  Experiments and computer simulations have established that liquid water's
surfaces can deviate in important ways from familiar bulk behavior. Even in the
simplest case of an air-water interface, distinctive layering, orientational
biases, and hydrogen bond arrangements have been reported, but an overarching
picture of their origins and relationships has been incomplete. Here we show
that a broad set of such observations can be understood through an analogy with
the basal face of crystalline ice. Using simulations, we specifically
demonstrate that water and ice surfaces share a set of structural features
suggesting the presence of nanometer-scale ice-like domains at the air-water
interface. Most prominent is a shared characteristic layering of molecular
density and orientation perpendicular to the interface. Similarities in
two-point correlations of hydrogen bond network geometry point to shared
ice-like intermolecular structure in the parallel direction as well. Our
results bolster and significantly extend previous conceptions of ice-like
structure at the liquid's boundary, and suggest that the much-discussed
quasi-liquid layer on ice evolves subtly above the melting point into a
quasi-ice layer at the surface of liquid water.
