Water on Earth originated from comets?

Using European Space Agency's Herschel Space Observatory, researchers found that "Comet Hartley 2" contains water more like that found on Earth than all other known comets.

This comet, which the spacecraft flew by in November 2010, originated in the disk-shaped Kuiper belt -- a region of the solar system beyond the orbit of Neptune, suggesting this is ultimately where much of Earth's water came from.

"When the Earth formed it was so hot that most volatiles escaped to space, so when the Earth cooled down it was dry," lead researcher Paul Hartogh, a planetary scientist at the Max Planck Institute for Solar System Research in Germany, said.

"Water and other volatiles must have been delivered at a later stage," Hartogh was quoted as saying by Space.Com.

Comets are natural candidates for the original sources of the world's seas, loaded with ice as they often are. However, computer models of how the solar system formed have suggested that asteroids actually were the source of most of the Earth's oceans, with comets delivering no more than 10 per cent of the planet's water.

For their research, published in the journal Nature, the scientists compared isotopes of the hydrogen found in the planet's water to isotopes of hydrogen seen in comets.

All isotopes of an element have the same number of protons, but each has a different number of neutrons -- for instance, regular hydrogen has no neutrons, while the hydrogen isotope known as deuterium has one neutron.

They used Herschel Space Observatory to observe the ratio of deuterium to regular hydrogen in comet Hartley 2 and found that water seen in the comet was very close to Earth's, with about 1,610 deuterium atoms per 10 million regular hydrogen atoms.

In Earth water, about 1,558 deuterium atoms are seen per 10 million regular hydrogen atoms.

"With our finding it may be that more than 10 per cent and perhaps all water on Earth possibly stems from comets," Hartogh said.

It may be that all bodies in the inner solar system get their water from these comets. Sampling a larger number of comets for their deuterium-hydrogen ratios could shed light on the matter, Hartogh said.

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