(a) Photograph of the Y-82094 ungrouped carbonaceous chondrite. The side of the cube is 1 cm. (b) An image of the polished thin section under an optical microscope. Credit:  National Institute of Polar Research.

One of the major classes of meteorites is the carbonaceous chondrites, volatile-rich primitive meteorites thought to have formed in the outer solar system. The components of carbonaceous chondrites — such as calcium-aluminum-rich inclusions, chondrules, and meteoritic matrix — formed in the early solar system and have remained relatively unaltered since that time. Therefore, these components are records of early solar system processes. Carbonaceous chondrites are classified into eight major groups based on their chemistry, mineralogy, and isotopic composition. However, there are about 100 ungrouped chondrite meteorites that show characteristics distinct from these major groups. These ungrouped chondrites may represent a wider range of materials from different meteoritic parent bodies that are not sampled by the major groups of carbonaceous chondrites.

A recent study led by Makoto Kimura of the National Institute of Polar Research in Japan reports on two unusual carbonaceous chondrites, Asuka (A)-9003 and Asuka (A) 09535, that show similar characteristics to a previously reported ungrouped meteorite, Yamato (Y)-82094. These three meteorites have the highest abundances of refractory inclusions and bulk oxygen isotopic compositions of carbonaceous (C) chondrites and are most similar to the CO and CV groups (named for the Ornans and Vigarano meteorites). However, they also have chondrule-to-matrix ratios similar to unequilibrated ordinary chondrites. The authors suggest these samples may represent a new group of carbonaceous chondrites tentatively called CA types after Asuka Station, a Japanese observation base in Antarctica.

The discovery and analysis of new types of primitive meteorites provide previously unexplored samples that may enhance our understanding of the environment of the early solar system. This work suggests a larger range of formation conditions in the early solar system than is recorded by the other major carbonaceous chondrite groups. Additionally, these meteorites provide samples that can be directly compared with samples returned from planetary missions. READ MORE