A light carbon isotope composition for the Sun.

Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, O and O, by approximately 80 and 100‰, respectively, relative to Earth's oceans, with inferred photospheric values of about -60‰ for both isotopes. Direct astronomical measurements of CO absorption lines in the solar photosphere have previously yielded a wide range of O isotope ratios. Here, we reanalyze the line strengths for high-temperature rovibrational transitions in photospheric CO from ATMOS FTS data, and obtain an O depletion of δO = -50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δC = -48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in C relative to the bulk material from which the solar system formed, possibly as a result of CO self-shielding or inheritance from the parent cloud.