The European Space Agency (ESA’s) Rosetta spacecraft is still chasing Comet 67P/Churyumov–Gerasimenko, and its recent findings may blow away theories about how water arrived on Earth.
One theory about how Earth’s oceans formed assumes that the Earth was so hot when it originally formed 4.6 billion years ago that its original water boiled off. New water had to be delivered from somewhere, most likely either comets or asteroids.
Rosetta’s findings tilt the balance in favor of asteroids. The spacecraft discovered the water vapor from the comet has a chemical “flavor” that’s very different from water vapor on Earth.
The “flavor” of water in this case refers to the proportion of deuterium – a form of hydrogen with an additional neutron – to normal hydrogen. The flavor should theoretically be different depending on where the water was in relation to the sun in the early solar system. Determining where is not a straightforward process, of course, as comets can get pushed out of their original orbits by the gravitational pull of planets.
Of the 11 comets previously tested, only one, Comet 103P/Hartley 2, had water with the same balance as that found on Earth. That comet was, like Comet 67P/Churyumov–Gerasimenko, a “Jupiter Family” comet, which means that scientists believe they originated from roughly the same place in the Kuiper Belt beyond Neptune. The fact that Rosetta’s target actually has a deuterium ratio three times higher than Hartley 2 and Earth’s water throws a wrench in the idea that comets from that location all contain Earth-like water.
On the other hand, meteorites from the Asteroid Belt do match Earth water.
“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 Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, or ROSINA and lead author of the paper reporting the results in the journal Science this week.
“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.”
ESA’s Rosetta project scientist, Matt Taylor, said Rosetta is bound to keep changing our understanding of the solar system as it continues its mission.
“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 Taylor.