Some – Perhaps Most - Water in the Earth must result from Adsorption on to Grains in the Accretion Disk

Muralidharan et al. [1] have shown that up to 8 Earth oceans of water could be associatively adsorbed on to grains in the accretion disk at 1 AU. Here we expand on that work using atomistic and electronic-level calculations to investigate dissociative adsorption, and use density functional theory to investigate the difference in D/H ratios between water on Earth and in the accretion disk. We find that dissociative adsorption leads to even stronger bonding than associative adsorption, demonstrating that adsorption must be a significant source of terrestrial planetary water. Using density functional theory we show that HDO may be preferentially retained relative to H2O in adsorption/desorption kinetics. Introduction: Comets, hydrous asteroids, phyllosilicates migrating from the asteroid belt, and hydrous minerals forming in the inner solar system have all been proposed as possible sources of inner solar system water, but there remain unresolved issues with these sources [2]. Recently, Drake [2] proposed that microscopic-level processes such as molecular adsorption of water in the accretion disk could in fact lead to the delivery of water to the inner solar system planets. Previously, adsorption on to grains had been dismissed due to the misconception that adsorption energies were too low to capture and retain gaseous water at high temperatures. We have shown [1,3] that the direct adsorption of water on to forsterite grains, the major silicate phase in protoplanetary disks, could be a significant source of terrestrial planetary water. Specifically, we [3,4] have carried out atomistic and electronic-structure calculations to map out the adsorption energy landscapes of water on forsterite surfaces and showed that water molecules could strongly chemisorb (via chemical bonds) on to forsterite (especially the {100} surfaces in a non-dissociative fashion. Next, we [1] used kinetic Monte Carlo (KMC) simulations in conjunction with the results of [3] to show that water could be retained even at conditions corresponding to the accretion disk (Fig. 1). We concluded that adsorption of gaseous water on to dust grains would start from the early stages of accretion. Many conservative assumptions were made in the above work. For example, only associative adsorption was considered. But forsterite surfaces are characterized by underbonded O and Mg atoms [4], leading to the surfaces being very reactive and consequently making dissociation of a water molecule into H and OH energetically favored. Here we calculate the dissociative adsorption energy landscapes of water on different forsterite surfaces and compare these results with associative adsorption [3]. If the resultant dissociative adsorption energy landscapes are similar to, or much lower than, the previous results, then adsorption is an even more significant source of terrestrial planetary water. This work also addresses why the D/H ratio of water on Earth is higher than in the accretion disk. Currently there is no quantitative theory. Using firstprinciples density functional theory (DFT), we calculate the activation barriers for adsorption/desorption of H2O and HDO molecules in order to see if differences could lead to preferential retention of HDO. If HDO is preferentially retained, as adsorption on to grains in the accretion disk would be a plausible alternative to the hypothesis that Earth’s water came from exogeneous sources such as comets or wet asteroids. Figure 1: Variation in the amount of adsorbed water expressed in terms of Earth-Oceans at different temperatures Models and Methods: Dissociative adsorption energy calculations: Using interatomic potential parameters derived for bulk Mg2SiO4 [4], the different surfaces ({100}, {010}, and {110}) were relaxed using the L-BFGS method [5] (which accurately locates the lowest surface energy configuration). Each surface is characterized by the presence of underbonded O and Mg atoms, with the number of underbonded atoms being a function of the surface geometry. Next, in order to calculate the adsorption energy, using an appropriate potential for water [4], a water molecule was dissociated on the relaxed surfaces such that an underbonded surface oxygen was protonated, while the OH group was bonded to a surface Mg atom. Then, the hydroxylated surface was relaxed using the L-BFGS energy-minimizer and the adsorption energy was calculated according to Eq. 1, [ ] ) ( 2O H s w s ads E E E E + − = + (1) where, Eads corresponds to the dissociative adsorption energy, Es+w is the total potential energy of the energy1882.pdf 40th Lunar and Planetary Science Conference (2009)