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New Rosetta Data Suggest Earth’s Water Didn’t Come From Comets

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New Rosetta Data Suggest Earth’s Water Didn’t Come From Comets

Take a virtual trip around the Solar System and you’ll come to one conclusion pretty quickly: Earth is odd. One of the most significant oddities about our pale blue dot is the vast quantity of liquid water. Astronomers and other scientists have proposed various arguments for how Earth ended up with huge, stable liquid oceans — and now, thanks to research from the Rosetta probe, we’ve got evidence that one prominent theory may not be correct.

The European Space Agency’s (ESA) Rosetta spacecraft has found the water vapor from its target comet to be significantly different from that found on Earth. The discovery fuels the debate on the origin of our planet’s oceans. The measurements were made in the month following the spacecraft’s arrival at Comet 67P/Churyumov-Gerasimenko on August 6. It is one of the most anticipated early results of the mission because the origin of Earth’s water is still an open question.

Rosseta and Philae

Rosseta and Philae

The young Earth was a hot place — so hot that most of its surface water evaporated.

At the time, about four billion years ago, the solar system was swarming with asteroids and comets. They pelted Earth’s surface, prompting scientists to hypothesise that maybe it was these objects that helped supply Earth with its oceans. Because comets are known to contain water, they seemed a likely source.

If this were the case, then the chemical signature of Earth’s water would match what is found on comets. Water, of course, is H2O: two parts hydrogen and one part oxygen. It can be made from regular hydrogen, which consists of a proton and an electron, or from a type of hydrogen called deuterium, which has an added neutron.

If Earth’s water came from comets, then the ratios of deuterium to hydrogen would be the same in both Earth and comets.

The comet Rosetta is studying, comet 67P is a Jupiter-family (comet from the Kuiper belt) and one of the mission’s goals is to help determine whether comets like 67P could have produced Earth’s oceans. The answer, however, appears to be no. Rosetta’s measurements revealed a deuterium-to-hydrogen ratio about three times higher than that found on Earth, even higher than in the Oort cloud comets.

“This surprising finding could indicate a diverse origin for the Jupiter-family comets – perhaps they formed over a wider range of distances in the young Solar System than we previously thought,” said Dr Kathrin Altwegg of the University of Bern, Switzerland, who is the principal investigator for ROSINA and the lead author of a paper published in the journal Science.

“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 Earth’s water,” said Matt Taylor from ESA.

“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.”

This new data isn’t exactly a slam-dunk against the theory of panspermia, which generally holds that life (or molecules vital to the eventual formation thereof) were distributed to Earth from another source, but it does imply that there’s no currently known smoking gun that would tie Earth’s water supply to a vital extraterrestrial source.


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