Evidence for Impact Mixing Among Rocky Planetesimals in the Early Solar System Due to Early Giant Planet Migration

Chemical and structural characterization of relict olivine grains in NWA 12320. Credit: Rider-Stokes et al., 2023.

The structure and dynamic evolution of the early solar system have been debated, particularly with regard to the extent, mechanism, and timing of mixing between the inner and outer solar system. Dynamical models such as the Grand Tack propose that Jupiter, due to orbital resonance with Saturn, migrated into the inner solar system within the first few million years of the solar system’s history, causing rocky planetesimals to be scattered inward. It then migrated back outward, causing some rocky planetesimals to be scattered out into the main asteroid belt. However, there has been little observational evidence to test the Grand Tack model.

A new study by Ben Rider-Stokes (Open University) and co-authors reports oxygen isotope measurements of angrites, a group of differentiated meteorites representing some of the oldest material from the inner solar system. They analyzed some angrites that appear to have been impact-melted and found that the groundmass of these samples shows a distinctly different oxygen isotope signature (negative Δ17O) from bulk rock samples and olivine crystals. These results suggest that angrites (or at least some of them) experienced large-scale impact melting and mixing with an impactor derived from a different region of the protoplanetary disk. Although it has been proposed that angrites record the mixing of inner and outer solar system material, a chondritic impactor cannot explain these O isotope results. The author suggests that a differentiated, inner solar system body having a positive Δ17O impacted the angrite parent body, which had a Δ17O value close to that of bulk Earth. If this is correct, it implies that displacement of the angrite parent body from the inner solar system into the main asteroid belt occurred early and may represent evidence for the Grand Tack migration. This hypothesis is novel, as most studies designed to test the Grand Tack focus on mixing between inner and outer solar system bodies. READ MORE