Micro- and nanoscale studies of insoluble organic matter and C-rich presolar grains in Murchison and Sutter's Mill in preparation for Bennu sample analysis

Abstract

Samples of B-type asteroid (101955) Bennu returned by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft will provide unique insight into the nature of carbonaceous asteroidal matter without the atmospheric entry heating or terrestrial weathering effects associated with meteoritic samples. Some of the Bennu samples will undergo characterization by X-ray computed tomography (XCT). To protect the pristine nature of the samples, it is important to understand any adverse effects that could result from irradiation during XCT analysis. We analyzed acid-insoluble residues produced from two powdered samples of the Murchison carbonaceous chondrite, one control and one XCT-scanned, to assess the impact on insoluble organic matter (IOM) and presolar grains. Using a suite of in situ analytical techniques (field-emission scanning electron microscopy, optical and ultraviolet fluorescence microscopy, microprobe two-step laser mass spectrometry, and nanoscale secondary ion mass spectrometry), we found that the two residues had indistinguishable chemical, molecular, and isotopic signatures on the micron to submicron scale, indicating that an X-ray dosage of 180 Gy (the maximum dose to be used during preliminary examination of Bennu materials) did not damage the IOM and presolar grains. To explore the use of acid-insoluble residues to infer parent body processes in preparation for Bennu sample analysis, we also analyzed a residue produced from the Sutter's Mill carbonaceous chondrite. Multiple lines of evidence, including severely degraded UV fluorescence signatures and D-rich hotspots, indicate that the parent body of Sutter's Mill was heated to >400°C. This heating event was likely short lived because the abundance of presolar SiC grains, which are destroyed by thermal metamorphism and prolonged oxidation, was consistent with those in Murchison and other unheated chondrites. The results of these in situ analyses of acid-insoluble residues from Murchison and Sutter's Mill provide complementary detail to bulk analyses.