A close-up examination of Comet 67P/Churyumov–Gerasimenko reveals dancing gravel, whirling icy debris and transient, movable depressions on its smooth terrain, courtesy of photos from the Rosetta mission.

Comet 67P emits ancient molecular oxygen from its nucleus

Comet 67P/Churyomov-Gerasimenko – arguably the most-studied comet in history – has yielded a cosmic surprise: It emits molecular oxygen drawn from its nucleus.

Comet comas – the expanding gaseous atmospheres around the solid nuclei of comets – were known to contain mostly water, carbon monoxide and carbon dioxide, but the European Space Agency’s Rosetta mission’s ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) sensor found abundant molecular oxygen. 

Now, an international group of scientists – which includes Cornell’s Jonathan Lunine and led by the Johns Hopkins Applied Physics Laboratory’s Adrienn Luspay-Kuti – suggests that in addition to the surface ice, an ancient reservoir of molecular oxygen is stored in its nucleus. This discovery also could shed light on how early organic matter and molecules found their way on to solid bodies in the solar system.

The research, “Dual Storage and Release of Molecular Oxygen in Comet 67P/Churyumov-Gerasimenko,” was published March 10 in Nature Astronomy.

“Scientists were surprised at how much molecular oxygen was measured in this comet, relative to the water and the other gases,” said Lunine, professor and chair in the Department of Astronomy in the College of Arts and Sciences. “No one predicted that.

“We were seeing outgassing of molecular oxygen from the depths of the nucleus, not just from the surface, but from deep down,” he said.

Comet 67P is large and it rounds our sun every 6.5 years. Like other comets, it is presumed to be composed of material that formed the planets some 4.5 billion years ago at the birth of our solar system.

Rosetta was a complex, rare and scientifically patient mission – flying in formation with the comet and sending a probe to its surface. The mission launched in 2004 and the spacecraft received a gravity-assist (a gravitational slingshot maneuver used to accelerate a spacecraft) from Earth in 2005, an assist from Mars in 2007, and two more assists from Earth in 2007 and 2009. Waiting for comet to swing by the sun, Rosetta entered hibernation in 2011 and awoke in 2014. From there, it became the comet’s constant two-year companion.

The new paper brought in Lunine’s expertise in chemistry. He modeled the process for how the molecular oxygen was trapped in the comet’s icy surface and how it emerged from the nucleus.

As Comet 67P swings past the sun, it starts warming up about a year before perihelion – the closest the comet gets to the sun.

The relationships between the outgassing (emission) of molecular oxygen, carbon monoxide and carbon dioxide change due to the sun’s warming influence and subsequent cooling after the comet flew by the sun on its outward-bound trip. Effectively, the oxygen gets trapped on the comet’s near-surface layers, while a larger, older inventory remains inside the comet.

“It’s kind of an illusion. In reality, the comet doesn’t have this high oxygen abundance, at least not as far as its formation goes,” Luspay-Kuti said. “But it has accumulated oxygen that gets trapped in the upper layers of the comet, which then gets released all at once.”

“Both of these reservoirs are important, resolving this mystery about the apparent overabundance of oxygen in the near surface layers, and actually identifying a deep source of that gas in the comet,” Lunine said.

In the early history of the solar system, comets were delivered to the planets thanks to gravitational perturbations, Lunine said. “There is evidence that comets contributed to the original inventory of organics on the Earth and maybe some of our water.

“To the extent that we can understand how our own rocky planet got these gases that are rich in organics, we get a perspective on how our planet became habitable and how this might happen in planetary systems around other stars.”

Lunine’s contribution to the research was funded by NASA.

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Becka Bowyer