Comet's water 'like that of Earth's oceans'
08 Oct BBC
Comet Hartley 2 contains water more like that found on Earth than all
the comets we know about, researchers say.
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.
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.
"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 are 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 argues that the result shows that the
distinction between the potential water sources may need 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
origin 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?.
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