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

Hard X‐ray, quantitative, fluorescence elemental imaging was performed on the ID22NI nanoprobe and ID22 microprobe beam lines of the European Synchrotron Research facility (ESRF) in Grenoble, France, on eight interstellar candidate impact features in the framework of the NASA Stardust Interstellar Preliminary Examination (ISPE). Three features were unambiguous tracks, and the other five were identified as possible, but not definite, impact features. Overall, we produced an absolute quantification of elemental abundances in the 15 ≤ Z ≤ 30 range by means of corrections of the beam parameters, reference materials, and fundamental atomic parameters. Seven features were ruled out as interstellar dust candidates (ISDC) based on compositional arguments. One of the three tracks, I1043,1,30,0,0, contained, at the time of our analysis, two physically separated, micrometer‐sized terminal particles, the most promising ISDCs, Orion and Sirius. We found that the Sirius particle was a fairly homogenous Ni‐bearing particle and contained about 33 fg of distributed high‐Z elements (Z > 12). Orion was a highly heterogeneous Fe‐bearing particle and contained about 59 fg of heavy elements located in hundred nanometer phases, forming an irregular mantle that surrounded a low‐Z core. X‐ray diffraction (XRD) measurements revealed Sirius to be amorphous, whereas Orion contained partially crystalline material (Gainsforth et al. 2014). Within the mantle, one grain was relatively Fe‐Ni‐Mn‐rich; other zones were relatively Mn‐Cr‐Ti‐rich and may correspond to different spinel populations. For absolute quantification purposes, Orion was assigned to a mineralogical assemblage of forsterite, spinel, and an unknown Fe‐bearing phase, while Sirius was most likely composed of an amorphous Mg‐bearing material with minor Ni and Fe. Owing to its nearly chondritic abundances of the nonvolatile elements Ca, Ti, Co, and Ni with respect to Fe, in combination with the presence of olivine and spinel as inferred from XRD measurements, Orion had a high probability of being extraterrestrial in origin.