Why Is It So Difficult to Identify the Onset of Ice Premelting?

Premelting of ice at temperatures below 0 degrees C is of fundamental importance for environmental processes. Various experimental techniques have been used to investigate the temperature at which liquid-like water first appears at the ice vapor interface, reporting onset temperatures from 160 to 2 degrees C. The signals that identify liquid-like order at the ice vapor interface in these studies, however, do not show a sharp initiation with temperature. That is at odds with the expected first-order nature of surface phase transitions, and consistent with recent large-scale molecular simulations that show the first premelted layer to be sparse and to develop continuously over a wide range of temperatures. Here we perform a thermodynamic analysis to elucidate the origin of the continuous formation of the first layer of liquid at the ice vapor interface. We conclude that a negative value of the line tension of the ice liquid vapor three-phase contact line is responsible for the continuous character of the transition and the sparse nature of the liquid-like domains in the incomplete first layer.