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
Authors: Flynn George J. , Sutton Steven R. , Lai Barry , Wirick Sue , Allen Carlton , Anderson David , Ansari Asna , Bajt Saša , Bastien Ron K. , Bassim Nabil , Bechtel Hans A. , Borg Janet , Brenker Frank E. , Bridges John , Brownlee Donald E. , Burchell Mark , Burghammer Manfred , Butterworth Anna L. , Changela Hitesh , Cloetens Peter , Davis Andrew M. , Doll Ryan , Floss Christine , Frank David , Gainsforth Zack , Grün Eberhard , Heck Philipp R. , Hillier Jon K. , Hoppe Peter , Hudson Bruce , Huth Joachim , Hvide Brit , Kearsley Anton , King Ashley J. , Leitner Jan , Lemelle Laurence , Leroux Hugues , Leonard Ariel , Lettieri Robert , Marchant William , Nittler Larry R. , Ogliore Ryan , Ong Wei Ja , Postberg Frank , Price Mark C. , Sandford Scott A. , Tresseras Juan‐Angel Sans , Schmitz Sylvia , Schoonjans Tom , Silversmit Geert , Simionovici Alexandre , Sol Vicente A. , Srama Ralf , Stadermann Frank J. , Stephan Thomas , Sterken Veerle , Stodolna Julien , Stroud Rhonda M. , Trieloff Mario , Tsou Peter , Tsuchiyama Akira , Tyliszczak Tolek , Vekemans Bart , Vincze Laszlo , Von Korff Joshua , Westphal Andrew J. , Wordsworth Naomi , Zevin Daniel , Zolensky Michael E. ,   >30,000 Stardust@home dusters
Published: August 2014

The 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.