Water activity recorded in asteroids offers a perspective on their habitability. Aqueous alteration, namely the interaction of liquid water with minerals, occurred in the parent body of the C-type asteroid Ryugu. However, Ryugu is not a wet body, and the processes that led to the loss of liquid water along with its chemical environment remain unclear. We report the presence of sodium carbonates, chlorides and sulfates in Ryugu samples, which indicate that alkaline, salt-rich water once flowed through its parent body. Highly concentrated brine probably formed through evaporation or freezing of the liquid water in the final stages of aqueous alteration. Similar processes may have occurred in carbonaceous asteroids in the Solar System, although terrestrial weathering of meteorites might obscure evidence of salt precipitation. Sodium salts could be crucial for comparing the evolved water in carbonaceous bodies and alkaline subsurface oceans in the dwarf planet Ceres and the moons of Jupiter and Saturn.

小行星中记录的水活动为它们的宜居性提供了视角。水蚀变，即液态水与矿物的相互作用，发生在 C 型小行星 Ryugu 的母体中。然而，Ryugu 并不是一个湿体，导致液态水及其化学环境损失的过程仍不清楚。我们报告了 Ryugu 样品中存在碳酸钠、氯化物和硫酸盐，这表明碱性、富含盐分的水曾经流过其母体。高浓度盐水可能是通过在水蚀变的最后阶段蒸发或冻结液态水而形成的。类似的过程可能发生在太阳系的碳质小行星中，尽管陨石的陆地风化可能会掩盖盐分沉淀的证据。钠盐对于比较矮行星谷神星以及木星和土星卫星的碳质天体和碱性地下海洋中的逸出水可能至关重要。