An evaporite sequence from ancient brine recorded in Bennu samples

Article An evaporite sequence from ancient brine recorded in Bennu samples https://doi.org/10.1038/s41586-024-08495-6 Received: 31 July 2024 Accepted: 5 December 2024 Published online: 29 January 2025 Open access Check for updates T. J. McCoy1,30 ✉, S. S. Russell2,30, T. J. Zega3, K. L. Thomas-Keprta4, S. A. Singerling5, F. E. Brenker5, N. E. Timms6, W. D. A. Rickard7, J. J. Barnes3, G. Libourel8, S. Ray1,29, C. M. Corrigan1, P. Haenecour3, Z. Gainsforth9, G. Dominguez10, A. J. King2, L. P. Keller11, M. S. Thompson12, S. A. Sandford13, R. H. Jones14, H. Yurimoto15, K. Righter11,16, S. A. Eckley4, P. A. Bland6, M. A. Marcus17, D. N. DellaGiustina3, T. R. Ireland18, N. V. Almeida2, C. S. Harrison2, H. C. Bates2, P. F. Schofield2, L. B. Seifert11, N. Sakamoto19, N. Kawasaki15, F. Jourdan6,7, S. M. Reddy6, D. W. Saxey7, I. J. Ong3, B. S. Prince3, K. Ishimaru3, L. R. Smith3, M. C. Benner3, N. A. Kerrison3, M. Portail20, V. Guigoz20, P.-M. Zanetta21, L. R. Wardell1, T. Gooding1, T. R. Rose1, T. Salge2, L. Le4, V. M. Tu4, Z. Zeszut3, C. Mayers6, X. Sun7, D. H. Hill3, N. G. Lunning11, V. E. Hamilton22, D. P. Glavin23, J. P. Dworkin23, H. H. Kaplan23, I. A. Franchi24, K. T. Tait25, S. Tachibana26, H. C. Connolly Jr.3,27,28 & D. S. Lauretta3 Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth1 and detected by remote sensing on icy bodies in the outer Solar System2,3. The mineralogical evolution of these brines is well understood in regard to terrestrial environments4, but poorly constrained for extraterrestrial systems owing to a lack of direct sampling. Here we report the occurrence of salt minerals in samples of the asteroid (101955) Bennu returned by the OSIRIS-REx mission5. These include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine that existed early in the history of Bennu’s parent body. Discovery of diverse salts would not be possible without mission sample return and careful curation and storage, because these decompose with prolonged exposure to Earth’s atmosphere. Similar brines probably still occur in the interior of icy bodies Ceres and Enceladus, as indicated by spectra or measurement of sodium carbonate on the surface or in plumes2,3. Brines (over 3.5 wt% dissolved solids) are environments in which life could have evolved or might persist in the Solar System6, and are targets for spacecraft exploration. Evaporation or freezing can lead to the formation of brines from which a variety of minerals (for example, carbonates, sulfates and halides) precipitate. On Earth, such mineral deposits are a major source of technologically critical elements7. On Mars, brine freezing points extend to approximately −20 °C, prolonging the liquid state of water8. Icy outer Solar System bodies contain subsurface brines, sometimes as oceans. Evidence of subsurface brines is found on Saturn’s moon Enceladus9 and the dwarf planet Ceres3, the largest body in the asteroid belt. Our knowledge of brines beyond Earth is hampered by a lack of samples. Remote-sensing observations of Mars, Ceres and Enceladus limit our ability to determine precipitated phases in minor to trace abundances, unravel the age and timing of fluid evolution and precipitation and determine the compositions of the associated fluids. Evaporite phases known from meteorites are extremely limited. These include sulfates and halides in Martian nakhlites10, intrusive igneous rocks that experienced secondary alteration, and potentially indigenous halite in ordinary chondrites11. Early analyses of samples from Bennu recorded evidence of pervasive aqueous alteration—including hydrated phyllosilicate clay minerals, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA. 2Planetary Materials Group, Natural History Museum, London, UK. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA. 4Jacobs Technology, NASA Johnson Space Center, Houston, TX, USA. 5Schwiete Cosmochemistry Laboratory, Goethe University, Frankfurt, Germany. 6Space Technology and Science Centre, School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia. 7John de Laeter Centre, Curtin University, Perth, Western Australia, Australia. 8Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France. 9Space Sciences Laboratory, University of California, Berkeley, CA, USA. 10Department of Physics, California State University, San Marcos, CA, USA. 11Astromaterials Research & Exploration Science (ARES), NASA Johnson Space Center, Houston, TX, USA. 12Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA. 13NASA Ames Research Center, Moffett Field, CA, USA. 14Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK. 15Natural History Sciences, Hokkaido University, Sapporo, Japan. 16Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, USA. 17Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 18School of the Environment, University of Queensland, St Lucia, Queensland, Australia. 19Creative Research Institution, Hokkaido University, Sapporo, Japan. 20Université Côte d’Azur, CNRS, CRHEA, Valbonne, France. 21CNRS, Université Jean Monnet Saint-Étienne, Saint-Etienne, France. 22Southwest Research Institute, Boulder, CO, USA. 23NASA Goddard Space Flight Center, Greenbelt, MD, USA. 24Faculty of Science, Technology, Engineering & Mathematics, Open University, Milton Keynes, UK. 25Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada. 26Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan. 27Department of Geology, Rowan University, Glassboro, NJ, USA. 28Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA. 29Present address: Los Alamos National Laboratory, Los Alamos, NM, USA. 30These authors contributed equally: T. J. McCoy, S. S. Russell. ✉e-mail: 1 3 1072 | Nature | Vol 637 | 30 January 2025 a b c D c ph M mt Na Si P Ca Fe Fig. 1 | Carbonate occurrences and textures in Bennu samples. a, Multiphase particle exhibiting successive growth of calcite (c), magnetite (mt) and zoned Mg,Na phosphate (ph), set in a matrix of phyllosilicates (multielement X-ray map; sample no. OREX-803065-0). b, HR-CL image of zoned carbonate. A core of Fe,Mn-bearing magnesite (M) appears dark (non-luminescent) and is surrounded by multiple dolomite overgrowths (D) exhibiting a range of luminescence intensity (dull to bright) around 650 nm due to Mn variations. Phase identification confirmed by SEM (sample no. OREX-800045-106). c, X-ray map of molecular carbonate acquired using scanning transmission X-ray microscopy by integration of the area under (...truncated)