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Rosetta spacecraft finds water on Earth didn’t come from comets

It’s a mystery that has baffled scientists for decades: Where did Earth’s water come from?

Some scientists believed comets might have been the original source of the Earth’s oceans. But a study published this week in the journal Science is sending scientists back to the drawing board. In its first published scientific data, the ROSINA mass spectrometer on board the Rosetta probe found that water on Comet 67P/Churyumov–Gerasimenko doesn’t match the water on Earth.

The result is surprising, says Kathrin Altwegg, principle investigator for ROSINA at the University of Bern and one of the authors of the study. For decades, scientists had ruled out comets from the Oort Cloud at the very edge of our solar system as the source of Earth’s water.

But three years ago, an analysis of water on the Hartley 2 comet near Jupiter found a perfect match to the Earth’s oceans. That finding led scientists to believe that Earth’s water could have come from much closer comets, either near Jupiter or in the Kuiper Belt just beyond Neptune. Comet 67P/Churyumov-Gerasimenko is one of those Jupiter family comets, which scientists believe originated in the Kuiper Belt.

“That was a big surprise, but now we are back to what I expected,” she said. “I think it’s very nice to see the diversity we have in Kuiper Belt, to see that not everything is as simple as it seemed.”

To find the origin of Earth’s water, scientists analyze the water’s “fingerprint”, says Claudia Alexander, project scientist of the U.S. Rosetta Project at NASA’s Jet Propulsion Laboratory. Water has a chemical isotopic signature, which works just like a fingerprint. Planets, comets, even minerals all have a fingerprint, Alexander says, and scientists are looking for a match to Earth’s.

On Earth, water is mostly two parts hydrogen and one part oxygen — H2O. But there’s also “heavy” water, Alexander explained, which is made with deuterium — a hydrogen atom with a neutron. That heavy water is what the Rosetta spacecraft found on Comet 67P/Churyumov–Gerasimenko. It’s also a closer match to the water scientists have found on other comets, ruling them out as Earth’s water source, Alexander said.

“The clues don’t quite all add up,” she said.

Altwegg agrees, saying that it’s not likely the other Kuiper Belt comets have a match to Earth’s water, but further studies would be helpful.

“You would have to assume that 67P is the exception in the Kuiper Belt,” she said. “We need more missions to Kuiper Belt comets, which would be fabulous.”

There are several ideas to explain the origin of Earth’s water, Alexander said. Some believe that water has been on Earth since its formation, that it was beaten out of other minerals as the planet formed. Others think “wet planetesimals” near Jupiter — which were like planetary Silly Putty, loose sticky blobs of rock and ice, Alexander said — collided with Earth in the early formation of the solar system.

Alexander believes the answer could be a combination of any of these ideas. The upcoming Dawn mission in 2015 will study the water on the asteroid Ceres, near Jupiter. If it’s a match for Earth’s water, it may be another clue, Alexander said. But the Rosetta finding is a huge step in solving the mystery, she said.

“I think this is a big deal…For me, I’ve not always been a believer in the story that comets brought the water,” Alexander said. “In some respects, I’m somewhat relieved (this finding) doesn’t confirm it. It’s more complicated than that. I think we need more forensic evidence to settle the score.”