Rosetta has been studying Comet 67P/Churyumov–Gerasimenko for over a year and has detected an abundance of different gases pouring from its nucleus. Water vapour, carbon monoxide and carbon dioxide are the most prolific, with a rich array of other nitrogen-, sulphur- and carbon-bearing species, and even ‘noble gases’ also recorded.

Oxygen is the third most abundant element in the Universe, but the simplest molecular version of the gas, O₂, has proven surprisingly hard to track down, even in star-forming clouds, because it is highly reactive and readily breaks apart to bind with other atoms and molecules.

For example, oxygen atoms can combine with hydrogen atoms on cold dust grains to form water, or a free oxygen split from O₂ by ultraviolet radiation can recombine with an O₂ molecule to form ozone (O₃).

Despite its detection on the icy moons of Jupiter and Saturn, O₂ had been missing in the inventory of volatile species associated with comets until now.

“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 Kathrin Altwegg of the University of Bern, and principal investigator of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA.

“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 team analysed more than 3000 samples collected around the comet between September 2014 and March 2015 to identify the O₂. They determined an abundance of 1–10% relative to H₂O, with an average value of 3.80 ± 0.85%, an order of magnitude higher than predicted by models describing the chemistry in molecular clouds.

The amount of molecular oxygen detected showed a strong relationship to the amount of water measured at any given time, suggesting that their origin on the nucleus and release mechanism are linked. By contrast, the amount of O₂ seen was poorly correlated with carbon monoxide and molecular nitrogen, even though they have a similar volatility to O₂. In addition, no ozone was detected.

Over the six-month study period, Rosetta was inbound towards the Sun along its orbit, and orbiting as close as 10–30 km from the nucleus. Despite the decreasing distance to the Sun, the O₂/H₂O ratio remained constant over time, and it also did not change with Rosetta’s longitude or latitude over the comet.

In more detail, the O₂/H₂O ratio was seen to decrease for high H₂O abundances, an observation that might be influenced by surface water ice produced in the observed daily sublimation–condensation process.

The team explored the possibilities to explain the presence and consistently high abundance of O₂ and its relationship to water, as well as the lack of ozone, by first considering photolysis and radiolysis of water ice over a range of timescales.

In photolysis, photons break bonds between molecules, whereas radiolysis involves more energetic photons or fast electrons and ions depositing energy into ice and ionising molecules – a process observed on icy moons in the outer Solar System, and in Saturn’s rings. Either process can, in principle, lead to the formation and liberation of molecular oxygen.

Radiolysis will have operated over the billions of years that the comet spent in the Kuiper Belt and led to the build-up of O₂ to a few metres depth. But these top layers must all have been removed in the time since the comet moved into its inner Solar System orbit, ruling this out as the source of the O₂ seen today.

More recent generation of O₂ via radiolysis and photolysis by solar wind particles and UV photons should only have occurred in the top few micrometres of the comet.

“But if this was the primary source of the O₂ then we would have expected to see a decrease in the O₂/H₂O ratio as this layer was removed during the six-month timespan of our observations,” says Andre Bieler of the University of Michigan and lead author of the paper describing the new results in the journal Nature this week.

“The instantaneous generation of O₂ also seems unlikely, as that should lead to variable O₂ ratios under different illumination conditions. Instead, it seems more likely that primordial O₂ was somehow incorporated into the comet’s ices during its formation, and is being released with the water vapour today.”

In one scenario, gaseous O₂ would first be incorporated into water ice in the early protosolar nebula stage of our Solar System. Chemical models of protoplanetary discs predict that high abundances of gaseous O₂ could be available in the comet forming zone, but rapid cooling from temperatures above –173ºC to less than –243ºC would be required to form water ice with O₂ trapped on dust grains. The grains would then have to be incorporated into the comet without being chemically altered.

“Other possibilities include the Solar System being formed in an unusually warm part of a dense molecular cloud, at temperatures of 10–20ºC above the –263ºC or so typically expected for such clouds,” says Ewine van Dishoeck of Leiden Observatory in the Netherlands, co-author of the paper.

“This is still consistent with estimates for the comet formation conditions in the outer solar nebula, and also with previous findings at Rosetta’s comet regarding the low abundance of N₂.”

Alternatively, 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,” adds Kathrin.

“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.

“Abundant molecular oxygen in the coma of 67P/Churyumov–Gerasimenko,” by A. Bieler et al is published in the 29 October 2015 issue of the journal Nature.