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. 
 
 The result, published in Nature, hints at the idea that much of the Earth's water could have initially come from cometary impacts. 
 
 Just a few million years after its formation, the early Earth was rocky and dry; most likely, something brought the water that covers most of the planet today. 
 
 Water has something of a molecular fingerprint in the amount of deuterium it contains, and only about half a dozen comets have been measured in this way - and all of them have exhibited a deuterium fraction twice as high as the oceans. 
 
 Asteroids, by contrast, give rise to the meteors and meteorites that arrive on Earth, making their deuterium fraction more well-established. 
 
 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. 
 
 Clouded measure 
 
 Until now, all of the comets that have been measured have been so-called Oort Cloud objects, believed to have been formed early in the Solar System's history in the region of the giant planets Neptune and Uranus and kicked out to a great distance as they bumped into the planets and each other. 
 
 Comet Hartley 2 is the first "Kuiper Belt" object to undergo the deuterium analysis. Kuiper belt objects formed not far outside our Solar System, and comets that originate there have much shorter orbits than those from the Oort Cloud. 
 
 Image caption Comets carry much more water than asteroids, but it may or may not be like the water found on Earth 
 
 An international team using the Herschel telescope to peer at the comet, they found it had a deuterium fraction much closer to that of our oceans. 
 
 Report co-author Ted Bergin of the University of Michigan said that opened up the possibility that comets at least contributed to our water supply. 
 
 "The reservoir of Earth ocean-like material is much larger than we thought, and it encompasses cometary material, which we hadn't recognised," he told BBC News. 
 
 "We have to think really hard and try to get a better understanding of what is going in our Solar System, and whether you can really rule out comets as the source of Earth's water." 
 
 They might not be ruled out, but they are not the definitive answer either; much of what we believe happened in the early Solar System is based on computer models. 
 
 James Greenwood of Wesleyan University in the US said such models may need adjusting in light of the new evidence - and that more such studies are needed to assess whether many Kuiper Belt objects are like Hartley 2. 
 
 "If the short-period comets are all like this one comet, then this could be a significant source of our early water," he told BBC News. 
 
 "It opens up a new can of worms for us." 
 
 Alessandro Morbidelli of the Observatory of Cote d'Azur argues that the result shows that the distinction between the potential water sources may need to be called into question. 
 
 "In the past, scientists thought that these asteroids and comets were completely different classes of bodies. Now, several new results show that primitive asteroids and comets are brothers and sisters," he told BBC News. 
 
 "This new view changes at least the semantics of the question on the origin of the Earth's water. The question becomes more technical: 'from which region of the disc and by which dynamical mechanism came the (objects) that delivered the water to the Earth?'" 
 
 Herschel has some time left to address the question, but what all the researchers agree is that the Atacama Large Millimeter Array of telescopes in Chile - which has just shown off its first results - will soon be able to resolve these questions with never-before-seen sensitivity. ||||| The comet Hartley is seen in this undated image courtesy of NASA. 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. 
 
 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. 
 
 The intense heat of the planet immediately after it formed means any initial water would have quickly evaporated and scientists believe the oceans emerged around 8 million years later. 
 
 The puzzle is where the water, which is vital for life on Earth, came from. 
 
 Past analysis of water-ice from far-flung comets suggested they could have delivered no more than 10 percent of today's oceans because the chemical "fingerprints" did not match up. 
 
 But research from Paul Hartogh of Germany's Max Planck Institute for Solar System Research and colleagues published on Wednesday showed a comet called 103P/Hartley 2 has the same chemical composition as the Earth's oceans. 
 
 The finding substantially increases the amount of water that could have originated from comets, which are made up of rock and ice with a characteristic tail of gas and dust. Previous models of the early Earth implied most water came from asteroids. 
 
 In the case of Hartley 2, researchers using infrared instruments on the Hershel Space Observatory found that ice on the comet has a near identical "D/H" ratio to seawater. D/H measures the proportion of deuterium -- or heavy hydrogen, which has an extra neutron -- compared to ordinary hydrogen in water. 
 
 "It was a big surprise when we saw the ratio was almost the same as what we find in the Earth's oceans," Hartogh told Reuters. 
 
 "It means it is not true any more that a maximum of 10 percent of water could have come from comets. Now, in principle, all the water could have come from comets." 
 
 Hartogh, whose research was published online in Nature, believes Hartley 2, whose current orbit around the sun does not extend much beyond Jupiter, started life in a different part of the solar system than other comets studied. 
 
 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. 
 
 (Reporting by Ben Hirschler; Editing by Janet Lawrence) ||||| Getty Images 
 
 The moon is home to some of the most lyrically named bodies of water that never existed. The Sea of Tranquility is familiar enough, but what about the Ocean of Storms, the Sea of Nectar, the Lake of Forgetfulness, the Bay of Rainbows? Altogether, the lunar map features 20 seas, 14 bays, 20 lakes and one ocean. That’s both poetic and ironic, because a world that’s positively drenched in aquatic names has not a drop of actual water. 
 
 Or that’s what we always thought. In recent years, however, evidence for a watery moon has been mounting—particularly from NASA’s Lunar Reconnaissance Orbiter and its sister probe, the Lunar Crater Observation and Sensing Satellite. The moon is hardly a sodden place—it’s wetter than the Sahara Desert, but that’s about it. Still, that’s huge compared to what we once believed. Now, a paper in Science explains in greater detail than ever how water got to so unlikely a place and how it survived the fiery violence that gave birth to the moon in the first place. 
 
 The prevailing—and now all-but universally accepted—theory for how the Earth-moon system came to be was first proposed in 1975 and quickly became known as the Big Whack theory. About 4.5 billion years ago, a Mars-sized planetesimal careered through the solar system and collided with the infant Earth, sending up a massive cloud of molten and vaporous debris. The hit-and-run world sailed on its way and the cloud of debris slowly accreted into the moon, eventually retreating to a stable orbit roughly 239,000 mi. (385,000 km) away, where it has remained ever since. 
 
 “The angular momentum, the distance, the size of the moon,” says Brown University geochemist Alberto Saal, the lead author of the Science paper, “there is no other model that can explain the physics of the Earth-Moon system than the Big Whack.” 
 
 (MORE: The Universe’s Biggest, Baddest Gamma Ray Burst Ever) 
 
 Physics isn’t chemistry, however, and none of this explained with any precision the elements and compounds—particularly water—that would have survived aboard the early moon. The inner solar system as a whole ought to be a very dry place. When the planets and moons that orbit the sun were first accreting, it took a very particular environment for water molecules to form. Anything too close to the sun would simply be too hot and energetic for H2O molecules to assemble themselves. You’d have to get about 3 astronomical units away (1 AU is the distance from the sun to the Earth) to reach what Saal calls the “snow line,” the place water could form. That boundary is roughly between Mars and Jupiter. 
 
 Nothing, however, would prevent water from being imported from the wet zone into the dry zone aboard comets or water-rich meteors known as carbonaceous chondrites. In recent years, comets have gained traction as the source of Earthly water. They’re mostly made of water ice and rock after all, and we now understand just how many billions—perhaps trillions—of them there are surrounding the solar system in the formations known as the Kuiper Belt and the Oort Cloud. 
 
 But there’s a good case to make for the carbonaceous chondrites too. They’re hardly the sodden snowballs comets are, and the little bit of water they do contain is integrated deep in their matrices. But there are a great, great many of the meteors flying around out there, and there’s nothing to say a downpour of rocks, rather than two or three comets, could not have watered us too. The growing study of exoplanets is helping to explain just how this could happen. 
 
 (MORE: Hopeful New Signs of Duplicate Earths) 
 
 “In 2011,” Saal says, “there were observations in other solar systems of large planets like Jupiter that probably migrated in towards their suns and then migrated back out, due to gravitational resonance. This would affect overall gravitational fields causing materials from deeper out to move in.” That material would likely include meteors. That same scenario could have played out here too, and there’s a relatively straightforward way to prove it. 
 
 Water comes in more than one form. So-called heavy water is not simple H2O, but 2H2O, containing a hydrogen isotope, known as deuterium. Unlike ordinary hydrogen, with just a proton in its nucleus, deuterium also contains a neutron. Heavy water and ordinary water are present on comets, carbonaceous chondrite meteors and Earth, but in different ratios, known as their deuterium/hydrogen, or D/H ratio. Studies since 1998 have shown that Earth’s D/H ratio is more similar to that of the meteors than of the comets, making a strong case for meteoric origins for Earth’s water. That case is not closed: a 2011 study of Comet Hartley 2 drew the opposite conclusion. Still, at the moment, the meteor camp is winning. 
 
 (PHOTOS: Windows on Infinity: Pictures from Space) 
 
 What Saal and his colleagues succeeded in doing was analyzing the D/H ratios in tiny beads of volcanically cooked glass in lunar samples brought back by Apollo 15 and Apollo 17 to see how closely they parallel Earth’s. The answer: exceedingly closely. “The moon’s D/H rations are similar to Earth’s and they’re both similar to the chondrites’,” says Saal. “What difference there is is probably due to the impact.” 
 
 So the moon’s water came from the Earth and maybe the impactor. But the fact that any water at all survived that collision is something of a surprise, since water’s extreme volatility should have caused it simply to vanish into space. Saal, however, cites research by Caltech astrophysicist David Stevenson and others that the blast might have created a sort of envelope of hot gasses, protecting the water while the moon accreted. 
 
 Of course, just as the Earth and moon drew apart in distance over time, so too did they part ways developmentally. The Earth’s greater gravity allowed it to retain an atmosphere, protecting the water that was there, as well as any that barreled in later. The moon could keep enough only to achieve its decidedly humbler wetter-than-the-Sahara status. Lunar gravity may turn the tides in our distant oceans, but the moon’s own seas and lakes and bays were—and always will be—mere phantasms. 
 
 (FROM THE ARCHIVES: Moon: From the Good Earth to the Sea of Rains)
– 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."