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Earth's surface is dominated by oceans. But where did all that water come from?

 

Asteroids¹ and comets smashing into the early Earth have long been thought to be a promising² source. But measurements of Halley's Comet and five others threw cold water on the idea that comets provided a large share of the oceans. Because the chemical signatures of the comets did not match the oceans. Specifically, the ratio of heavy hydrogen, or deuterium, to regular hydrogen was too high in the comets.

 

But now astronomers³ have gotten a glimpse of a comet with a different origin, and it matches the oceans much better. They used the Herschel space telescope to examine Comet Hartley 2, which originated in the Kuiper Belt. Halley and its ilk came from a distant swarm⁴ of comets called the Oort Cloud.

 

At a planetary science meeting last week in France, and in a study in Nature, the researchers announced that Comet Hartley 2 has the same deuterium-to-hydrogen ratio as ocean water. 

 

So maybe comets did play a major role in delivering Earth's oceans. Something to ponder next time you have a day at the beach.

 

Thanks for the minute. For Scientific American's 60-Second Science, I'm John Matson.

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