[MIS17-P07] Relationship between visible reflectance at 550 nm and carbon contents in carbonaceous chondrites: At-tempt to estimate the bulk carbon contents of asteroid Ryugu’s surface
As the samples, three primitive carbonaceous CM/CI chondrites (Murchison, Murray, and Yamato 980115), five thermally metamorphosed CM/CI chondrites (Jbilet Winselwan, Yamato 982086, Yamato 793321, Belgica 7904, and Yamato 86720), and three carbonaceous chondrites (Murchison, LAP 04721, and Tagish Lake) experimentally heated at different temperatures (300, 400, 500, 600 and 900°C) for 50 hrs, and a simulant of Phobos were used. Their grain sizes and/or porosity were made constant. Reflectance spectra of the meteorites were analyzed by VERTEX 70v, Bruker (Tohoku Univ.). Total carbon contents of the meteorites were analyzed by a CHN elemental analyzer (Flash EA1112) (Kochi Univ.).
For the primitive CM/CI chondrites and thermally metamorphosed CM/CI chondrites, roughly linear correla-tions were observed between total carbon contents and reflectance at 550 nm (R(550)) and 390 nm (R(390)). Thus, the meteorites with higher carbon contents show lower reflectance. However, some meteorites (Murchison, Murray, Y-982086, Jbilet Winselwan, Y-793321) with similar carbon contents showed variations in reflectance. This is probably due to the difference in chemical compositions between primitive CM/CI chondrites and thermally meta-morphosed CM/CI chondrites. The thermally metamorphosed CM/CI chondrites containing IOM with higher aro-maticity (Yabuta et al. 2005) show lower reflectance compared to the primitive CM/CI chondrites. Based on the relationship, asteroid Ryugu, whose mean R(550) is 0.019, may contain higher than 3% of carbon.
In the correlation plots between total carbon contents and R(550) and R(390) for the experimentally heated car-bonaceous chondrites, overall, meteorites with higher carbon contents show lower reflectance. However, individual meteorites showed different behaviors during heating. For instance, the reflectance values of Murchison were low-ered while the total carbon contents of Murchison did not change during heating up to 400°C. During heating at 500 - 600°C, the reflectance values of Murchison were raised again while the total carbon contents of Murchison decreased only slightly. At 900°C, the total carbon contents of Murchison suddenly decreased with increasing reflec-tance. These variations in reflectance are related to not only total carbon contents but also chemical change of IOM with increasing temperature, such as aromatization, pyrolytic degradation, and gasification. LAP 04721 and Tagish Lake showed similar trends, but their variations in reflectance differed depending on their original carbon contents and compositions. Based on the relationship, R(550) of Ryugu is the most similar to those of the heated Tagish Lake at 400-600°C or heated Murchison at 400°C. The result appears to be consistent with the conclusion obtained by the near infrared spectral pattern of the surface of Ryugu (Kitazato et al. 2019).