Scientists from the Rosetta mission have found oxygen in the atmosphere of comet 67P/Churyumov-Gerasimenko, a discovery that could change our understanding of how the solar system formed.
The molecular oxygen (O2) was detected by the ROSINA mass spectrometer, one of a suite of instruments aboard the Rosetta spacecraft that has been traveling with the comet since August 2014.
The revelation came as quite a shock.
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"The first time we saw it, we all went a little bit into denial because molecular oxygen was really not expected to be found on a comet," said Kathrin Altwegg of the University of Bern in Switzerland, the project leader for ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis.
"It does not sound that spectacular, but it is actually the most surprising discovery we have made so far on 67P," she said of the comet, which is about to pass the orbit of Mars.
Molecular oxygen is common on Earth, but it is rarely seen elsewhere in the universe. In fact, astronomers have detected molecular oxygen outside the solar system only twice, and never on a comet.
Oxygen is highly reactive, meaning it likes to bond to other kinds of atoms. Therefore, it was previously thought that all the oxygen present at the dawn of the solar system would have combined with the abundant hydrogen present at the time to form H20, or water.
Even if some pure oxygen managed to make it out of the primordial cloud of dust and gas from which our solar system emerged, computer models suggest that in the 4.5 billion years since then, the oxygen would have had ample opportunity to be reprocessed, especially in the chaotic early days of the solar system formation.
"All the models say it shouldn't be there and it shouldn't survive for such a long time," said Andre Bieler, who studies cometary science at the University of Michigan.
But instead, the ROSINA instrument found that oxygen is the fourth most abundant compound in the gaseous coma of 67P after water, carbon monoxide and carbon dioxide.
"It is not only that we have oxygen, we have a lot of oxygen," Altwegg said.
To make sure the O2 readings were not caused by an instrumental defect, the team checked oxygen levels at different distances from the comet. The closer to the comet the spacecraft flew, the more oxygen it detected. As it flew farther away, it detected less.
One way that molecular oxygen can form in space is through a process called radiolysis. This occurs when energetic particles coming off the sun break up the bonds of water ice. Experiments have shown that hydrogen can diffuse out of this process, leaving the oxygen with no other molecules with which to react, Bieler said.
Like all objects in the solar system, comet 67P has been hit with this high-energy radiation for billions of years, but it is unlikely that radiolysis can explain all the oxygen in the comet, the researchers said. High-energy particles from the sun would penetrate only a few meters into the comet's surface. Yet each time the comet orbits the sun, it sheds between one and 10 meters from its circumference.
In a Nature paper describing the discovery, the researchers note that the ratio of water to oxygen in the comet's atmosphere remained the same over the many months that ROSINA was collecting measurements. This suggests that the oxygen is present throughout the body of the comet and not just at its surface, Bieler said.
And if that's the case, the next logical conclusion is that molecular oxygen was present at the time the comet formed.
"Observing molecular oxygen anywhere in nature is rare because it is so reactive," said Christopher Snead, who studies comet dust at UCLA and was not involved with the paper. "The implication that this is primordial material is very interesting. It starts to give you a peek into the environment in which the solar system formed."
The findings lead to two puzzles for computer modelers to tackle. The first: What conditions were necessary for molecular oxygen to get trapped in the icy crystals of a comet like 67P? The second: How did that oxygen remain in its pure state for so long?
"This work tells us that the building process of our solar system had to be very gentle for those ice grains to never have really been heated up or reprocessed," Bieler said.
Another implication of the finding is that looking for molecular oxygen on distant bodies may not be the best way to look for extraterrestrial life.
"As far as we knew the combination of methane and O2 was a hint that you had life, but on our comet we have both methane and O2, but we don't have life, so it is probably not a very good bio-signature" Altwegg said.
But comet scientists say they are thrilled to have more evidence that the objects they study have preserved ancient materials that predate even our solar system.
"The cometary community has always said that comets are some of the least-processed bodies in the solar system," Bieler said. "Now we have evidence that at least a significant part of this comet survived the whole formation of our solar system."
That makes it a very pristine object indeed.