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 < 30 per amu between fast and slow wind for most elements. Analysis of solar-wind noble gases extracted from lunar and asteroidal soils, when compared with the terrestrial atmospheric composition, also suggests solar-wind fractionation consistent with Coulomb drag theories. Observations of solar and solar-wind compositions are reviewed for nearly all elements from hydrogen to iron, as well as the heavy noble gases. Other than Li and the noble gases, there is presently no evidence for differences among stable isotopes between terrestrial and solar photosphere compositions. Although spacecraft observations of solar-wind isotopes have added significantly to our knowledge within the past decade, more substantial breakthroughs are likely to be seen within the next several years with the return of long-exposure solar-wind samples from the Genesis mission, which should yield much higher precision measurements than in situ spacecraft instruments. (C) 2004 Elsevier B.V. All rights reserved.