Nitrogen isotopes in the recent solar wind from the analysis of Genesis targets: Evidence for large scale isotope heterogeneity in the early solar system

摘要

We have analyzed nitrogen, neon and argon abundances and isotopic ratios in target material exposed in space for 27 months to solar wind (SW) irradiation during the Genesis mission. SW ions were extracted by sequential UV (193 nm) laser ablation of gold-plated material, purified separately in a dedicated line, and analyzed by gas source static mass spectrometry. We analyzed gold-covered stainless steel pieces from the Concentrator, a device that concentrated SW ions by a factor of up to 50. Despite extensive terrestrial N contamination, we could identify a non-terrestrial, N-15-depleted nitrogen end-member that points to a 40% depletion of 15 N in solar-wind N relative to inner planets and meteorites, and define a composition for the present-day Sun (N-15/N-14 = [2.26 +/- 0.67] X 10(-3) 2 sigma), which is indistinguishable from that of Jupiter's atmosphere. These results indicate that the isotopic composition of nitrogen in the outer convective zone of the Sun has not changed through time, and is representative of the protosolar nebula. Large N-15 enrichments due to e.g., irradiation, low temperature isotopic exchange, or contributions from N-15-rich presolar components, are therefore required to account for inner planet values.