) samples show lower overall reflectance than normally packed and flat-surface powdered samples. Some B-class asteroids exhibit selected spectral properties consistent with CI chondrites, although perfect spectral matches have not been found. Because many CI chondrite spectra exhibit absorption features that can be related to specific mineral phases, the search for CI parent bodies can fruitfully be conducted using such parameters.