Heating of carbonaceous materials: Insights into the effects of thermal metamorphism on spectral properties of carbonaceous chondrites and asteroids

1,2,3Achen Duan,1,4Yunzhao Wu,5Edward A. Cloutis,1,2Jinfei Yu,1Shaolin Li,1,2Yun Jiang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13750]
1Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210023 China
2School of Astronomy & Space Sciences, University of Science and Technology of China, Hefei, 230026 China
3Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing, 210023 China
4CAS Center for Excellence in Comparative Planetology, China
5Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, R3B 2E9 Manitoba, Canada
6State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China
7CAS Center for Excellence in Comparative Planetology, China
Published by arrangement with John Wiley & Sons

Carbonaceous chondrites (CCs) are important materials for understanding the early evolution of the solar system and delivery of organic material to the early Earth. Spectral analysis of CCs can establish the relationship between them and their possible parent asteroids, which helps to determine the surface composition of the asteroid. In this paper, the 0.3–26 μm reflectance spectra of a series of coals ranging from lignite to anthracite (Earth analogs of organic matter contained in CCs), a coal heated to various durations and temperatures, and reflectance spectra of CM2 meteorites were analyzed in conjunction with compositional information to derive spectral–compositional relationships. All types of coals have strong aromatic absorptions (3.28 and 5–6.5 μm) and aliphatic “triplet” absorptions (3.38, 3.41, and 3.48 μm). In contrast, CM2 meteorites have obvious aliphatic absorptions and lack aromatic absorptions. The reason is the weak absorption coefficients of aromatic materials and the overlap with strong OH/H2O absorption. Absorptions in the coal spectra are strongly related to elemental H/C ratio. When the H/C ratio is >0.55, the absorption intensity of an aliphatic increases linearly with the increase of H/C. For heated coal, increasing heating time above 1 h at 450 °C causes the disappearance of the aliphatic “triplet” absorptions. Similarly, heating Murchison meteorite to 400 °C for 1 week causes all the organic absorptions to disappear. This implies that in remote sensing detections, only asteroids (e.g., with CM and CI carbonaceous chondrites compositions) that experienced low thermal metamorphism (<400 °C) are suitable as potential targets for detecting organic compounds using features in the 3–4 µm region.

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