Measuring the elemental abundance and isotopic signature of solar wind xenon collected by the Genesis mission

Abstract

Laser heating and ablation techniques are evaluated for quantitative extraction (for isotopic analysis) of implanted solar wind xenon from (silicon collector) targets flown on the solar wind sample return mission Genesis. The potential to correct for contributions from other sources by concurrent analyses of unflown silicon samples is also evaluated and appears to be a valid approach. Xenon measurements were made using the RELAX mass spectrometer. Both infra-red laser heating and ultra-violet laser ablation methods are evaluated. The preferred protocol uses a ∼350 mJ 355 nm laser beam to ablate the full surface of square samples typically 3–5 mm in dimension. A single, initial laser shot is used to remove surface adsorbed contamination, which is analysed. 30 consecutive shots then extract the implanted solar wind xenon for analysis. Further analyses are made after each of four 30-shot extractions, and one final 150-shot extraction. There is no correlation between intrinsic xenon concentrations derived from the first series of 30 shots and from the final 150 shots. More than 90% of the implanted solar wind is extracted in the first series of 30 laser shots. Intrinsic xenon concentrations vary by a factor of ∼10 in these analyses. These data indicate that, for the major isotopes (excluding ¹²⁴Xe and ¹²⁶Xe), the solar wind composition sampled by the Genesis mission is identical to that determined from lunar regolith samples irradiated by the solar wind within the past ∼100 Ma, with at most minor (<5‰ per amu) fractionation favouring the heavy isotopes. Since our data show that the major isotope composition of solar wind xenon deduced from the young lunar regolith is accurate (within a linear mass fractionation), we confirm that the Earth's atmosphere is depleted in the heavy isotopes relative to this composition.