Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p51a0903s&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P51A-0903
Physics
6213 Dust
Scientific paper
Ther'e is no consensus on the origin of water in the inner solar system. One group of theories envisages the delivery of water to the Earth by means of comets and asteroids after the planet acquired about 85% of its mass formation. However, isotopic and geochemical fingerprints seem to indicate that comets and asteroids alone could not have been the principal source of water for the Earth (Morbidelli et al. 2000; Kleine et al. 2002). These discrepancies could be avoided if the Earth acquired its water locally. We explore the role of adsorption onto grains prior to planetary accretion as a possible new mechanism that could bring water to the Earth. Atomistic simulation can be employed to investigate the interaction between volatiles and materials found in the nebula. We have been exploring how water adsorbs on olivine with the goal of understanding the energetic involved in this process. Volatiles (including water) and fine grained dust coexisted in the nebula for millions of years, opening the possibility for these two components to interact. The importance of characterizing the interaction between water gas and the surface of olivine lies in the possibility of explaining the presence of water in the inner solar system due to adsorption of water onto the nebular dust before accretion. Monte Carlo simulation of adsorption onto a flat surface showed that this mechanism can store up to 3 times the Earth's oceans on dust grains in the pre-accretion disk (Stimpfl et al. 2004). This model, however, did not take into account the specific surface interactions between water gas and the crystalline surface, nor did it rigorously investigate the role of porosity. To fill this gap, we are currently performing energy minimization and molecular dynamics simulations of the system water and olivine using the code GULP and DLpOLY, respectively. Bulk olivine is modelled using periodic boundary conditions and a well tested parameterized potential model for the short ranged repulsion between ions. Formal charges are adopted for the long ranged Coulombic interactions. The virtual crystal was then cleaved by removing the PBC in the positive z direction and thus creating a free surface. After the top layer relaxed, we divided the surface in a grid of (x,y) pairs. On top of each pair we generated a water molecule and minimized the energy for the system water and olivine, where the water molecule could reach the minimum surface energy site by minimizing (x,y,z). This reveled that the structure of the most stable incorporation sites involved the water's oxygen atom in coordinating with the under-coordinated surface magnesium ions and the water's hydrogen atoms bonding with surface oxygen ions. The values for surface and adsorption energies obtained in this study agree with previous investigations by de Leeuw et al. (2000). Furthermore, we generated maps of the surface potential energy by keeping (x,y) fixed and allowing the water molecule to rotate and move in the z direction. We explored the potential energy surface for water above four olivine planes: {010}, {100}, {011} and {110}. The stable positions for water incorporation on the surface coincide with the position of the potential energy wells and give a minimum energetic barrier to desorption of adsorbed water molecules of 160 kJmol-1. Energy barriers for diffusion across the surface are about 70 kJmol-1 and correspond to a state with the water's oxygen atom being closer to surface oxygen ions than surface cations.
Deymier Pierre
Drake Michael J.
Stimpfl Marilena
Walker Marilyn A.
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