The first comet containing water similar to Earth's oceans has been discovered by a team of astronomers using the European Space Agency's Herschel Space Observatory.
The comet 103P/Hartley 2 hints at the idea that much of the Earth's water could have initially come from cometary impacts.
The study's objective was to measure the quantity of deuterium, a rare type of hydrogen, present in the comet's water. The research by Paul Hartogh of Germany's Max Planck Institute for Solar System Research and colleagues was publishd in the journal Nature.
Hartley 2 is called a Jupiter family comet, because its orbit takes it near the largest planet.
Computer simulations suggest that Hartley 2 originated from the Kuiper belt; it is the first Kuiper Belt object to undergo deuterium analysis. The belt is a region of the solar system beyond the planets extending from the orbit of Neptune to approximately 50 astronomical units from the Sun.
Comet Hartley 2 had a deuterium fraction - similar to that found in meteorites - much closer to that of Earth's oceans.
It is known that due to the intense heat of Earth immediately after it was formed, any initial water would have quickly evaporated. Scientists believe the oceans emerged around 8 million years later. So the question is where Earth's water came from.
Earlier analysis of water-ice from far-flung comets suggested they could have delivered no more than 10 per cent of today's oceans. But the present finding substantially increases the amount of water that could have originated from comets.