Rosetta casts doubts on origin of Earth’s oceans

Analysis of 67P’s water shows it differs greatly from that found on our planet.


Rosetta arrived at 67P on 6 August and will stay with the comet for the next year.

Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The water vapour found by ESA’s Rosetta spacecraft on its target comet is distinctly different to that found on Earth, throwing doubt on the origin of our planet’s oceans.

Analysing the water content of the comet was one of the mission’s prime objectives, to help determine how water was brought to our planet.

When the Earth first formed, it was so hot that any water should have boiled away into space, yet two-thirds of our planet’s surface is covered by oceans. It’s hypothesised that this water must have been brought in after our planet cooled down. And comets are among the most likely candidates for doing so.

The measurements, made by ESA’s Rosetta orbiter after its arrival at comet 67P/Churyumov-Gerasimenkio, were to determine the comet’s ‘flavour’ – the proportion of deuterium (hydrogen with an additional neutron) to normal hydrogen. This proportion is a unique identifier of the water’s origin.

It found that the composition of 67P contains distinctly more deuterium that Earth. Of the 11 comets for which this measurement has been made, only one has been found with water matching than found on Earth, a Jupiter-family comet designated 103P/Hartley 2.

However, meteorites originating from the Asteroid Belt have been found to contain water matching Earth’s. Despite the fact that these space rocks have a much lower water content, asteroid impacts are much more common than cometary ones, meaning that a large number of them could have slowly built up the oceans over time.

“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,” says Rosetta project scientist Matt Taylor.

“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."


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