Scientists were perplexed when the European Space Agency’s Rosetta probe observed copious molecular oxygen exploding from comet 67P/Churyumov-Gerasimenko (67P) in 2015. They’d never seen a comet spew oxygen, much alone so much of it. The most troubling consequences were that researchers had to account for so much oxygen, which required them to rethink what they thought they understood about the chemistry of the early solar system and how it originated.
However, a recent study headed by planetary scientist Adrienn Luspay-Kuti at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, suggests that Rosetta’s finding may not be as odd as scientists initially thought. Instead, research shows that the comet contains two interior stores that make it seem as though there is more oxygen than there is.
“It’s a bit of an illusion,” Luspay-Kuti said. “In actuality, the comet does not have this tremendous oxygen abundance, at least not in terms of formation, but it has collected oxygen that becomes trapped in the higher layers of the comet and then expelled all at once.”
While plentiful on Earth, molecular oxygen (two oxygen atoms that are doubly bonded to each other) is very rare elsewhere in the cosmos. Because it easily attaches to other atoms and molecules, particularly the globally abundant elements hydrogen and carbon, oxygen exists in just a few molecular clouds. Many scientists concluded that any oxygen in the protosolar nebula that produced our solar system had been similarly picked away.
When Rosetta discovered oxygen leaking from comet 67P, everything changed. Nobody had ever seen oxygen in a comet before, and being the fourth most common component in the luminous coma of the comet (after water, carbon dioxide, and carbon monoxide), it demanded an explanation. Many experts believe the oxygen came off the comet with water, leading them to believe it was either primordial – meaning it was bound up with water at the creation of the solar system and accumulated in the comet when it subsequently developed – or produced from water after the comet had formed.
However, Luspay-Kuti and her colleagues were doubtful. As the comet’s dumbbell form gradually rotates, each “bell” (or hemisphere) faces the Sun at different points, implying that the comet has seasons and that the oxygen-water link may not always be there. Volatiles might possibly switch on and off in short time periods when they thaw and refreeze with the seasons.
You can see it now, but you can’t see it now
Taking advantage of these seasons, the researchers reviewed molecular data from short- and long-time intervals shortly before the comet’s southern hemisphere entered summer and then again just as it exited summer. According to their research, which was published on March 10 in Nature Astronomy, the scientists discovered that when the southern hemisphere rotated away from the Sun, the relationship between oxygen and water vanished. Because the quantity of water ejected by the comet had reduced dramatically, the oxygen seemed to be intimately related to carbon dioxide and carbon monoxide, which the comet was still producing.
“There’s no way it should be conceivable based on the prior explanations,” Luspay-Kuti remarked. “If oxygen was primordial and linked to water in its genesis, there should never be a period when oxygen significantly correlates with carbon monoxide and carbon dioxide but not with water.”
Instead, the scientists argued that the comet’s oxygen comes from two reservoirs: one deep beneath the comet’s stony core consisting of oxygen, carbon monoxide, and carbon dioxide, and a shallower pocket closer to the surface where oxygen chemically interacts with water ice molecules.
Because oxygen, carbon dioxide, and carbon monoxide all evaporate at extremely low temperatures, a deep store of oxygen, carbon monoxide, and carbon dioxide ice is continually discharging gases. However, when oxygen travels from the comet’s core to the surface, part of it chemically combines with water ice (a significant component of the comet’s nucleus) to generate a second, shallower oxygen reservoir. However, since water ice vaporizes at a considerably higher temperature than oxygen, the oxygen remains trapped until the Sun sufficiently warms the surface and vaporizes the water ice.
As a result, oxygen may collect in this subsurface reservoir for lengthy periods of time until the comet surface is ultimately warmed enough for water ice to melt, generating a plume significantly richer in oxygen than the comet itself.
“In other words,” Luspay-Kuti added, “the oxygen abundances detected in the comet’s coma do not always represent the abundances observed in the comet’s core.”
As a result, the comet would oscillate with the seasons between highly associating with water (when the Sun warms the surface) and strongly associating with carbon dioxide and carbon monoxide (when the surface faces away from the Sun and the comet is sufficiently far), as Rosetta observed.
“This isn’t just one explanation: it’s the only explanation since there aren’t any others,” said Olivier Mousis, a planetary scientist at France’s Aix-Marseille Université and research co-author. “These patterns recorded by Rosetta would not be noticed if oxygen were just flowing from the surface.”
The main consequence, he says, is that the oxygen in comet 67P is oxygen that accreted during the beginning of the solar system. It’s simply that it’s a fraction of what people expected.
Luspay-Kuti plans to go further into the subject by investigating the comet’s minor molecular species, such as methane and ethane, and their relationships with molecular oxygen and other key species. She believes this will aid researchers in determining the sort of ice into which the oxygen was integrated.
“You still have to figure out how to integrate the oxygen into the comet,” Luspay-Kuti said, noting that the quantity of oxygen in the comet is larger than in other molecular clouds. She did, however, predict that the majority of researchers would breathe a sigh of relief at the study and its findings.