A metastable state mediates the surface disordering of ice Ih

Ice premelting, the formation of a quasi-liquid layer on ice surfaces below the bulk melting point, plays a crucial role in various processes, ranging from glacier dynamics to ice friction and surface chemistry. Despite intensive research, the microscopic structure of the premelting layer and underlying molecular mechanisms remain poorly understood. In this work, we studied the temperature- and pressure-dependent structural disordering of crystalline Ih (0001) surface near the onset of premelting on the atomic scale by qPlus-based cryogenic atomic force microscopy. The linear correlation between the density of planar local structure (PLS) and the fraction of disordered surface region showed that the PLS mediated early-stage premelting by serving as a metastable seeding state. Notably, the associated surface disordering is cooperative, extending over an area of roughly ∼2 nm² around a PLS. We further found a striking structural similarity between the kinetic-trapped regime below the surface crystallization temperature (T_c) and the premelting-dominated regime above T_c. As the deposition pressure increased, the characteristic temperature dependence was preserved, with only T_c shifting to higher values due to kinetic effects. Finally, we proposed a surface phase diagram for ice Ih (0001) based on our experimental observations.