Last week, it was alcohol on Comet Lovejoy. Yesterday, it was oxygen molecules on Comet 67P/Churyumov-Gerasimenko. Oxygen joins a slew of other gases discovered by Rosetta since reaching the comet last August including carbon monoxide, carbon dioxide, several noble gases and more.

While oxygen is one of the most abundant elements in the Universe, finding it outside of Earth is a challenge. It’s been detected on the frozen moons of Jupiter and Saturn, but never on a comet. That’s because it is highly reactive and easily breaks apart to bind with other molecules such as hydrogen.

Kathrin Altwegg, the principal investigator of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA), touches on the surprising detection.

“We weren’t really expecting to detect O2 at the comet – and in such high abundance – because it is so chemically reactive, so it was quite a surprise,” says Altwegg.

Altwegg added, “It’s also unanticipated because there aren’t very many examples of the detection of interstellar O2. And thus, even though it must have been incorporated into the comet during its formation, this is not so easily explained by current Solar System formation models.”

The Science

Between September 2014 and March 2015, more than 3,000 samples were collected around the comet. After analyzing these samples, the researchers determined the oxygen was 1-10% relative to H2O. The average value was between 3-4%.

This is much higher than models predict. And the amount of molecular oxygen seen shows a strong relationship to the amount of water measured at any one point. This suggests that their origin in the nucleus and how it is released are linked. A similar relationship was not seen with carbon monoxide and nitrogen. And no ozone was detected.

Here’s an image showing the O2/H2O ratio during the Sept 2014 – Mar 2015 period.

comet 67p oxygen

There’s little long-term variation. The short spikes you see are tied to the comet’s daily water-ice cycle, which I wrote about it in an earlier post.

Explaining the oxygen

Current models of how the universe formed say that oxygen, as its own compound, should have disappeared before Comet 67P took shape. Altwegg and her team never considered the possibility that oxygen could survive for billions of years without combining with other molecules.

The ESA press release explains how the O2 could have remained on the comet for so long.

Radiolysis of icy dust grains could have taken place prior to the comet’s accretion into a larger body. In this case, the O2 would remain trapped in the voids of the water ice on the grains while the hydrogen diffused out, preventing the reformation of O2 to water, and resulting in an increased and stable level of O2 in the solid ice.

Incorporation of such icy grains into the nucleus could explain the observed strong correlation with H2O observed at the comet today.

“Regardless of how it was made, the O2 was also somehow protected during the accretion stage of the comet: this must have happened gently to avoid the O2 being destroyed by further chemical reactions,” says Altwell.

That throws a wrench into our current understanding of how the Solar System evolved.

“Current Solar System formation models do not predict conditions that would allow this to occur,” the researchers write in their study.


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