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.

水合小行星可能是类地行星的主要水源。然而，人们对小行星水合程度的控制知之甚少。在这里，我们报告了在 C 型小行星 162173 Ryugu 的样本中发现的多个裂缝和静脉组，这些裂缝和静脉组在其水性蚀变期间充当远端流体迁移的通道。龙宫的早期矿脉饰有草莓状磁铁矿，而晚期矿脉则引起围岩的交代作用。矿脉和裂缝均具有尖头几何形状和复杂的交叉几何形状，与实验过程中形成的冻融裂缝一致。我们表明，冻融可以有效地将直径达 300 公里的 C 型小行星破裂，因此对于含冰小行星中的流体向外迁移至关重要。因此，冻融很可能决定整个宇宙小行星中矿物质水的分布。