Comets May Not Have Brought Water To Earth, After All

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The Rosetta spacecraft has detected some strange water vapor streaming from Comet 67P — water that's significantly different to what we have here on Earth. The startling discovery challenges the popular assumption that much of our water was delivered here by comets.

By 'weird' water we're talking about major differences in the deuterium-to-hydrogen ratio (D/H). Comet 67P has water that's not only different from what we have here on Earth, it's also markedly different from water observed in other Solar System bodies.


The measurements, which are some of the most highly anticipated of the mission, were taken by Rosetta's ROSINA's DFMS double focusing mass spectrometer between 8 August and 5 September 2014.


Deuterium is an isotope of hydrogen that carries an additional neutron. Scientists study the ratio of deuterium to hydrogen in water because it's a way to determine where in the Solar System an object came from. But it may also be able to tell scientists what proportion of asteroids and comets contributed to Earth's oceans.

Indeed, a leading hypothesis on the origins of Earth's water suggests that it was delivered by comets and asteroids after our planet had cooled down. But the relative contribution of each class of object to our planet's water supply is still debated.


The ESA explains:

Previous measurements of the deuterium/hydrogen (D/H) ratio in other comets have shown a wide range of values. Of the 11 comets for which measurements have been made, it is only the Jupiter-family Comet 103P/Hartley 2 that was found to match the composition of Earth's water, in observations made by ESA's Herschel mission in 2011.

By contrast, meteorites originally hailing from asteroids in the Asteroid Belt also match the composition of Earth's water. Thus, despite the fact that asteroids have a much lower overall water content, impacts by a large number of them could still have resulted in Earth's oceans.

It is against this backdrop that Rosetta's investigations are important. Interestingly, the D/H ratio measured by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, or ROSINA, is more than three times greater than for Earth's oceans and for its Jupiter-family companion, Comet Hartley 2. Indeed, it is even higher than measured for any Oort cloud comet as well.

"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," says Kathrin Altwegg, principal investigator for ROSINA and lead author of the paper reporting the results in the journal Science this week.


To summarize, this finding potentially rules out the possibility that Jupiter-family comets exclusively contain Earth ocean-like water, and adds credence to the suggestion that asteroids were the main delivery mechanism for Earth's oceans.

Images: ESA

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