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Using a powerful space telescope, scientists have found evidence that the water in the Earth's oceans could have been delivered by comets.

During observations made with the Herschel Space Telescope last year, astronomers found water ice on a comet called Hartley 2. Water has been found on six other comets, but its molecular composition - specifically, its deuterium-to-hydrogen ratio - made them unlikely as a source of Earth's water. Most scientists have assumed that water arrived on this planet by way of asteroid impacts.

But the ice on Hartley 2 is composed of water that matches the chemistry of the Earth's oceans. The handful of water-bearing comets previously discovered, the researchers believe, came from the Oort cloud, a band of comets about a light-year away from the sun.

Hartley 2 probably came from the Kuiper belt, 1,000 times closer to Earth. This could explain the very different composition of its water ice.

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