Water like that of Earth's oceans found in comet

The seas may have arrived from the sky


Ice found in a comet supports the theory that the earth's oceans were delivered to the planet in the form of crashing comets.

It is thought that the Earth's oceans formed about 8 million years after the planet did, and scientists have long suggested comets as a cause.

Many comets contain ice - unlike asteroids which are largely rock and metal - but the water contained in the Hartley 2 comet is particularly interesting to scientists because it has the same isotopic composition as the water found in our oceans.

Both have similar D/H ratios. The D/H ratio is the proportion of deuterium, or heavy hydrogen, in the water. A deuterium atom is a hydrogen with an extra neutron in its nucleus.

This is the first time ocean-like water has been detected in a comet.

“We were all surprised,” said University of Michigan astronomy professor Ted Bergin, who helped make the discovery.

“Life would not exist on Earth without liquid water, and so the question of how and when the oceans got here is a fundamental one,” said Professor Bergin.

“It’s a big puzzle, and these new findings are an important piece.”

The discovery comes from the HiFi, the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. It has analysed six other comets but only the ice from Hartley 2 matched the earth's water make-up. Hartley 2 comes from the Kuiper belt, not too far beyond Pluto. The other five come from the far-off Oort Cloud. The Kuiper belt is about 30 times farther from the Sun than the Earth is, the the Oort cloud is more than 5,000 times farther out.

The paper Ocean-like water in the Jupiter-family comet 103P/Hartley 2 is published in the journal Nature. ®

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