The first detailed scientific results from the Rosetta space mission, which successfully orbited a comet and then soft-landed a small robotic probe on its surface, have punctured one of the most plausible theories of how the Earth became a blue planet with vast oceans of water.
Scientists had until now believed the most likely explanation for the presence of water was one or more comets dumping it on the Earth when they collided on the planet a few hundred million years after it was created.
However, results from an instrument on board Rosetta, which will continue to orbit the comet 67P/Churyumov-Gerasimenko for another year, show comets are not likely to be the source of the terrestrial water on which life on Earth depends. Instead, it’s more likely to have come through asteroid collisions several hundred million years later.
The water on Earth has a particular signature in terms of its ratio of molecular isotopes – normal water and “heavy” water – and the water found on 67P has a ratio three times greater. This almost certainly rules out comets as the source of the water on Earth, said Kathrin Altwegg of the University of Bern, the principal investigator of the Rosetta instrument that measured the comet’s water isotopes.
In pictures: European Space Agency's Rosetta mission
In pictures: European Space Agency's Rosetta mission
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Image of Comet 67P/CG taken by the Philae lander from a distance of approximately 3km from the surface
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Rosetta's lander Philae took this parting shot of its mothership shortly after separation
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Parting shot of the Philae lander after separation, captured by one of Rosetta's cameras
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A technician celebrates after the successful landing of the Philae lander, in the control room at the ESA headquarters in Darmstadt
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Scientists celebrate at a mission observation centre in Toulouse, southern France as they receive information that Philae has landed on the Churyumov-Gerasimenko comet
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Astronomer Klim Ivanovych Churyumov, who discovered the comet 67P/ Churyumov-Gerasimenko in 1969, reacts after the successful landing of the Philae lander on the comet
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A model demonstrates how the landing device Philae, of the space probe Rosetta, stands on the comet 67P/Churyumov-Gerasimenko at the press center of the satellite control center of the European Space Agency in Darmstadt, Germany
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An artist impression of Rosetta's lander Philae on the surface of comet
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Picture taken on October 28 by the navigation camera on Rosetta shows the boulder-strewn neck region of comet 67/P Churyumov-Gerasimenko. It was captured from a distance of 9.7 km from the center of the comet
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Picture taken on October 24 shows a raised plateau on the larger lobe of the comet
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The probe is supposed to fly to a comet and put down a small laboratory on the top of it
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A scientist from the European Space Agency with an airworthy copy of space probe 'Rosetta' in the control center in Darmstadt, Germany
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Maneuvers designed for the actual space probe are simulated with the replica. 'Rosetta' will be woken up from an energy saving hibernation after 957 days
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A European Space Agency employee sits in the control room for the Rosetta mission in Darmstadt, Germany
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Scientists at the European Space Agency are expecting their comet-chasing probe Rosetta to wake from almost three years of hibernation
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Europe's Rosetta probe on a NASA mission
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NASA is participating in the European Space Agency's Rosetta mission, whose goal is to observe one such space-bound icy dirt ball from up close for months on end
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An impression of the Philae lander
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ESA probe Rosetta with Mars in the background. The three-tonne probe blasted off aboard an an Ariane V rocket from Kourou, French Guiana in 2004
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Rosetta orbiter deploying the Philae lander to comet 67P/Churyumov–Gerasimenko, the spacecraft measures 32 m across including the solar arrays, while the comet nucleus is thought to be about 4 km wide
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The spacecraft, festooned with 25 instruments between its lander and orbiter (including three from NASA), is programmed to 'wake up' from hibernation
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An Ariane V carrying the three-tonne probe Rosetta blasting off from Kourou, beginning a decade-long quest to hunt a comet in the depths of the Solar System and shadow it around the Sun in a bid to tease out secrets of how life began on Earth
In a study to be published in the journal Science, Professor Altwegg and her colleagues said that the threefold higher ratio of heavy water to normal water suggests that the family of comets to which 67P belongs “precludes the idea that this reservoir is solely composed of Earth ocean-like water”.
She said: “Models show that the Earth was quite hot after its formation so there was probably no water left on the surface of the Earth.”
Temperatures on the Earth were almost certainly too hot for water to have existed for several hundred million years. This suggests the water was brought here after the planet had cooled down to temperatures that would prevent it being boiled away into space, Professor Altwegg said.
Three years ago, scientists analysing the ratio of water isotopes on the comet Hartley 2 found that it perfectly matched the water-isotope ratio found on Earth, which supported the idea that comets had brought the water to Earth, she said.
However, the results from Rosetta now show that in fact comets generally have very different water isotopes and so are unlikely to have brought today’s water to Earth.
A more likely source now appears to be the rocky asteroids that bombarded the Earth about 3.8 billion years ago, about a billion or so years after the planet was created. These space objects are dry now because they have been orbiting the Sun for so long, but at that time they were wet, Professor Altwegg said.
“There was a lot of impact in the early times, and it’s very likely that some of the terrestrial water was brought at that time and so the only water that’s remaining is actually [from] asteroids or asteroid-like bodies from the inner Solar System,” she said.Reuse content