What Bennu's samples reveal to us.
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 more than 4.5 billion years ago. Analysis of the samples has already revealed new information about the geochemical history of Bennu and the early stages of the formation of the solar system.
This research, published in Nature Geoscience on August 22 and September 11, 2025, was conducted by an international team involving scientists from Université Côte d'Azur and the CNRS, notably from the Joseph-Louis Lagrange Laboratory (CNRS/Observatoire de la Côte d'Azur/Université Côte d'Azur) at the Center for Research on Heteropitaxy and its Applications (CNRS/Université Côte d'Azur). It highlights the major role of fluids in the evolution of the asteroid, providing essential clues about 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 in Nice, scientists have discovered evidence that these minerals have been altered by an aqueous fluid that has evolved over time from neutral to alkaline pH. This process caused the dissolution of certain minerals and the re-precipitation of new ones at around 20-30°C, conditions similar to those observed on the asteroid Ryugu (another primitive asteroid sampled by JAXA's Hayabusa2 mission) and in primitive carbonaceous Ivuna-type (CI) meteorites.
These discoveries, combined with other published findings, confirm that celestial bodies such as Bennu were rich in aqueous fluids shortly after their formation, providing crucial clues about 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 deep understanding of asteroids, the basic “building blocks” that ultimately contributed to the formation of Earth and, perhaps, the emergence of life.
Researcher contacts
Guy Libourel, enseignant-chercheur d’Université Côte d’Azur au laboratoire Joseph-Louis Lagrange (Observatoire de la Côte d’Azur ,Université Côte d’Azur, CNRS), libou@oca.eu, Co-I OSIRIS-REx et coordinateur géographique (France-Europe)
Marc Portail, ingénieur de recherche du CNRS au CRHEA (Université Côte d’Azur, CNRS), Marc.Portail@crhea.cnrs.fr
G.L & M.P remercient le CNES, l’ANR, Université Côte d’Azur et la fédération Doeblin pour leurs soutiens financiers.
Find out more
Bennu up close and mineralogical. Nat. Geosci. 18, 811 (2025). https://do>i.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
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