Comet 67P/Churyumov-Gerasimenko as seen by the European Space Agency's Rosetta spacecraft during approach earlier this year. ESA

Scientists reviewing data from the European Space Agency's Rosetta spacecraft flying along with Comet 67P/Churyumov-Gerasimenko have concluded that asteroids, not comets, most likely delivered the lion's share of the water making up Earth's oceans, according to a paper published Wednesday in the journal Science.

The conclusion supports earlier observations that ruled out comets residing in the remote Oort Cloud as a source of Earth's water and indicates comets found closer to the sun in the Kuiper Belt, a population of icy relics born beyond the orbit of Neptune, likely formed in different places and somehow migrated to their current positions.

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The conclusions are based on the observed ratio of deuterium, an isotope of hydrogen with one proton and one neutron in its nucleus, to normal hydrogen, with a nucleus made up of a single proton, in water molecules streaming away from 67P/Churyumov-Gerasimenko. For comets to be a source of Earth's oceans, the D/H ratio must match up.

"The Earth has a D/H ratio which is around three heavy molecules in 10,000 water molecules," Kathrin Altwegg, principal investigator with Rosetta's ROSINA instrument at the University of Bern, told reporters during a teleconference Tuesday. "It's very little, but it's very characteristic for the Earth."

During the solar system's formation some 4.6 billion years ago, extreme temperatures would have removed any water from Earth's surface but computer models indicate the planet was bombarded by asteroids and comets during its first billion years, Altwegg said, "so the question now is, who brought this water? Was it comets or was it something else?"

Based on earlier studies, including a flyby of Halley's comet in 1986, scientists decided that Oort Cloud comets could not be the source of terrestrial water. The deuterium-to-hydrogen ratio in water released from Halley, believed to have originated in the Oort Cloud, was twice that of seawater on Earth.

Planetary scientists subsequently gave up on the theory that comets seeded Earth's oceans during a period of heavy bombardment some 800 million years or so after Earth's formation, Altwegg said. Then, three years ago, ESA's Herschel Space Observatory observed a Kuiper Belt comet -- Hartley 2 -- during a close approach and remotely measured a D/H ratio almost exactly in line with Earth's.

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Rosetta comet probe reveals clues about source of Earth's water