Comet Hartley has had its own close encounter with the Deep Impact probe Comet Hartley 2 contains water more like that found on Earth than prior comets seem to have, researchers say.
A study using the Herschel space telescope aimed to measure the quantity of deuterium, a rare type of hydrogen, present in the comet's water.The comet had just half the amount of deuterium seen in comets.
The result, published in Nature, hints at the idea that much of the Earth's water could have initially came from cometary impacts.
Just a few million years after its formation, the early Earth was rocky and dry; something must have 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 a half-dozen comets have been measured in this way.
All of them have exhibited a deuterium fraction twice as high as the oceans, so the current theory holds that asteroids were likely to be the carriers for water; meteorites that they give rise to have roughly the same proportion of deuterium that the Earth's oceans contain.
Clouded measure However, until now, all of those 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.
The assumption has thus been that if anything brought the water to Earth, it must have been asteroids and meteors - despite the fact that comets carry significantly larger amounts of water.
Comets carry a great deal more water than asteroids - but only maybe like that of our oceans Like the meteorites, 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.
"The reservoir of Earth ocean-like material is much larger than we thought, and it encompasses cometary material, which we hadn't recognised," he told BBC News.
"We have to think really hard and try to get a better understanding of what is going in our Solar System and whether you can really rule out comets as the source of Earth's water."
They might not be ruled out, but they're not the definitive answer either; much of what we believe happened in the early Solar System is based on computer models.
James Greenwood of Wesleyan University in the US said such models may need adjusting in light of the new evidence - and that more such studies are needed to assess whether many Kuiper Belt objects are like Hartley 2.
"If the short-period comets are all like this one comet, then this could be a significant source of our early water," he told BBC News.
"It opens up a new can of worms for us."
Alessandro Morbidelli of the Observatory of Cote d'Azur argued that the result shows that perhaps the distinction between the potential water sources is to be called into question
"In the past, scientists thought that these asteroids and comets were completely different classes of bodies. Now, several new results show that primitive asteroids and comets are brothers and sisters," he told BBC News.
"This new view changes at least the semantics of the question on the orgin of the Earth's water. The question becomes more technical: from which region of the disc and by which dynamical mechanism came the (objects) that delivered the water to the Earth?"
Herschel has some time left to address the question, but what all the researchers agree is that the Atacama Large Millimeter Array of telescopes in Chile - which has just shown off its first results - will soon be able to resolve these questions with never-before-seen sensitivity.
http://www.bbc.co.uk/news/science-environment-15181123
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