Oxygen on Rosetta’s comet: Scientists explore possible explanation

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Scientists at University of Bern discovered presence of molecular oxygen on comet 67P/Churyumov–Gerasimenko – a first of its kind discovery that changes our understanding of comets to a great deal.

The molecular oxygen was detected using Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument and according to Kathrin Altwegg, principal investigator of ROSINA, they weren’t expecting O2 at the comet at all and they were quite surprised with the finding.

The team behind the finding explored the possibilities to explain the presence and consistently high abundance of O2, its relationship to water, and the lack of ozone using photolysis and radiolysis of water ice over a range of timescales.

Researchers explain that in the process of photolysis, photons break bonds between molecules, whereas in radiolysis energetic photons or fast electrons and ions deposit 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.

The team explains that the process of 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 radiolysis would have been the primary source of O2, then there should have been a decrease in the O2/H2O ratio as this layer was removed during the six-month timespan of our observations explains Andre Bieler of the University of Michigan and lead author of the paper describing the new results in the journal Nature.

“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.”

One of the scenarios that scientists considered is that 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.

The other possibility that scientist considered is that the Solar System could have formed in an unusually warm part of a dense molecular cloud, at temperatures of 10–20ºC above the –263ºC or so revealed 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.