  The planetary building blocks that formed in the terrestrial planet region
were likely very dry, yet water is comparatively abundant on Earth. We review
the various mechanisms proposed for the origin of water on the terrestrial
planets. Various in-situ mechanisms have been suggested, which allow for the
incorporation of water into the local planetesimals in the terrestrial planet
region or into the planets themselves from local sources, although all of those
mechanisms have difficulties. Comets have also been proposed as a source,
although there may be problems fitting isotopic constraints, and the delivery
efficiency is very low, such that it may be difficult to deliver even a single
Earth ocean of water this way. The most promising route for water delivery is
the accretion of material from beyond the snow line, similar to carbonaceous
chondrites, that is scattered into the terrestrial planet region as the planets
are growing. Two main scenarios are discussed in detail. First is the classical
scenario in which the giant planets begin roughly in their final locations and
the disk of planetesimals and embryos in the terrestrial planet region extends
all the way into the outer asteroid belt region. Second is the Grand Tack
scenario, where early inward and outward migration of the giant planets
implants material from beyond the snow line into the asteroid belt and
terrestrial planet region, where it can be accreted by the growing planets.
Sufficient water is delivered to the terrestrial planets in both scenarios.
While the Grand Tack scenario provides a better fit to most constraints, namely
the small mass of Mars, planets may form too fast in the nominal case discussed
here. This discrepancy may be reduced as a wider range of initial conditions is
explored. Finally, we discuss several more recent models that may have
important implications for water delivery to the terrestrial planets.
