Water. You turn on the faucet, and out it comes. But where did water on Earth originally come from? Some scientists believe that oceans may have been formed (about 8 million years after the formation of planet Earth itself) by water dumped from passing comets, and the recent discovery of a comet containing water with a similar chemical composition to the water all around us on Earth has given support to that theory.
Hartley 2, a comet about 1.25 miles long, contains ice with the same D/H ratio (the proportion of “heavy hydrogen”) as water found in oceans here on Earth. It’s the first time that Earth-like water has been detected in a comet. Hartley 2 probably comes to us from the Kuiper Belt, a sector of space about 30 times Earth’s distance from the sun. Six other comets studied by the scientists contained water with a different composition, and probably originated in the Oort Cloud, about 5,000 times farther away.