Placing asteroid Bennu’s organic solids in molecular and elemental context with those in aqueously altered carbonaceous chondrites

We present the first investigation into the molecular structure of organic solids (insoluble organic matter, IOM) in samples of the carbonaceous asteroid (101955) Bennu returned by the OSIRIS-REx mission. We used¹H and¹³C solid-sate nuclear magnetic resonance (ssNMR) to analyze three subsamples of aggregate Bennu material. However, the IOM isolated from two of the three subsamples exhibited substantial magnetic inhomogeneity, due to contaminant magnetic grains. The resulting magnetic interference degraded NMR signals for both¹H and¹³C and likely introduced spectral distortions. The third subsample was pretreated with 6 N HCl prior to IOM isolation and exhibited minimal (i.e., typical) magnetic interference. In this subsample’s IOM, we find a very low fraction of aromatic carbon, and a high fraction of aliphatic hydrogen, relative to IOM from Bennu’s closest meteoritic analogs, the petrologic type 1 and 2 carbonaceous chondrites. Elemental analysis–isotope ratio mass spectrometry (EA-IRMS) further reveals a high H/C × 100 atomic values, relative to type 1 and 2 chondritic IOM. These data indicate that Bennu’s organic solids, at least in this aggregate sample, suffered minimal to no molecular evolution from thermal perturbation throughout this material’s long history—starting with accretion of a planetesimal, followed by disruption and gravitational reassembly to form a rubble-pile asteroid, and ultimately migration from the Main Belt to a near-Earth orbit. The state of molecular evolution recorded in IOM places a strong constraint on the magnitude of temperature and pressure derived from impact events that yielded the rubble-pile asteroid Bennu.