  Endogenous or exogenous, dry or wet, various scenarios have been so far
depicted for the origin of water on our Solar Systems rocky bodies. Hydrated
silicates found in meteorites and in interplanetary dust particles together
with observations of abundant water reservoirs in the habitable zone of
protoplanetary disks are evidences that support aqueous alteration of silicate
dust grains by water vapor condensation in a nebular setting. We investigate
the thermodynamics (temperature and pressure dependencies) and kinetics
(adsorption rates and energies, surface diffusion and cluster formation) of
water adsorption on surfaces of forsterite grains. (abbreviated) The
protoplanetary disk model (ProDiMo) code is tuned to simulate the
thermochemical disk structure of the early solar nebula at three evolutionary
stages. Pressure, temperature and water vapor abundance within 1 au from the
protosun were extracted and used as input for a Monte Carlo code to model water
associative adsorption using adsorption energies that resemble the forsterite
[100] crystal lattice. Hydration of forsterite surfaces by water vapor
adsorption could have occurred within the nebula lifetime already at a density
of 1e8 cm-3, with increasing surface coverage for higher water vapor densities.
Full surface coverage is attained for temperatures lower than 500 K, while for
hotter grain surfaces water cluster formation plays a crucial role.
(abbreviated) This work shows that water cluster formation enhances the water
surface coverage and enables a stable water layer to form at high temperature
and low water vapor density conditions. Finally, surface diffusion of
physisorbed water molecules shortens the timescale for reaching steady state,
enabling phyllosilicate formation within the solar nebula timescale.
