Water ice, abundant in the outer solar system, is volatile in the inner solar system. On the largest airless bodies of the inner solar system (Mercury, the Moon, Ceres), water can be an exospheric species but also occurs in its condensed form. Mercury hosts water ice deposits in permanently shadowed regions near its poles that act as cold traps. Water ice is also present on the Moon, where these polar deposits are of great interest in the context of future lunar exploration. The lunar surface releases either OH or H₂O during meteoroid showers, and both of these species are generated by reaction of implanted solar wind protons with metal oxides in the regolith. A consequence of the ongoing interaction between the solar wind and the surface is a surficial hydroxyl population that has been observed on the Moon. Dwarf planet Ceres has enough gravity to have a gravitationally-bound water exosphere, and also has permanently shadowed regions near its poles, with bright ice deposits found in the most long-lived of its cold traps. Tantalizing evidence for cold trapped water ice and exospheres of molecular water has emerged, but even basic questions remain open. The relative and absolute magnitudes of sources of water on Mercury and the Moon remain largely unknown. Exospheres can transport water to cold traps, but the efficiency of this process remains uncertain. Here, the status of observations, theory, and laboratory measurements is reviewed.

外太阳系中大量存在的水冰在内太阳系中是易变的。在太阳系内部最大的无空气天体（水星、月球、谷神星）上，水可以是一种外层物质，但也可能以凝聚形式存在。水星在其两极附近的永久阴影区域拥有水冰沉积物，这些区域充当冷阱。月球上也存在水冰，这些极地沉积物在未来的月球探索中具有重要意义。月球表面释放 OH 或 H ₂O 在流星雨期间，这两种物质都是由植入的太阳风质子与风化层中的金属氧化物反应产生的。太阳风和地表之间持续相互作用的结果是在月球上观察到的表面羟基群。矮行星谷神星有足够的引力来形成受引力束缚的水外逸层，并且在其两极附近也有永久阴影区域，在其寿命最长的冷阱中发现了明亮的冰沉积物。冷被困水冰和分子水外逸层的诱人证据已经出现，但即使是基本问题仍然悬而未决。水星和月球上水源的相对和绝对量级在很大程度上仍然未知。Exospheres 可以将水输送到冷阱，但这个过程的效率仍然不确定。这里，