Despite being the third most abundant element in the Universe, molecular oxygen, or O₂, is relatively rare off Earth. That's why it raised a few eyebrows at ESA when the space agency's Rosetta spacecraft discovered oxygen molecules jetting out of the nucleus of Comet 67P/Churyumov–Gerasimenko. According to the Rosetta team, the oxygen is outgassing in such abundance that its presence may date back to the formation of the comet over 4.6 billion years ago.

The discovery is based on observations made by the unmanned orbiter from September 2014 to March 2015. During this time, Rosetta traveled to a distance of between 10 and 30 km (6.2 to 18.6 mi) from the comet's nucleus and took over 3,000 samples. When the gases were measured using the spacecraft's Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA), the scientists found an abundance of oxygen (O₂) measuring 1 to 10 percent relative to H₂O, with an average value of 3.80 ± 0.85 percent. According to the team, this is an order of magnitude higher than amounts predicted by current models of molecular clouds and their chemistry.

The team found that the presence of the oxygen was closely associated with the amount of water vapor present and that no ozone was detected. In addition, the oxygen was appearing evenly over the comet nucleus and the O₂/H₂O ratio remained constant even as 67P heated up as it approached the Sun and outgassing increased. The only decrease occurred as the water vapor from inside the nucleus was supplemented with water that froze and sublimated in the course of the cometary night.

What makes the presence of so much O₂ so intriguing to scientists is that it shouldn't be there. The gases spraying from 67P are mainly water vapor, carbon monoxide, carbon dioxide, and various others in smaller quantities, but the presence of oxygen in any quantity, is unexpected. This is because oxygen in its simplest molecular form of a gas, O₂, is very reactive and tends to bond with other chemicals and form new molecules, such as water, or it will break down in the presence of sunlight and react with itself to form ozone (O₃).

According to ESA, the oxygen sampled probably dates back to the formation of the comet. If the oxygen was younger, it would need to have been generated by either photolysis, where photons breaks the molecular bonds to release the oxygen, or radiolysis, where energetic photons or fast electrons do so. Over billions of years in the frozen wastes of the Kuiper Belt, this could have created a shallow layer of oxygen-bearing ice, but that should have boiled away when 67P migrated to the inner Solar System. Any layer that was created after this would have shown a marked decrease in oxygen as the comet approached the Sun over the last year, which indicates that the oxygen is coming from deep inside the nucleus.

Comet 67P on October 18, 2015(Credit: ESA)

The team has narrowed the origin of the cometary oxygen down to one of two scenarios. In the first, the free gaseous oxygen present in the disk of primordial dust and gas that formed the Solar System suddenly cooled from -173º C (-279º F) to under -243º C (-405º F) in the region where comets formed. This caused ice particles to form with oxygen trapped inside, where it was isolated from other chemicals with which it could react.

The second theory is that the oxygen was formed by radiolysis of icy dust grains that were buried inside the nucleus as the early comet merged with other bodies. As it did so, the oxygen was trapped, while the hydrogen released gently filtered out.

"This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of Solar System evolution," says Matt Taylor, ESA’s Rosetta project scientist.

The team's results were published in Nature.

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