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, (17)O and (18)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 (18)O depletion of δ(18)O = −50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δ(13)C = −48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in (13)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.