Did the SEA come from the sky?

Much of Earth's oceans may have been created by comets, which whacked into the infant planet billions of years ago, bringing precious loads of ice, a study published Wednesday in the journal Nature suggests.

The evidence to support this comes from a signature of the ratio of heavy hydrogen, or deuterium, in water.

Ice on a comet called 103P/Hartley 2, analysed by an infrared instrument aboard Europe's Herschel space telescope as it swung by Earth in October and November 2010, has the same deuterium ratio as water on Earth.

Primal leftovers from the building of the Solar System, comets are mixtures of ice and dust that have been dubbed "dirty snowballs."

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