Argon, krypton, and xenon in the bulk solar wind as collected by the Genesis mission

Abstract

We present bulk solar wind isotopic and elemental ratios for Ar, Kr, and Xe averaged from up to 14 individual analyses on silicon targets exposed to the solar wind for ~2.3 years during NASA's Genesis mission. All averages are given with 1σ standard errors of the means and include the uncertainties of our absolute calibrations. The isotopic ratios ^(86)Kr/^(84)Kr and ^(129)Xe/^(132)Xe are 0.303 ± 0.001 and 1.06 ± 0.01, respectively. The elemental ratios ^(36)Ar/^(84)Kr and ^(84)Kr/^(132)Xe are 2390 ± 120 and 9.9 ± 0.3, respectively. Average fluxes of ^(84)Kr and ^(132)Xe in the bulk solar wind in atoms/(cm^2 s) are 0.166 ± 0.009 and 0.017 ± 0.001, respectively. The flux uncertainties also include a 2% uncertainty for the determination of the extracted areas. The bulk solar wind ^(36)Ar/^(38)Ar ratio of 5.50 ± 0.01 and the ^(36)Ar flux of 397 ± 11 atoms/(cm^2s) determined from silicon targets agree well with the ^(36)Ar/^(38)Ar ratio and the ^(36)Ar flux determined earlier on a different type of target by Heber et al. (2009). A comparison of the solar wind noble gas/oxygen abundance ratios with those in the solar photosphere revealed a slight enrichment of Xe and, within uncertainties a roughly uniform depletion of Kr–He in the solar wind, possibly related to the first ionization potentials of the studied elements. Thus, the solar wind elemental abundances He–Kr display within uncertainties roughly photospheric compositions relative to each other. A comparison of the Genesis data with solar wind heavy noble gas data deduced from lunar regolith samples irradiated with solar wind at different times in the past reveals uniform ^(36)Ar/^(84)Kr ratios over the last 1–2 Ga but an increase of the ^(84)Kr/^(132)Xe ratio of about a factor of 2 during the same time span. The reason for this change in the solar wind composition remains unknown.

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