WASHINGTON — The mystery of where Earth’s water came from got murkier Wednesday when some astronomers essentially eliminated one of the chief possibilities: comets.
During the past few months, the European Space Agency’s Rosetta space probe 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.
“The question is: Who brought this water? Was it comets or was it something else?” asked Kathrin Altwegg of the University of Bern in Switzerland, lead author of a study published in the journal Science.
Something else, probably asteroids, Altwegg concluded. But others disagree. Many scientists have long thought that Earth had water when it first formed but that it boiled off, so that the water on the planet now had to have come from an outside source.
