Earth’s Water May Have Come from Comets, Asteroids or Something Else Entirely

To discover the origin of the oceans, scientists are investigating our solar system’s farthest reaches and earliest moments

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Standing on the seashore, watching waves roll in from over the horizon—it is easy to see the ocean as something timeless. Our ancient ancestors certainly did. In numerous creation myths, a watery abyss was present before the emergence of land and even light. Today we realize that Earth's global ocean has not been around forever. Its water—as well as every drop of rain, every gust of humid air and every sip from your cup—is a memory from eons ago, when the seas literally fell from the sky.

All the water in our solar system can be traced to the giant primordial cloud of gas and dust that collapsed to form the sun and planets more than four and a half billion years ago. The cloud was rich with hydrogen and oxygen, the two atomic ingredients for water, H2O. That enrichment is no surprise because hydrogen and oxygen are also the first and third most abundant elements in the universe (chemically inert helium is the second). Most of the gas was sopped up by the sun and the gas-giant planets, which formed earlier than the rocky planets. Much of the remaining oxygen bonded with other atoms, such as carbon and magnesium, but the hydrogen and oxygen left over were sufficient to produce several times more water than rock in our solar system.

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