Paris: Much of Earth's oceans may have been created by comets, which whacked into the infant planet billions of years ago, bringing precious loads of ice, a study published in the journal Nature suggests.

The evidence to support this comes from a signature of the ratio of heavy hydrogen, or deuterium, in water.

Ice on a comet called 103P/Hartley 2, analysed by an infrared instrument aboard Europe's Herschel space telescope as it swung by Earth in October and November 2010, has the same deuterium ratio as water on Earth.

Primal leftovers from the building of the Solar System, comets are mixtures of ice and dust that have been dubbed "dirty snowballs."

They are lonely wanderers, looping around the Sun in orbits that are measured in a years or sometimes centuries, although a tiny gravitational tug by a nearby body can deflect their course.

Astrophysicists have long puzzled over the source for Earth's water.

Earth was so scorching hot in its early years that all volatiles, including water, evaporated.

Water, 3.9 billion years ago, would only have existed in abundance in the chillier outer regions of the Solar System.

This is why fingers have until now pointed in the direction of asteroids, rocks that orbit in a belt between Mars and Jupiter, as the big source.

"Current theories came to the result that less than 10 percent of Earth's water originated from comets," said Paul Hartogh of the Max Planck Institute for Solar System Research (MPS) in Germany, who led the study.

"For the first time, our results imply that comets may have played a much more important role," his colleague, Miriam Rengel, added.

The icy bombardment would have occurred some eight million years after Earth was formed.

Hartley 2 is probably traceable to comets that originated in the Kuiper Belt, which starts in the region of Pluto, says the paper.

Six other comets measured by Herschel in recent years have very different deuterium ratios. They are believed to have originated from the far distant Oort Cloud, which is 5,000 times farther out from the Sun.