 Image caption Comet Hartley 2 was also the subject of the Deep Impact probe study 
 
 Comet Hartley 2 contains water more like that found on Earth than all the comets we know about, researchers say. A study using the Herschel space telescope aimed to measure the fraction of deuterium, a rare type of hydrogen, present in the comet's water. Like our oceans, it had half the amount of deuterium seen from other comets. Meteoritic material has roughly the same proportion of deuterium that the Earth's oceans contain, and so the assumption has been that if water arrived from elsewhere, it came from asteroids. It probably formed in the Kuiper belt, which lies about 30 to 50 times further from the sun than the Earth, while the others come from the Oort Cloud, some 5,000 times further away. LONDON (Reuters) - Astronomers have found the first comet with ocean-like water in a major boost to the theory that the celestial bodies were a significant source of water for a thirsty early Earth.
– Most of the water that fills the world's oceans—and makes up more than half of the human body—may have started out as comets, not asteroids as current theories hold, according to new research. Scientists using the Herschel telescope found that unlike other comets studied, the comet Hartley 2 contains water with the same chemical signature as water on our planet, suggesting that cometary impacts brought the water that covered the rocky and dry early Earth, the BBC reports. Hartley 2 is believed to have formed in the Kuiper belt, not far outside the solar system. Comets studied earlier came from the much more distant Oort cloud and had a different chemical composition, leading scientists to believe only 10% of Earth's water could have come from comets. "It was a big surprise when we saw the ratio was almost the same as what we find in the Earth's oceans," the lead researcher tells Reuters. "It means it is not true any more that a maximum of 10% of water could have come from comets. Now, in principle, all the water could have come from comets."