SIMS studies of Allende projectiles fired into Stardust-type aluminum foils at 6 km/sec

Abstract

We have explored the feasibility of degrees C, N, and O isotopic measurements by NanoSIMS and of elemental abundance determinations by time-of-flight secondary ion mass spectrometry (TOF-SIMS) on residues of Allende projectiles that impacted Stardust-type aluminum foils in the laboratory at 6 km/sec. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with microcraters that were produced during the encounter of the Stardust space probe with comet 81P/Wild-2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF-SIMS. Crater sizes were between 3 and 190 micrometers. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and degrees C and O isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, which is the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of solar system origin. TOF-SIMS studies are restricted to materials from the crater rim. The element ratios of the major rock-forming elements in the Allende projectiles are well-characterized by the TOF-SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits chemical information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially degrees C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.