Survival of Ancient Lunar Water Affected by Topographic Degradation of Old, Large Complex Craters

Lunar water is redistributed by various processes. Topographic diffusion affects the transport of water and contributes to its preservation in subsurface layers. Here, we analyze 16 complex craters (similar to 3.2 - similar to 4.2 Ga) larger than 20 km in diameter near the lunar south pole to quantify their degradation states. The results show that the diffusive rates of late Imbrian craters are similar to those of similarly aged simple craters, while Nectarian and pre-Nectarian craters are degraded less efficiently. Within a complex crater, the mass accumulation by topographic diffusion and ejecta blankets from other complex craters preserves water, ranging up to 1 wt% on average. However, impact mixing and internal heat further limit the stability of ancient water to subsurface regions with ages younger than 3.9 Ga and at depth from a few meters to 10s of meters.