Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the
Moon andthe Earth were essentially dry immediately after the formation of the Moon—by a giant impact on the proto-Earth—
and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is
supported by U–Pb and I–Xe chronologies, which showthat water delivery peaked ,100 million years afterthe isolation of the
Solar System. Introduction of water into the terrestrial mantle triggered plate tectonics, which may have been crucial for the
emergence of life. This mechanism may also have worked for the young Venus, but seems to have failed for Mars.