[Paper Review] On water delivery in the inner solar nebula: Monte Carlo simulations of forsterite hydration

Martina D'Angelo, S. Cazaux+3 more|arXiv (Cornell University)|Aug 19, 2018

Astro and Planetary Science77 references12 citations

TL;DR

This study uses Monte Carlo simulations to model water vapor adsorption on forsterite grains in the inner solar nebula, showing that surface hydration can occur efficiently at temperatures below 500 K and enhanced by water cluster formation and surface diffusion. The results indicate that phyllosilicate formation via nebular condensation is feasible within the solar nebula timescale, potentially delivering up to 10 Earth oceans of water through agglomeration of hydrated dust grains.

ABSTRACT

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.

Motivation & Objective

• To investigate the thermodynamics and kinetics of water vapor adsorption on forsterite surfaces in the early solar nebula.
• To determine the radial and thermal conditions in the protoplanetary disk where efficient hydration of silicate grains could occur.
• To assess the role of water cluster formation and surface diffusion in enhancing water coverage and reducing reaction timescales.
• To estimate the total water delivery potential to Earth via agglomeration of hydrated 0.1 µm grains into planetesimals.
• To evaluate how grain growth and size distribution affect water retention and phyllosilicate formation efficiency.

Proposed method

• The ProDiMo code was used to model the thermochemical structure of the early solar nebula at three evolutionary stages, extracting temperature, pressure, and H₂O vapor abundance profiles within 1 au of the protosun.
• Monte Carlo simulations were applied to model water associative adsorption on a forsterite [100] crystal lattice using adsorption energies representative of the surface.
• The simulations tracked adsorption rates, surface coverage, cluster formation, and lateral diffusion of physisorbed water molecules under varying temperature and water vapor density conditions.
• The model assumed surface-limited reactions with no bulk diffusion initially, providing a conservative lower bound on phyllosilicate formation rates.
• A stoichiometric reaction for forsterite hydration to brucite and serpentine was used to estimate minimum phyllosilicate surface densities.
• Water fraction on grains was calculated based on grain size, surface site occupancy, and mass balance, with sensitivity to grain size distribution assessed.

Experimental results

Research questions

• RQ1 At what temperature and water vapor density conditions in the inner solar nebula can forsterite surface hydration by water vapor reach full coverage?
• RQ2 How does water cluster formation influence surface coverage and stability at high temperatures and low vapor densities?
• RQ3 To what extent does surface diffusion of physisorbed water molecules reduce the timescale for reaching steady-state hydration?
• RQ4 What is the maximum amount of water that could be delivered to Earth via agglomeration of hydrated 0.1 µm dust grains?
• RQ5 How does grain growth and size distribution affect the efficiency of water retention and phyllosilicate formation compared to single-sized grains?

Key findings

• Complete surface coverage of forsterite by water occurs at temperatures between 300 K and 500 K, with full hydration achieved under sufficient water vapor density.
• At temperatures above 600 K, surface coverage drops below 30%, but water cluster formation significantly enhances adsorption efficiency, enabling stable hydration even at low vapor densities.
• Surface diffusion of physisorbed water molecules reduces the timescale to reach steady-state hydration by three orders of magnitude compared to a Simple Collision Theory model.
• The model predicts that agglomeration of hydrated 0.1 µm grains could deliver up to 10 Earth oceans of water to a planetesimal, assuming ideal conditions.
• If grain growth occurs first in a dry environment and water vapor processes the grains afterward, the water delivery potential decreases by two orders of magnitude.
• The lower limit for water fraction adsorbed on grain surfaces is ~10⁻⁵, rising to ~10⁻³ for uniformly 0.1 µm grains, indicating strong dependence on grain size distribution.