Helium, neon, and argon composition of the solar wind as recorded in gold and other Genesis collector materials

Abstract

We report compositions and fluxes of light noble gases in the solar wind (SW), extracted by stepped pyrolysis and amalgamation from gold collector materials carried on the Genesis Solar Wind Sample Return Mission. Results are compared with data from other laboratories on SW–He, Ne and Ar distributions implanted in Genesis aluminum, carbon, and silicon collectors and extracted by laser ablation. Corrections for mass-dependent losses (“backscatter”) of impinging SW ions due to scattering from the collector material are substantially larger for gold than for these lower atomic weight targets. We assess such losses by SRIM simulation calculations of SW backscatter from gold which are applied to the measurements to recover the composition of the incident SW.Averaged results of integrated stepped pyrolysis and single-step amalgamation measurements, with 1σ errors, are as follows: for SW–Ne and Ar isotope ratios (3He/4He was not measured), 20Ne/22Ne=14.001±0.042, 21Ne/22Ne=0.03361±0.00018, 36Ar/38Ar=5.501±0.014; for SW element ratios, 4He/20Ne=641±15, 20Ne/36Ar=51.6±0.5; and for SW fluxes in atomscm−2s−1 at the Genesis L1 station, 4He=1.14±0.04×107, 20Ne=1.80±0.06×104, 36Ar=3.58±0.11×102. Except for the 21Ne/22Ne and 20Ne/36Ar ratios, these values are in reasonable accord (within ∼1–3σ) with measurements on different collector materials reported by one or both of two other Genesis noble gas research groups. We further find, in three stepped pyrolysis experiments on gold foil, that He, Ne and Ar are released at increasing temperatures without elemental fractionation, in contrast to a pyrolytic extraction of a single non-gold collector (Al) where the release patterns point to mass-dependent thermal diffusion. The pyrolyzed gold foils exhibit enhancements, relative to sample totals, in 20Ne/22Ne and 21Ne/22Ne ratios evolved at low temperatures. The absence of elemental fractionation in pyrolytic release from gold implies that these isotope ratio enhancements, involving much smaller mass differences, do not result from preferential diffusive release of the lighter Ne isotopes. This effect, not predicted by SRIM calculations, has also been observed in stepped acid-etch releases from a different Genesis collector material in another laboratory.