Where did the Earth’s oceans come from?
European probe sheds light on the source of Earth’s water
ESA’S ROSETTA SPACECRAFT has challenged current thinking about the origins of water on Earth. It has shown that the make-up of comet water is often different to that on Earth, hinting that asteroids, rather than comets, were the major source of Earth’s oceans.
Scientists believe that when the Earth formed 4.6 billion years ago, it was so hot that its water boiled off. But today our planet has vast oceans, so where did all this water come from? The likely sources are comets and asteroids that hit Earth after it cooled. However, their relative contribution to our planet’s water supply is still unclear.
Rosetta, which arrived at Comet 67P/ Churyumov-Gerasimenko last August, has added some clarity. It measured a key feature of the comet’s water: its ratio of deuterium – an isotope of hydrogen with an extra neutron – to normal hydrogen. The ratio turns out to be more than three times higher than that of Earth’s oceans.
Scientists conclude that asteroids, rather than comets, probably delivered most of our water. Rocky bodies in the asteroid belt between Mars and Jupiter have similar water composition to that on Earth.
“As Rosetta follows the comet on its orbit around the Sun throughout next year, we’ll be keeping a close watch on how it evolves and behaves,” says Matt Taylor, ESA’s Rosetta project scientist. “This will give us unique insight into the mysterious world of comets and their contribution to our understanding of the evolution of the Solar System.”
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