Hydrogen Isotopic Fractionation in the Solar Nebula and Implications for Water in Earth and Inner Solar System Bodies

The origin of Earth's water requires explanation of both its abundance and its elevated D/H ratio relative to the solar nebula. Previous work (Asaduzzaman et al., 2015) demonstrated that dissociative chemisorption of water on olivine grains in the solar nebula could supply several ocean equivalents of water to the growing Earth. However, this mechanism has been challenged on isotopic grounds, as it does not inherently explain the enrichment of D/H ratios relative to solar values. Here, I quantify hydrogen isotopic fractionation between nebular gas and a set of condensed phases using a combination of quantum chemical calculations and statistical thermodynamics. Equilibrium fractionation factors are determined as functions of temperature for (i) chemisorbed hydroxyls on olivine surfaces, (ii) structural hydroxyls in apatite and pyroxenes (including pigeonite), and (iii) representative organic compounds. The results show that deuterium is preferentially partitioned into condensed phases, with enrichment factors increasing systematically with decreasing temperature. The calculated enrichment factors reach values comparable to those observed in terrestrial water, carbonaceous chondrites, Mars, and asteroid Vesta at temperatures of ∼175-300 K. Kinetic constraints based on hydrogen diffusion in olivine indicate that isotopic equilibration between gas and grain surfaces is plausible within the lifetime of the solar nebula. These findings demonstrate that equilibrium gas-grain isotopic fractionation can naturally generate elevated D/H ratios in water adsorbed onto silicate grains. This study offers an explanation to prior criticism of adsorption-based models by showing that nebular water can acquire terrestrial-like isotopic compositions through low-temperature equilibration with grain surfaces. Chemisorption-mediated accretion therefore provides a plausible complementary mechanism for explaining both the abundance and isotopic composition of water in Earth and other inner solar system bodies, and may reduce part of the required contribution from carbonaceous chondrites.