An ice film with thickness greater than 10 Angstrom was crystallized on a clean Pt(111) surface. Its external surface structure was investigated at 90 K by dynamical low-energy electron diffraction (LEED), followed by molecular dynamics simulations and ab initio quantum chemical calculations. The results favor the common hexagonal ice Ih structure over other forms of ice, with (0001) termination. A full-bilayer termination is found, but with much enhanced amplitudes of motion of the O atoms in the outermost layer of H2O molecules, even at 90 K, so that these molecules were undetected experimentally by LEED.