Origin of water in the terrestrial planets

I examine the origin of water in the terrestrial planets. Late-stage delivery of water from asteroidal and cometary, sources appears to be ruled out by isotopic and molecular ratio considerations, unless either comets and asteroids currently sampled spectroscopically and by meteorites are unlike those falling to Earth 4.5 Ga ago, or our measurements are not representative of those bodies. However, the terrestrial planets were bathed in a gas of H, He, and O. The dominant gas phase species were H-2, He, H2O, and CO. Thus, grains in the accretion disk must have been exposed to and adsorbed H-2 and water. Here I conduct a preliminary analysis of the efficacy of nebular gas adsorption as a mechanism by which the terrestrial planets accreted wet. A simple model suggests that grains accreted to Earth could have adsorbed 1-3 Earth oceans of water. The fraction of this water retained during accretion is unknown, but these results suggest that examining the role of adsorption of water vapor onto grains in the accretion disk bears further study.