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. Read More