The observations were made by the ROSINA instrument and are reported this week in the journal Science. Our full story is published on the ESA web portal – read it here.

The measurements were made in the month following the spacecraft’s arrival at Comet 67P/Churyumov–Gerasimenko’s on 6 August. It is one of the most anticipated early results of the mission, because the origin of Earth’s water is still an open question. A leading hypothesis is that comets and asteroids played a role in delivering it to Earth, but the relative contribution of each class of object to our planet’s water supply is widely debated.

The key to determining where the water originated is in its ‘flavour’, in this case the proportion of deuterium – a form of hydrogen with an additional neutron – to normal hydrogen.

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.

“Our finding also 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 Earth’s oceans.”

This is just an excerpt from our story published on the ESA web portal today; read our full report – including more background information and graphics – here: Rosetta fuels debate on origin of Earth’s oceans