^1,2A. Garenne,^1,2P. Beck,^3,4,5,6G. Montes-Hernandez, ^1,2O. Brissaud, ^1,2B. Schmitt, ^1,2E. Quirico, ^1,2L. Bonal, ⁷C. Beck, ^8,9K.T. Howard
¹Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France
²CNRS, IPAG, F-38000 Grenoble, France
³Univ. Grenoble Alpes, ISTerre, F-38000 Grenoble, France
⁴CNRS, ISTerre, F-38000 Grenoble, France
⁵IRD, ISTerre, F-38000 Grenoble, France
⁶IFSTTAR, ISTerre, F-38000 Grenoble
⁷Univ. Savoie, ISTerre Science Institue EARTH, 73376, Le Bourget du Lac, France
⁸Kingsborough Community College, 2001 Oriental Blvd., Brooklyn, NewYork, NT 11235, USA
⁹Department of Earth and Planetary Sciences, American Museum of Natural History.

In this study, we measured bidirectional reflectance spectra (0.5-4.0 μm) of 24 CMs, five CRs, one CI, one CV, and one C2 carbonaceous chondrites. These meteorites are known to have experienced an important variability in their relative degrees of aqueous alteration degree (Rubin et al., 2007 and Howard et al., 2009; 2011; Alexander et al., 2013). These measurements were performed on meteorite powders inside an environmental cell under a primary vacuum and heated at 60°C in order to minimize adsorbed terrestrial water. This protocol allows controlling of atmospheric conditions (i.e. humidity) in order to avoid contamination by terrestrial water. We discuss various spectral metrics (e.g. reflectance, band depth, single-scattering albedo, …) in the light of recent bulk composition characterization (Howard et al., 2009, Howard et al., 2015, Alexander et al., 2012, Beck et al., 2014 and Garenne et al., 2014). This study reveals variability of reflectance among meteorite groups. The reflectance is not correlated with carbon or hydrogen abundance neither with measured grain size distribution. We suggest that it is rather controlled by the nature of accreted components, in particular the initial matrix/chondrule proportion. Band depth, integrated band depth, mean optical path length, normalized optical path length, effective single–particle absorption thickness were calculated on the so called 3-μm band for reflectance spectra and for single scattering albedo spectra. They were compared with hydrated phase proportions from previous study on the same meteorites by thermogravimetric analyses and infrared spectroscopy in transmission. We find that normalized optical path length (NOPL) is the most appropriate to quantify water abundance, with an absolute error of about 5 wt.%. These datasets also reveal a variability of the band shape between 2.8 and 2.9 μm, which is interpreted as reflecting variation in the chemical composition and structure of phyllosilicates. This chemical variation could also be used to quantify the aqueous alteration degree between meteorite groups. The combination of reflectance at 2 μm and the depth of 3-μm band can be combined, to classify carbonaceous chondrites in reflectance in term of primary composition (e.g. matrix/chondrule ratio, carbon content) and secondary processes (e.g. aqueous alteration, thermal metamorphism). This could be used to decipher the nature of aqueous alteration in C-complex asteroids.

Reference
Garenne A, Beck P, Montes-Hernandez G, Brissaud O, Schmitt B, Quirico E, Bonal L, Beck C, Howard KT (2015) Bidirectional reflectance spectroscopy of carbonaceous chondrites: Implications for water quantification and primary composition. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.09.005]
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