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Earth's water formed close to home

Watery origins A new study claims most of Earth's water didn't originate in the frozen outer reaches of the solar system, but came from the asteroid belt.

The research led by Dr Conel Alexander from the Carnegie Institution in Washington DC has important implications for our understanding of how the solar system evolved.

Their findings appear today in the journal Science.

Astronomers believe water was distributed throughout much of the still developing early solar system.

But the primordial Earth was a molten mass, far too hot to retain any water condensing out of the dust cloud from which the planets formed.

The existing hypothesis suggests both comets and a type of primitive meteorite called carbonaceous chondrites formed beyond Jupiter, and then migrated inward, bringing water and organic material to Earth.

Astronomers can determine how far from the Sun an object formed by measuring the ratio of hydrogen to its heavy isotope deuterium contained within.

Objects that formed farther out should generally have higher deuterium content in their ice than objects which formed closer to the Sun.

Close to home

Alexander and colleagues measured the hydrogen, carbon and nitrogen isotopic compositions in 85 chondrite meteorite samples and compared the results to the known composition of comets.

"We found carbonaceous chondrites were unlikely to have formed in the same regions of the Solar System as comets because they have much lower deuterium to hydrogen ratios," says Alexander.

"Instead it looks like carbonaceous chondrites formed in the asteroid belt between Mars and Jupiter."

According to Alexander, the hydrogen to deuterium ratios in carbonaceous chondrites also closely matches the ratios found in water on Earth.

"That means most of the volatile elements on Earth such as water, arrived in carbonaceous chondrites from the asteroid belt, not from comets," he says.

Alexander points out that comets crashing on Earth would have delivered the whole comet, not just the water.

"There's going to be lots of organic material in the comet including lots of deuterium," he says.

"When we compare the estimated deuterium to hydrogen ratio for the whole comet to that of the Earth, there isn't a very good match."