  Formation and evolution of water in the Solar System and the origin of water
on Earth constitute one of the most interesting questions in astronomy. The
prevailing hypothesis for the origin of water on Earth is by delivery through
water-rich small Solar system bodies. In this paper, the isotopic and chemical
evolution of water during the early history of the solar nebula, before the
onset of planetesimal formation, is studied. A gas-grain chemical model that
includes multiply-deuterated species and nuclear spin-states is combined with a
steady-state solar nebula model. To calculate initial abundances, we simulated
1 Myr of evolution of a cold and dark TMC1-like prestellar core. Two
time-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 the
radial outward increase of the H2O D/H ratio is shallower in the
chemo-dynamical nebular model compared to the laminar model. This is related to
more efficient de-fractionation of HDO via rapid gas-phase processes, as the 2D
mixing model allows the water ice to be transported either inward and thermally
evaporated or upward and photodesorbed. The laminar model shows the Earth water
D/H ratio at r ~<2.5 AU, while for the 2D chemo-dynamical model this zone is
larger, r ~<9 AU. Similarly, the water D/H ratios representative of the
Oort-family comets, ~2.5-10 x 10-4, are achieved within ~2-6 AU and ~2-20 AU in
the laminar and the 2D model, respectively. We find that with regards to the
water isotopic composition and the origin of the comets, the mixing model seems
to be favored over the laminar model.
