Watering the Earth

Water is one of the key molecules of life, and a fundamental solvent of our own human life form. The planet that spawned our watery origins, Earth, presently carries enough surface water in vapor or liquid form to cover the entire planet to a depth of about 3 km. The fact that nearly three-quarters of the Earth's surface is covered by seas triggered writer Arthur C. Clarke to question why our planet is called *Earth*, when it could more aptly be called *Ocean*. Driven by our watery origins, we naturally look for other life forms in the universe at the "water hole" (wavelengths 18-21 cm). Simultaneously, we search, and find, water in planetary atmospheres, comets, asteroids, interplanetary dust, and molecular clouds. Water drives our questions about terrestrial and extraterrestrial life and we wonder how we came to exist on a planet so rich in water in the first place. So then, how _did_ our planet Earth get its water? The short answer is: 'we still don't know'. Despite our living embedded in the Earth environment, the origin of our atmosphere is one of the most puzzling enigmas in the planetary sciences. The processes and sources that contributed to its formation require knowledge of the formation of the solar nebula, Earth and its planetary neighbors, and each of their subsequent interactions, including the smaller members of the clan: asteroids, meteorites and comets. Timing and location is everything in this story and our main tool for trying to understand the puzzle will be elemental isotopic abundance measurements.