Early results from Europe's Rosetta spacecraft challenge a long-held theory that comets delivered water to early Earth, a study released on Wednesday shows.
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.
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)
