Topologically disordered mesophase at the topmost surface layer of crystalline ice between 120 and 200 K

The structure and dynamics of an ice surface upon melting are of paramount importance in a variety of phenomena on earth and in the universe because undercoordinated water molecules at an ice surface have peculiar physical and chemical properties distinct from fully coordinated molecules in bulk. Using surface-specific sum-frequency generation spectroscopy and molecular dynamics simulations, we demonstrate that a topologically disordered hydrogen-bond network emerges at the topmost surface layer of crystalline ice Ih(0001) at $\ensuremath{\sim}120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ that is much lower than the conventionally believed premelting temperature of $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The ice surface undergoes a cascade of structural transitions from a low-temperature solid phase to high-temperature quasiliquid phase via the topologically disordered mesophase between 120 and 200 K. Because the lower limit of temperature of the earth's atmosphere is $\ensuremath{\sim}120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ around the mesopause, the topmost surface layers of crystalline ice on earth are unlikely to be the perfectly ordered solid.