Rosetta Pours Cold Water on Cometary Origins of Earth’s Oceans

12-10-2014 | Lee Billings

In the Enuma Elish, a Babylonian epic that recounts the creation of the world, the heavens and the Earth emerge from a primordial abyss of brackish water. According to the biblical book of Genesis, water existed before land, life, and even light itself. Our ancient ancestors realized, just as we do today, that water is fundamental to life. But even though they could conceive creation stories for the Earth, moon, and stars, many cultures at the dawn of recorded history seemed baffled by the origin of our planet’s water.

Thousands of years later, the genesis of Earth’s oceans remains one of the key missing pieces in our modern creation stories. We know that water’s precursors, hydrogen and oxygen, were abundant in the giant, cold molecular cloud that collapsed to form the sun and its planets just over 4.5 billion years ago. When the cloud collapsed, it became a twirling, incandescent disk of gas and dust with the glowing, nascent sun at its center. Small pieces of whirling debris collided and stuck together in the disk, gradually growing to form full-fledged worlds. Water ice was abundant in the cold outer disk, but near the sun, where all the rocky planets formed, the disk was far too hot to sustain more than a wisp of moisture. The Earth and its rocky siblings probably formed relatively dry. So where do Earth’s oceans come from?

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