The comet being studied by Europe's Rosetta spacecraft contains molecular oxygen, a surprising discovery that will force scientists to rethink details of how the solar system formed.
Scientists expected that the highly reactive gas would have long ago combined with hydrogen, but they found molecular oxygen (abbreviated O2) consistently outgassing from comet 67P/Churyumov-Gerasimenko.
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"It is the most surprising discovery we have made so far in 67P," said Rosetta scientist Kathrin Altwegg, with the Physics Institute and Center for Space and Habitability at the University of Bern in Germany.
"The first time we really saw it I think we all went a little bit into denial because ... oxygen was not among the molecules suspected in a cometary coma," Altwegg said. "All models show that molecular oxygen will react with the hydrogen and will no longer be present."
The discovery also may complicate an evolving strategy to look for signs of extraterrestrial life by scanning the atmospheres of distant planets for telltale chemical signatures. Molecular oxygen, along with methane, is a key bio-signature of life on Earth.
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"If we look at exoplanets, our goal of course will be to detect biosignatures, to see if the planet contains life. And as far as I know, so far the combination of methane and O2 was a hint that you have life underneath it. On the comet, we have both methane and O2, but we don't have life. So it's probably not a very good biosignature," Altwegg said.
Scientists measured the amounts of O2 coming from 67P for months before publishing their results this week in Nature. They found that the levels of O2, relative to water, remained stable as solar heating made the comet more active. That was the proverbial smoking gun that 67P's molecular oxygen was more than skin deep.
"This oxygen has to be present in the whole body. If it were only on the top surface, we would see a decrease over time of the oxygen-to-H20 (water) ratio," said André Bieler, a research fellow and Rosetta scientist at the University of Michigan's Department of Atmospheric, Oceanic and Space Sciences.
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That realization led scientists to conclude that the O2 was present when the comet formed, a period that dates back to soon after the birth of the solar system some 4.6 billion years ago.
"This tells us something about the accretion -- the building process -- of our solar system. It had to very gentle. This ice has never been heated up enough to get reprocessed. It seems like it's a pretty pristine material still," Bieler said.
"Now the question is how did it get there and how did it survive for such a long time?" he added.