Scientists have, for the first time, found water like that found in the Earth's oceans on a comet.

The new evidence strongly suggests that a significant portion of the Earth's seawater arrived as a result of collisions with ice-packed comets.

Astronomers at the University of Michigan found the comet Hartley 2 carries ice with the same chemical composition as our oceans.

"We were all surprised," said Professor Ted Bergin.

"Life would not exist on Earth without liquid water, so the questions of how and when the oceans got here is a fundamental one (sic)," he said.





"It's a big puzzle and these new findings are an important piece."

The scientists made the discovery with infrared instruments on the Hershel Space Observatory , a telescope which orbits 930,000 miles away from Earth.

They measured the ratio of hydrogen and deuterium - or heavy hydrogen - in the frozen water, and found it was the same as in seawater.

Six other comets analysed by the observatory in recent years carried ice with different chemical compositions. As a result scientists concluded that comets could not have contributed more than 10% of the world's water.





But Hartley 2 was born in a different part of the solar system to the other six comets tested.

"The results show that the amount of material out there that could have contributed to Earth's oceans is perhaps larger than we thought," said Professor Bergin.

The research is published in the science journal Nature.