Evidence from 162173 Ryugu for the influence of freeze–thaw on the hydration of asteroids

Hydrated asteroids are likely to be main source of water for the terrestrial planets. The controls on the extent of asteroid hydration, however, are poorly understood. Here we report the discovery of multiple fracture and vein sets in a sample from the C-type asteroid 162173 Ryugu that acted as pathways for the migration of distal fluids during its aqueous alteration. Early veins in Ryugu are decorated with framboidal magnetite, while later veins caused metasomatism of wall rocks. Both veins and fractures have cuspate geometries and complex intersecting geometries consistent with freeze–thaw fractures formed during experiments. We show that freeze–thaw is effective in fracturing C-type asteroids to up to 300 km in diameter and is thus crucial in the outwards migration of fluids in ice-bearing asteroids. Freeze–thaw is likely, therefore, to determine the distribution of mineral-hosted water in asteroids throughout the Cosmos.