  Water condensed as ice beyond the water snowline, the location in the Sun's
natal gaseous disk where temperatures were below 170 K. As the disk evolved and
cooled, the snowline moved inwards. A low temperature in the terrestrial
planet-forming region is unlikely to be the origin of water on the planets, and
the distinct isotopic compositions of planetary objects formed in the inner and
outer disks suggest limited early mixing of inner and outer Solar System
materials. Water in our terrestrial planets has rather been derived from
H-bearing materials indigenous to the inner disk and delivered by water-rich
planetesimals formed beyond the snowline and scattered inwards during the
growth, migration, and dynamical evolution of the giant planets.
