Analysis of layered boulders on asteroid (101955) Bennu and their implications for fluid flow on the parent body

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.