"It is the most surprising discovery we have made so far, because oxygen was not among the molecules expected in a cometary coma," the University of Bern's Kathrin Altwegg said at a news conference Tuesday held by Nature. Altwegg, one of the new study's authors, is in charge of Rosetta's ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis). This instrument is used to "sniff" out the atmospheric composition of the comet.
About a year ago, she and the rest of the team now report, it sniffed out some O2. The gas enveloping a comet — known as its coma — is usually comprised of carbon monoxide, carbon dioxide and water. There are atoms of water in all three of those compounds, of course, but atmospheric atoms of oxygen — the stuff present in our own atmosphere — were unexpected.
"We all went a little bit into denial," she said. The result was so unexpected that the team wanted to be absolutely certain that the oxygen wasn't the result of some kind of instrumentation failure or contamination. But the oxygen, Altwegg explained, "follows the comet very well." The team has observed that oxygen levels are higher close to the comet, and disappear as the instrument pulls away.
Furthermore, her co-author André Bieler of the University of Michigan explained, the oxygen to hydrogen ratios have stayed level throughout their observations.
This is important, because Comet 67P was getting ever closer to the sun during that time. As it approached its closest pass of the sun (called its perihelion) in August, the comet became more "active" — with more and more of its ice sublimating into gas.
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Molecular oxygen has already been spotted on moons of Jupiter and Saturn. This moon oxygen comes about when high-energy particles strike compounds containing oxygen and free it.
But if the oxygen on the comet were due to some kind of recent process like this one, the researchers say, they would have seen a smaller amount of it (relative to the more common H2O) as the comet off-gassed and exposed virgin surface. The reason scientists love studying comets is that their inner bodies contain relatively pristine chemical imprints of their formation, giving us a hint at what the early solar system looked like.
"It must have been primordial," Bieler said at Tuesday's news conference. "The O2 must have been present before or at the formation of the comet."
Their best guess is that the oxygen predates the solar system itself, and was created when high-energy particles collided with grains of ice in the cold, dense region where our sun would one day be born — creating hydrogen and oxygen.
But no current models of the solar system's formation would allow for molecular oxygen to get locked away inside a comet. Oxygen is highly reactive, so it's always been assumed that any molecular oxygen present would bond with the abundantly available hydrogen. The researchers were shocked to see that oxygen could "survive" in its molecular form for billions of years.
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