Stardust Interstellar Preliminary Examination VII: Synchrotron X‐ray fluorescence analysis of six Stardust interstellar candidates measured with the Advanced Photon Source 2‐ID‐D microprobe

Abstract

AbstractThe NASA Stardust spacecraft exposed an aerogel collector to the interstellar dust passing through the solar system. We performed X‐ray fluorescence element mapping and abundance measurements, for elements 19 ≤ Z ≤ 30, on six “interstellar candidates,” potential interstellar impacts identified by Stardust@Home and extracted for analyses in picokeystones. One, I1044,3,33, showed no element hot‐spots within the designated search area. However, we identified a nearby surface feature, consistent with the impact of a weak, high‐speed particle having an approximately chondritic (CI) element abundance pattern, except for factor‐of‐ten enrichments in K and Zn and an S depletion. This hot‐spot, containing approximately 10 fg of Fe, corresponds to an approximately 350 nm chondritic particle, small enough to be missed by Stardust@Home, indicating that other techniques may be necessary to identify all interstellar candidates. Only one interstellar candidate, I1004,1,2, showed a track. The terminal particle has large enrichments in S, Ti, Cr, Mn, Ni, Cu, and Zn relative to Fe‐normalized CI values. It has high Al/Fe, but does not match the Ni/Fe range measured for samples of Al‐deck material from the Stardust sample return capsule, which was within the field‐of‐view of the interstellar collector. A third interstellar candidate, I1075,1,25, showed an Al‐rich surface feature that has a composition generally consistent with the Al‐deck material, suggesting that it is a secondary particle. The other three interstellar candidates, I1001,1,16, I1001,2,17, and I1044,2,32, showed no impact features or tracks, but allowed assessment of submicron contamination in this aerogel, including Fe hot‐spots having CI‐like Ni/Fe ratios, complicating the search for CI‐like interstellar/interplanetary dust.