Abstract
Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, 17O and 18O, 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 18O depletion of δ18O = −50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δ13C = −48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in 13C 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.
Highlights
Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, 17O and 18O, by approximately 80 and 100‰, respectively, relative to Earth’s oceans, with inferred photospheric values of about −60‰ for both isotopes
Measurement of solar wind implanted in lunar regolith silicate grains yielded a δ13C ~−105 ± 20‰, from which a bulk solar ratio of ~−150 to −100‰ was inferred[8] (C isotope δvalues are computed relative to the Vienna Pee Dee Belemnite (VPDB) standard with 13C/12C = 0.0112372
In order to resolve the discrepancies between astronomical observations of the photosphere and ion microprobe measurements of C isotopes in solar wind and TiC, we will first focus on solar O isotopes
Summary
Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, 17O and 18O, by approximately 80 and 100‰, respectively, relative to Earth’s oceans, with inferred photospheric values of about −60‰ for both isotopes. TiC and CAIs are high-temperature condensates, so it was argued that the TiC isotope ratio represents the bulk solar nebula, and the bulk Sun. Measurement of N isotopes in TiN grains in the same Isheyevo CAI yielded δ15N = −359 ± 5‰ relative to atmospheric N2 (15N/14N = 3.676 × 10−3), a value consistent with the bulk solar value inferred by Genesis[2]. The literature values for the oscillator strength (fvalue) scale for CO rovibrational transitions left a 60‰ range of uncertainty, spanning the 18O/16O ratios from terrestrial values to those inferred for the photosphere from Genesis, precisely the range of most interest[12] We resolve these differences in f-values, and present new photospheric O isotope ratios. We derive a self-consistent 13C/12C ratio for the photosphere, which defines the C isotope ratio for the initial solar system
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