The mystery of where Earth's water came from got murkier Wednesday when some astronomers essentially eliminated one of the chief suspects: comets.
Over the past few months, the European Space Agency's Rosetta space probe closely examined the type of comet that some scientists theorized could have brought water to our planet 4 billion years ago. It found water, but the wrong kind.
It was too heavy. One of the first scientific studies from the Rosetta mission found that the comet's water contains more of a hydrogen isotope called deuterium than water on Earth does.
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In 1986, a spacecraft came within about 400 miles of Halley's comet, an Oort Cloud comet, and analyzed its water. It proved to be heavier than Earth's. But three years ago, scientists examined the water in a Kuiper Belt comet, Hartley 2, and it was a perfect match for Earth's, so the comet theory was back, stronger than ever, Altwegg said.
