The Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) instrument aboard ESA’s Rosetta spacecraft has found that the composition of the water vapor from its target comet – the Jupiter family comet 67P/Churyumov-Gerasimenko – is significantly different to that found on our planet.
This graph displays the different values of the deuterium/ hydrogen ratio in water observed in various objects in the Solar System: the data points are grouped by color as planets and moons (blue), chondritic meteorites from the asteroid belt (grey), comets originating from the Oort cloud (purple) and Jupiter family comets (pink). Rosetta’s Jupiter-family comet is highlighted in yellow. Diamonds represent data obtained in situ; circles represent data obtained by astronomical methods. Image credit: ESA / ATG medialab / Rosetta / NavCam / Altwegg et al.
ROSINA’s double focusing mass spectrometer made the measurements in the month following the Rosetta’s arrival on August 6, 2014. 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 the proportion of deuterium – an isotope of hydrogen with an added neutron – to normal hydrogen.
ROSINA collected over fifty spectra between August 8 and September 5. The deuterium/ hydrogen ratio was derived from the measurements of HD16O/H216O.
Previous measurements in other comets have shown a wide range of values. Of the eleven 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.
Interestingly, the deuterium/ hydrogen ratio measured in the water vapor from 67P/Churyumov-Gerasimenko (5.3 x 10–4) is more than three times greater than for Earth’s oceans (1.56 × 10–4) and for 103P/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,” 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.
“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.”
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K. Altwegg et al. 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science, published online December 10, 2014; doi: 10.1126/science.1261952
