Context. The origin of the reservoirs of water on Earth is debated. The Earth's crust may contain at least three times more water than the oceans. This crust water is found in the form of phyllosilicates, whose origin probably differs from that of the oceans. Aims. We test the possibility to form phyllosilicates in protoplanetary disks, which can be the building blocks of terrestrial planets. Methods. We developed an exploratory rate-based warm surface chemistry model where water from the gas-phase can chemisorb on dust grain surfaces and subsequently diffuse into the silicate cores. We applied the phyllosilicate formation to a zero-dimensional chemical model and to a 2D protoplanetary disk model (PRODIMO). The disk model includes in addition to the cold and warm surface chemistry continuum and line radiative transfer, photoprocesses (photodissociation, photoionisation, and photodesorption), gas-phase cold and warm chemistry including three-body reactions, and detailed thermal balance. Results. Despite the high energy barrier for water chemisorption on silicate grain surfaces and for diffusion into the core, the chemisorption sites at the surfaces can be occupied by a hydroxyl bond (-OH) at all gas and dust temperatures from 80 to 700 K for a gas density of 2 × 104 cm-3. The chemisorption sites in the silicate cores are occupied at temperatures between 250 and 700 K. At higher temperatures thermal desorption of chemisorbed water occurs. The occupation efficiency is only limited by the maximum water uptake of the silicate. The timescales for complete hydration are at most 105 yr for 1 mm radius grains at a gas density of 108 cm-3. Conclusions. Phyllosilicates can be formed on dust grains at the dust coagulation stage in protoplanetary disks within 1 Myr. It is however not clear whether the amount of phyllosilicate formed by warm surface chemistry is sufficient compared to that found in Solar System objects.

Context. Endogenous or exogenous, dry or wet, various scenarios have been depicted for the origin of water on the rocky bodies in our solar system. 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 evidence that support aqueous alteration of silicate dust grains by water vapor condensation in a nebular setting. Aims. 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, constraining the location in the solar nebula where aqueous alteration of silicates by water vapor adsorption can occur efficiently and leads to the formation of phyllosilicates. We analyze the astrophysical conditions favorable for such hydration mechanism and the implications for water on solid bodies. Methods. 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 of 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 [1 0 0] crystal lattice. Results. Hydration of forsterite surfaces by water vapor adsorption could have occurred within the nebula lifetime already at a density of 10 ⁸ 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. Between 0.5 and 10 Earth oceans can arise from the agglomeration of hydrated 0.1 μm grains into an Earth-sized planet. However, if grain growth occurs dry and water vapor processes the grains afterward, this value can decrease by two orders of magnitude. Conclusions. 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.