Abundant supernova dust and heterogeneous aqueous alteration revealed by stardust in two lithologies of asteroid Bennu

The oldest constituents in chondritic samples are presolar grains that condensed in the outflows and explosions of dying stars. These grains divulge the types and concentrations of dust that seeded our Solar System. However, they are subject to destruction during planetesimal formation and alteration. We conducted a detailed study of presolar grains in fragments of asteroid Bennu to elucidate the alteration history of distinct lithologies—angular and hummocky—and the sources of stardust that were accreted by the parent asteroid. The presolar grain abundances support a history of substantial aqueous alteration. Nevertheless, we found organic-rich clasts within a hummocky particle having higher presolar silicate abundances, akin to some of the least altered chondritic meteorites, and presolar silicate, oxide and SiC grains that retain their crystallinity. These clasts illustrate that aqueous alteration was heterogeneous within the parent body and their properties may better represent the starting materials that accreted to form the protolith. In addition, the Bennu samples we analysed have a six-times greater proportion of C-rich supernova dust than other chondritic samples, injected perhaps from a nearby supernova. This observation adds to evidence that Bennu’s parent body sampled a region of the protoplanetary disk having a distinct mixture of starting materials.