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‰ (1 σ ). From the same analysis we find a carbon isotope ratio of δ 13 C = −48 ± 7‰ (1 σ ) 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. The Sun’s light stable isotopes compositions can help us understand how our solar system formed. Here, the authors find that solar C is depleted relative to bulk Earth indicating that the 13 C enrichment of the terrestrial planets is from CO self-shielding or inheritance from the parent cloud.