Department of Climate and Space Sciences and Engineering in the College of Engineering at the University of Michigan


First detection of molecular oxygen at a comet

Posted: October 28, 2015

First detection of molecular oxygen at a comet Comet 67P/Churyumov–Gerasimenko (Image Credit: ESA/Rosetta/MPS for OSIRIS Team)

ESA’s Rosetta spacecraft has made the first in situ detection of oxygen molecules outgassing from a comet, a surprising observation that suggests they were incorporated into the comet during its formation.

These findings were reported in a paper written by Climate & Space Research Fellow Dr. Andre Bieler for the journal Nature. Climate & Space Professors Mike Combi, Tamas Gombosi and K.C. Hansen were co-authors on the paper.

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.

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,” Bieler told 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.”

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..”

“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.

To read the full news release, please visit

This news was also reported on, The Guardian, Discovery News, the Washington Post and the Los Angeles Times.


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