Relative Abundances of Heavy Noble Gases from the Polished Aluminum Solar Wind Collector on Genesis

Introduction: Previous elemental ratios of light solar noble gases were determined from the SWC Experiments on Apollo [1] with collection time ranging from 77 minutes (July 1969) to 45 hours (April 1972). These exposures were too short to collect measurable amounts of less abundant heavy noble gases and contamination with fractionated solar wind-rich dust remained a problem. However, the 853 days exposure of Genesis solar wind collectors give us a good platform to measure the abundances of the heavy noble gases and refine those of the light. Here we report the current status of our Ar-Kr-Xe analysis of solar wind captured by Polished Aluminum Collector (PAC). Experimental: PAC was selected for these measurements for several reasons. First, it’s large area, critical for precision of the low-abundance heavy noble gases and, following the mission, is the only unbroken Genesis solar wind collector of significant area available. Second, our earlier experiments with Al and Si films deposited on sapphire revealed that most of the noble gas blank is associated with the interface between the coating and sapphire, and not within the film itself. The solid Al alloy of the PAC has extremely low volume-correlated noble gases. However, in contrast to these expectations, high Xe blanks were found, accompanied by unknown contaminants (possibly organic) at specific isotopes. We first attributed this contamination to a residue of fatty acids contained in the paste used to polish the surface of the PAC (which served as a thermal shield for the battery compartment). However, analyses of non-flight coupons of this material, polished using the same paste, did not demonstrate the same level of contamination nor Xe blank. Apparently the problem originates from the large amounts of solar wind hydrogen and helium implanted in the PAC. Hydrogen reacts with the active getter material, sintered in an inert atmosphere during manufacter, and the internal surfaces of the sample system, which may contain traces of the electropolishing solution and lubricants, releasing otherwise dormant noble gases and contaminants. Although the exact sources of hydrocarbon contamination and elevated Xe and Kr blanks are yet to be determined, the installation of a Pd-filter to remove hydrogen before it reacts with getters has allowed us significantly reduce the contamination and blank problems. Results: We have analyzed three areas of the PAC: 3.5 cm, 0.34 cm and 0.16 cm using UV-laser ablation with incrementally increased power density delivered to the PAC surface. Figure 1 demonstrates the ability of this technique to retrieve implanted solar Ar from different depths and to separate it from surface-correlated terrestrial Ar. In the two intermediate extraction steps the Ar/Ar ratio is 1.3 and 1.4, the purest solar Ar we yet observed (the solar ratio is <1, while the terrestrial ratio = 296).