  The reentrant layering transitions found in rare gas adsorption on solid
substrates have conflictually been explained either in terms of preroughening
(PR), or of top layer melting-solidification phenomena. We obtain adsorption
isotherms of Lennard-Jones particles on an attractive substrate by off lattice
Grand Canonical Monte Carlo (GCMC) simulation, and reproduce reentrant
layering. Microscopic analysis, including layer-by-layer occupancies, surface
diffusion and pair correlations, confirms the switch of the top surface layer
from solid to quasi-liquid across the transition temperature. At the same time,
layer occupancy is found at each jump to switch from close to full to close to
half, indicating a disordered flat (DOF) surface and establishing preroughening
as the underlying mechanism. Our results suggest that top layer melting is
essential in triggering preroughening, which thus represents the threshold
transition to surface melting in rare gas solids.
