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 Chemical analysis of water coming from Comet
67P/Churyumov-Gerasimenko, which Rosetta has been orbiting since
August, shows it has three times more deuterium - an atomic
variation of regular hydrogen - as hydrogen in water molecules on
Earth, said Rosetta scientist Kathrin Altwegg, with the University
of Bern.
Water is comprised of two hydrogen atoms bonded with one oxygen
atom. On Earth, three in 10,000 water molecules have the heavy
hydrogen isotope deuterium.
Unless 67P is a total oddball, Altwegg said the finding eliminates
comets as the source of Earth's water - and most likely its organics
as well.
Both water and carbon compounds were needed for life to evolve.
 The finding leaves asteroids as Earth's probable water bearers,
though the mini-planets that bombarded baby Earth likely bore little
resemblance to the dry, rocky bodies circling the sun beyond Mars
today.
"Asteroids could well have had much more water than they have
today," Altwegg said. "They have just lived in the vicinity of the
sun for 4.6 billion years."
Comet 67P hails from the Kuiper Belt region of the solar system,
located beyond Neptune's orbit 30 to 40 times farther from the sun
than Earth.
Three years ago, analysis of water in another Kuiper Belt comet
showed a chemical fingerprint that matched Earth's water. The
measurements from 67P, however, are so much higher that even if only
a few comets of its type smashed into Earth, Earth's deuterium ratio
would not be what it is today, Altwegg said.
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Previous studies had dismissed comets from even farther out in the
solar system, a region called the Oort Cloud, as the source of
Earth's water.
Also on Wednesday, scientists said the search for Rosetta's
companion probe, Philae, continues.
Philae made an unprecedented descent to the surface of the comet on
Nov. 12, bounced twice and settled in what appears to be a crater.
It ran through 2-1/2 days of preprogrammed science experiments
before its battery died.
Results of the studies, which include chemical analysis of samples
drilled out from the comet's body, have not yet been released.
In August, Rosetta became the first spacecraft to put itself in
orbit around a comet. It will continue to accompany 67P for about
another year.
(Reporting by Irene Klotz, editing by G Crosse)
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