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.

The observation of carbonate veins on asteroid Bennu supports the idea that large-scale water flow may have occurred in carbonaceous asteroids in the early solar system. We identified and analyzed 11 boulders with layered structures on asteroid Bennu's surface using high-resolution (centimeter-scale) image and altimetry data obtained by the OSIRIS-REx mission. The boulders' linear layer boundaries and parallel bedding follow the principle of original horizontality and suggest that they formed from sediment deposition by fluid flow on Bennu's parent body. We developed a simple model of the parent body (100-km diameter with the density of CM chondrite material) and found that the water flow velocity had to be at least 21.1 cm s-1 to transport the largest clast observed embedded in a layered rock, which is 85 cm in average length. The flow velocity could have been as high as 26.5 cm s-1 if a larger clast observed on top of a layered rock was once embedded therein. Our results strongly support open-system aqueous alteration on carbonaceous chondrite parent bodies.

K. Ishimaru1, D. S. Lauretta1, N. Porter1, D. R. Golish1, M. Al Asad2, R. L. Ballouz1, O. S. Barnouin3, K. N. Burke1, M. G. Daly4, D. N. DellaGiustina1, B. Rizk1, and K. J. Walsh5. 1Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ, 85721, USA, 2Department of Earth Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada, 3The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, 4The Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada, 5Southwest Research Institute, Boulder, CO, USA. (kana@orex.lpl.arizona.edu)

S. Lauretta1, N. Porter1, D. R. Golish1, M. M. Al Asad2, R.-L. Ballouz1, O. S. Barnouin3, K. N. Burke1, M. G. Daly4, D. N. DellaGiustina1, B. Rizk1, and K. J. Walsh5. 1Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ, 85721, USA, 2Department of Earth Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada, 3The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, 4The Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada, 5Southwest Research Institute, Boulder, CO, USA. (kana@email.arizona.edu)

The composition of asteroids and their connection to meteorites provide insight into geologic processes that occurred in the early Solar System. We present spectra of the Nightingale crater region on near-Earth asteroid Bennu with a distinct infrared absorption around 3.4 micrometers. Corresponding images of boulders show centimeters-thick, roughly meter-long bright veins. We interpret the veins as being composed of carbonates, similar to those found in aqueously altered carbonaceous chondrite meteorites. If the veins on Bennu are carbonates, fluid flow and hydrothermal deposition on Bennu's parent body would have occurred on kilometer scales for thousands to millions of years. This suggests large-scale, open-system hydrothermal alteration of carbonaceous asteroids in the early Solar System.