Scientists hail comet water finding
Water - but not as we know it - has been found around the comet that the Rosetta probe landed on last month.
The discovery, made by an instrument on board the Rosetta spacecraft, calls into question a leading theory about how the Earth got its oceans.
The finding suggests that rocky asteroids rather than icy comets may have been chiefly responsible for bringing water to Earth early in its history.
The Rosetta mission hit the headlines when the spacecraft deployed a robot lander, Philae, that made a dramatic descent to comet 67P/Churyumov-Gerasimenko (67P) on November 12.
Data from the lander, which bounced twice before coming to rest near the wall of a crater, are yet to yield scientific results.
But an instrument on the orbiting Rosetta mothership has come up with a surprise after analysing water vapour enveloping the comet.
Unlike the Earth's oceans, the vapour largely consists of water with a different atomic flavour containing deuterium, the "heavy" isotope of hydrogen. The amount of deuterium compared with normal hydrogen in the comet's water is three times greater than it is in water on Earth.
This suggests that so-called Jupiter-family comets, such as 67P, cannot be assumed to have created the oceans on Earth.
In contrast, meteorites falling on Earth that originate from the asteroid belt between Mars and Jupiter tend to match the composition of Earth ocean water.
Professor Kathrin Altwegg, principal investigator for Rosetti's Rosina mass spectrometer that carries out chemical analysis based on light signals, said: "Our finding .. rules out the idea that Jupiter-family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for the Earth's oceans."
Members of the Rosetta team reported the results in the journal Science.
The European Space Agency's British Rosetta project scientist Dr Matt Taylor said: "We knew that Rosetta's in situ analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science, and this outstanding observation certainly adds fuel to the debate about the origin of the Earth's water.
"As Rosetta continues to follow the comet on its orbit around the Sun throughout next year, we'll be keeping a close watch on how it evolves and behaves, which will give us unique insight into the mysterious world of comets and their contribution to our understanding of the evolution of the Solar System."
Scientists have long wondered about the oceans because the Earth was so hot when it formed 4.6 billion years ago that all its original water should have boiled off.
Yet today, two thirds of the Earth's surface is covered in water. The most likely explanation is that after the Earth cooled down, collisions with water-bearing objects such as comets or asteroids filled up the ocean basins.