Comet's water 'like that of Earth's oceans'

A study using the Herschel space telescope aimed to measure the fraction of deuterium, a rare type of hydrogen, present in the comet's water.

Like our oceans, it had half the amount of deuterium seen from other comets.

The result, published in Nature, hints at the idea that much of the Earth's water could have initially come from cometary impacts.

Just a few million years after its formation, the early Earth was rocky and dry; most likely, something brought the water that covers most of the planet today.

Water has something of a molecular fingerprint in the amount of deuterium it contains, and only about half a dozen comets have been measured in this way - and all of them have exhibited a deuterium fraction twice as high as the oceans.

Asteroids, by contrast, give rise to the meteors and meteorites that arrive on Earth, making their deuterium fraction more well-established.

Meteoritic material has roughly the same proportion of deuterium that the Earth's oceans contain, and so the assumption has been that if water arrived from elsewhere, it came from asteroids.

Clouded measure

Until now, all of the comets that have been measured have been so-called Oort Cloud objects, believed to have been formed early in the Solar System's history in the region of the giant planets Neptune and Uranus and kicked out to a great distance as they bumped into the planets and each other.

Comet Hartley 2 is the first "Kuiper Belt" object to undergo the deuterium analysis. Kuiper belt objects formed not far outside our Solar System, and comets that originate there have much shorter orbits than those from the Oort Cloud.

An international team using the Herschel telescope to peer at the comet, they found it had a deuterium fraction much closer to that of our oceans.

Report co-author Ted Bergin of the University of Michigan said that opened up the possibility that comets at least contributed to our water supply.

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