Oxygen leaking from comet surprises astronomers

A comet is leaking oxygen molecules that have been buried since the beginning of the solar system.

The Rosetta spacecraft detected O₂ around comet 67P/Churyumov-Gerasimenko, the first time these molecules have been seen around a comet. The oxygen is probably primordial, trapped in water ice as the comet was assembled roughly 4.6 billion years ago, researchers report in the Oct. 29 Nature. Andre Bieler, a planetary scientist at the University of Michigan in Ann Arbor, and colleagues detected the oxygen using a mass spectrometer on board Rosetta, which has been orbiting comet 67P since August 2014.

“This is the most surprising discovery we have made so far,” study coauthor Kathrin Altwegg said at an October 27 news briefing. Researchers did not expect to find oxygen in the fog of gas surrounding a comet. Oxygen is highly reactive, and theories about the formation of the solar system indicate that O₂ should have quickly interacted with hydrogen to form water. “When we first saw it, we all went into a little bit of denial,” said Altwegg, of the University of Bern in Switzerland.

Oxygen’s presence supports the long-held assumption that comets are pristine fragments from the dawn of the solar system. Comet 67P must have been put together gently, Bieler says, otherwise the ice-coated grains that make up its bulk would have been heated and the oxygen removed. Because the grains have not been heated, they are unprocessed time capsules — frozen samples that preserve the conditions that prevailed when the planets were forming.

Ultraviolet light from the sun and free-range electrons are probably responsible for creating the O₂ in the first place. High-energy photons and particles can zap water molecules, which in turn reform into molecules of oxygen (and hydrogen). The oxygen was then trapped within ice that collected on dust grains, which in turn came together to assemble the comet. There the oxygen stayed protected for nearly the age of the solar system. As recently as 1840, comet 67P was far enough out in the solar system to escape the sun’s destructive influence, but an encounter with Jupiter nudged it in closer. With each close approach to the sun, heat reaches into the comet, sublimates the ice, and liberates the O₂.

“It’s just fascinating that O₂ was detected in a comet and never has been before,” says Lori Feaga, a planetary scientist at the University of Maryland in College Park. That’s partly because molecular oxygen is impossible to detect using ground-based telescopes; the oxygen in Earth’s atmosphere gets in the way. And previous instruments flown on other comet-bound spacecraft were not sensitive enough to sense oxygen.

Plus, oxygen molecules are rare in the cosmos. Observations of gas clouds where stars are forming in the Milky Way have turned up gaseous oxygen in only two locations: the dark rho Ophiuchi cloud in the constellation Ophiuchus and the more famous Orion nebula. The scarcity of interstellar oxygen is another reason why comet 67P’s oxygen was unexpected.

“They were surprised and I was surprised,” says Paul Goldsmith, an astrophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who helped discover the O₂ in Orion. Rosetta has shown that O₂ hides within icy coatings on the dust grains that make up a comet. If oxygen is tucked away in similar grains found in star-forming clouds, oxygen might be more abundant than thought. “The picture of interstellar chemistry is not as simple as some people would make it,” he says.

If dust grains are helping create (and protect) oxygen, they’re probably important for building other molecules as well, Goldsmith notes. These dust grains are thought to be the place where hydrogen molecules form, for example. Understanding how quickly H₂ is assembled leads to better estimates of how efficiently clouds of atoms turn into clouds of molecules, he says. And it’s those dense molecular clouds that are the sites of future stars, planets and comets.