The first scientific results from Europe's Rosetta spacecraft, which successfully orbited a comet and then soft-landed a small robotic probe on its surface, have undercut one of the most reasonable theories of how the Earth became a blue planet with vast oceans of water.
Till now, the scientists had believed that the presence of water on earth was due to one or more comets dumping water 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, demonstrate comets are not likely to be the source of water on which life on earth depends. Instead, it is likely to have come through asteroid collisions several hundred million years later.
The latest study demonstrates that the chemical signature of water on comet 67P is unlike that on Earth. As 67P is believed to be typical of many comets that originate in the far reaches of the solar system, the findings counteract the view that comets were the main source of water for Earth?s oceans.
Some water is made from regular hydrogen atoms, which, in turn, are made from one proton and one electron. But in other cases, the hydrogen is replaced by its heavier isotope called deuterium, which also includes a neutron.
Rosetta's instruments found that 67P's water had a deuterium-hydrogen ratio that is three times that of Earth's water.
By studying the craters found on the moon and elsewhere, scientists hypothesize that between 4.1 and 3.8 billion years ago a number of asteroids and comets peppered earth and other planets, an event called "the late heavy bombardment." They argue that earth was seeded with water during this period.
Scientists had demonstrated that the chemical signature of water found in meteorites was the same as the signature of Earth's water. Moreover, asteroids also were formed between the orbits of Mars and Jupiter - which, in cosmic terms, isn't that far from Earth.
On the other hand, comets exist much further out in the solar system. Their orbital patterns are extended and more elongated than the orbits of asteroids, which makes them only occasional visitors to the inner reaches of the solar system.
With the latest Rosetta results, the pendulum has swung back to asteroids quite strongly. Geoff Blake, a cosmochemist at the California Institute of Technology, who wasn't involved in the Science study, said it rules out comets as the dominant source of earth's water.