Gradual Destruction of Refractory Stardust in Alkali Brines on Asteroid Bennu

Carbonaceous asteroids preserve presolar grains-refractory stardust that predates the Solar System-recording both their stellar origins and subsequent alteration on their parent asteroids. Here we examine the distribution of presolar grains and isotopically anomalous organic matter in mottled particles from samples returned by the OSIRIS-REx mission from asteroid Bennu. Although the mottled samples contain organic matter with carbon and nitrogen isotopic compositions similar to those of other Bennu samples, they lack presolar silicate or oxide grains and show a markedly reduced abundance of presolar silicon carbide. In this work, we show that these findings are consistent with the slow destruction of silicon carbide grains during late-stage alteration by a low-temperature, oxidizing, alkaline brine. These brines, coinciding with evaporite salt formation, selectively erased chemically robust presolar phases while preserving isotopically anomalous organic matter. This shows that evolved aqueous fluids can fundamentally alter the presolar record of primitive asteroids.