Isotopic Composition of the Noble Gases and of Some Other Elements in the Sun: A Review and a Discussion of Open Questions

The isotopic composition of highly volatile elements (noble gases, carbon, nitrogen, and oxygen) in the solar accretion disk is largely unconstrained by meteorite data and must be inferred independently. The best proxy is the present-day Sun, whose isotopic composition is most precisely obtained in the solar wind. Measurements from NASA’s Genesis mission provide high-precision isotopic data for most ultravolatile elements. We review these data for the noble gases, O and N in Genesis collectors, which sampled bulk solar wind for 2.3 years, as well as He, Ne, and Ar from solar wind in different velocity ranges. To obtain accurate isotopic compositions for the Sun, solar wind data require corrections for gravitational element settling in the Sun’s Outer Convective Zone (OCZ), and, more importantly, for isotopic fractionation upon formation and acceleration of the solar wind. Although a comprehensive theory to explain this fractionation is lacking, a model known as inefficient Coulomb drag (ICD) provides a plausible match to existing data. We discuss the evidence supporting this conclusion. A future determination of the carbon composition in solar wind might provide a further check. While ICD appears to reasonably characterize the isotopic fractionation between OCZ and solar wind for elements with masses up to about Ar, it may overestimate the fractionation of Kr and Xe. Precise analyses of the isotopic composition of Kr and Xe in Genesis collectors that sampled high-speed and low-speed solar wind might resolve this key question for the isotope cosmochemistry of xenon and krypton.