Data coming from a spacecraft orbiting a comet suggests it was more likely water came to Earth courtesy of asteroids, not comets, scientists say.

The European Space Agency's Rosetta spacecraft, examining Comet 67P/G-C -- the kind of comet some scientists theorize may have delivered water to the Earth 4 billion years ago -- has detected water, but it's the wrong kind, the researchers say.

Chemically, the water molecules being given off by the comet don't match water on Earth, an international research team is reporting in the journal Science.

Some spacecraft have previously found that water on a couple of comets, and most asteroids, matches that found on Earth in terms of its ratio of normal hydrogen to a heaver form known as deuterium.

In the case of Comet 67P/G-C, however, that ratio in its water is way off, with about three times as much deuterium as water on Earth, the researchers say.

"It's probably one of the highest deuterium-to-hydrogen ratios ever measured -- the heaviest water in any source or body," says research leader Kathrin Altwegg of the University of Bern in Switzerland.

That means if more comets have this non-matching kind of water, it brings scientists back to the question of the origin of water on the early Earth.

"The question is who brought this water: Was it comets or was it something else?" says Altwegg.

Comets are broadly divided into two types; near comets that come from the Kuiper Belt, beyond the obits of Neptune and Pluto, and far comets, that come from the even more distant Oort Cloud.

Halley's Comet, an Oort cloud comet, has had its water analyzed and it's heavier than what's found on Earth.

Then a Kuiper Belt comet observed 3 years ago had water that was a perfect match for Earth's water, leading Kuiper Belt comets to become the leading candidates for deliverers of Earth's water.

However, Comet 67P/G-C -- with its distinctly non-matching water -- is a Kuiper Belt comet, complicating any theory of those comets as the main source of Earth's water.

Either it's a major exception, with water content drastically different from other Kuiper Belt objects, or those comets are more diverse in terms of their water content and composition than previously thought, the researchers say.

"That probably rules out Kuiper Belt comets from bringing water to Earth," Altwegg says, suggesting asteroids were the more likely source.

Most of our knowledge of asteroid water is from pieces of asteroids that have fallen to Earth as meteorites; analysis of those meteorites shows deuterium levels that match those of Earth water, strongly supporting the idea of them being the more likely source, many scientists believe.

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