Bennu: key clues to the origins of the solar system

On September 24, 2023, NASA's OSIRIS-REx mission brought back to Earth 121.6 grams of samples from the asteroid Bennu, a primitive body rich in water and organic compounds. A veritable time capsule, this material offers a unique opportunity to explore the conditions that prevailed in the solar nebula over 4.5 billion years ago. Their analyses have already revealed unprecedented insights into Bennu's geochemical history and the early stages of solar system formation.

The research, published in Nature Geoscience on August 22 and September 11, 2025, was carried out by an international team involving scientists from the Université Côte d'Azur and the CNRS, notably from the Laboratoire de la Recherche et de la Recherche de la Côte d'Azur.Azur and CNRS, notably from the Joseph-Louis Lagrange laboratory (CNRS/Observatoire de la Côte d'Azur/Université Côte d'Azur) and the Centre de recherche sur l'hétéroepitaxie et ses applications (CNRS/ Université Côte d'Azur). They underline the major role of fluids in the evolution of the asteroid, providing essential clues as to the conditions that may have led to the synthesis of prebiotic organic molecules.

The international team is responsible for the detailed characterization of the Bennu samples. Mineralogical analyses, carried out using electron microscopy and X-ray diffraction, reveal that the samples are mainly composed of nanoscale hydrated silicates, such as serpentine and saponite. These minerals are interspersed with iron sulfides, magnetite and carbonates.

Thanks in particular to cathodoluminescence studies carried out by teams from Nice, scientists have discovered clues that these minerals have been altered by an aqueous fluid that has evolved over time from a neutral to an alkaline pH. This process led to the dissolution of certain minerals and the re-precipitation of new ones at around 20-30°C, conditions similar to those observed on theasteroid Ryugu (another primitive asteroid sampled by JAXA's Hayabusa2 mission) and in primitive Ivuna-type carbonaceous meteorites (CI).

These discoveries, combined with other published results, confirm that celestial bodies like Bennu were rich in aqueous fluids shortly after their formation, offering crucial clues as to the conditions that may have led to the synthesis of prebiotic organic molecules. Ultimately, these sample return missions and the microscopic analyses they enable are invaluable. They offer us a profound understanding of asteroids, the elementary "building blocks" that ultimately contributed to the formation of the Earth and, perhaps, to the emergence of life.

Research contacts

• Guy Libourel, teacher-researcher at Université Côte d'Azur's Joseph-Louis Lagrange laboratory (Observatoire de la Côte d'Azur ,Université Côte d'Azur, CNRS), libou@oca.eu, Co-I OSIRIS-REx and geographical coordinator (France-Europe)
• Marc Portail, CNRS research engineer at CRHEA (Université Côte d'Azur, CNRS), Marc.Portail@crhea.cnrs.fr

G.L & M.P would like to thank CNES, ANR, Université Côte d'Azur and the Doeblin federation for their financial support.

For further information

Bennu up close and mineralogical. Nat. Geosci. 18, 811 (2025). https://doi.org/10.1038/s41561-025-01799-w

Mineralogical evidence for hydrothermal alteration of Bennu samples. Zega, T.J., McCoy, T.J., Russell, S.S. et al. Nat. Geosci. 18, 832-839 (2025). https://doi.org/10.1038/s41561-025-01741-0

Composition of asteroid Bennu transformed by aqueous alteration. Nat. Geosci. 18, 819-820 (2025). https://doi.org/10.1038/s41561-025-01765-6