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Formation and evolution of water in the Solar System and the origin of wateron Earth constitute one of the most interesting questions in astronomy. Theprevailing hypothesis for the origin of water on Earth is by delivery throughwater-rich small Solar system bodies. In this paper, the isotopic and chemicalevolution of water during the early history of the solar nebula, before theonset of planetesimal formation, is studied. A gas-grain chemical model thatincludes multiply-deuterated species and nuclear spin-states is combined with asteady-state solar nebula model. To calculate initial abundances, we simulated1 Myr of evolution of a cold and dark TMC1-like prestellar core. Twotime-dependent chemical models of the solar nebula are calculated over 1 Myr:(1) a laminar model and (2) a model with 2D turbulent mixing. We find that theradial outward increase of the H2O D/H ratio is shallower in thechemo-dynamical nebular model compared to the laminar model. This is related tomore efficient de-fractionation of HDO via rapid gas-phase processes, as the 2Dmixing model allows the water ice to be transported either inward and thermallyevaporated or upward and photodesorbed. The laminar model shows the Earth waterD/H ratio at r ~<2.5 AU, while for the 2D chemo-dynamical model this zone islarger, r ~<9 AU. Similarly, the water D/H ratios representative of theOort-family comets, ~2.5-10 x 10-4, are achieved within ~2-6 AU and ~2-20 AU inthe laminar and the 2D model, respectively. We find that with regards to thewater isotopic composition and the origin of the comets, the mixing model seemsto be favored over the laminar model.

Chemo-dynamical deuterium fractionation in the early solar nebula: The origin of water on Earth and in asteroids and comets