Bennu Isotopes Match CI Meteorites, Placing Formation at Solar System Water-Ice Line

The Science Advances study by Maria Schönbächler’s group at ETH Zurich used thermal ionization mass spectrometry on Bennu grains returned by OSIRIS-REx. Isotopic homogeneity across Fe, Ti and Cr rules out late-stage mixing and instead points to rapid accretion in a narrow radial zone where inward-drifting inner-Solar-System dust met outward-migrating icy particles. This hybrid signature overturns the prior assumption that primitive asteroids formed beyond Jupiter’s orbit.

The same isotopic pattern appears in Ryugu samples and the rare CI chondrites, forming a distinct reservoir that never fed the terrestrial planets. Jupiter’s early growth likely truncated this reservoir, preventing further radial transport and preserving the observed chemical boundary. The finding reframes how water and organics were delivered to the inner Solar System.

Future missions targeting main-belt objects with similar spectra can now be prioritized by predicted isotopic match rather than dynamical class alone. Sample-return from a verified CI-like body would test whether the water-ice-line mixing zone was a single event or repeated over several million years.

The primary limitation remains the half-gram allocation; larger masses and spatially resolved analyses are required to confirm whether the homogeneity holds at the millimeter scale.