  There is a long-standing debate regarding the origin of the terrestrial
planets' water as well as the hydrated C-type asteroids. Here we show that the
inner Solar System's water is a simple byproduct of the giant planets'
formation. Giant planet cores accrete gas slowly until the conditions are met
for a rapid phase of runaway growth. As a gas giant's mass rapidly increases,
the orbits of nearby planetesimals are destabilized and gravitationally
scattered in all directions. Under the action of aerodynamic gas drag, a
fraction of scattered planetesimals are deposited onto stable orbits interior
to Jupiter's. This process is effective in populating the outer main belt with
C-type asteroids that originated from a broad (5-20 AU-wide) region of the
disk. As the disk starts to dissipate, scattered planetesimals reach
sufficiently eccentric orbits to cross the terrestrial planet region and
deliver water to the growing Earth. This mechanism does not depend strongly on
the giant planets' orbital migration history and is generic: whenever a giant
planet forms it invariably pollutes its inner planetary system with water-rich
bodies.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics