Solar and solar-wind isotopic compositions

With only a few exceptions, the solar photosphere is thought to have retained the mean isotopic composition of the original solar nebula, so that, with some corrections, the photosphere provides a baseline for comparison of all other planetary materials. There are two sources of information on the photospheric isotopic composition: optical observations, which have succeeded in determining a few isotopic ratios with large uncertainties, and the solar wind, measured either in situ by spacecraft instruments or as implanted ions into lunar or asteroidal soils or collection substrates. Gravitational settling from the outer convective zone (OCZ) into the radiative core is viewed as the only solar modification of solar-nebula isotopic compositions to affect all elements. Evidence for gravitational settling is indirect, as observations are presently less precise than the predictions of <10‰ effects for the isotopes of solid-forming elements. Additional solar modification has occurred for light isotopes (D, Li, Be, B) due to nuclear destruction at the base of the convection zone, and due to production by nuclear reactions of photospheric materials with high-energy particles from the corona. Isotopic fractionation of long-term average samples of solar wind has been suggested by theory. There is some evidence, though not unambiguous, indicating that interstream (slow) wind is isotopically lighter than high-speed wind from coronal holes, consistent with Coulomb drag theories. The question of fractionation has not been clearly answered because the precision of spacecraft instruments is not sufficient to clearly demonstrate the predicted fractionations, which are