ESA’s Rosetta orbiter has detected molecules of oxygen in the coma of comet 67P/Churyumov–Gerasimenko.
Rosetta has made the first in situ detection of oxygen molecules outgassing from a comet; this surprising observation suggests they were incorporated into the comet during its formation. This infographic depicts various strands of evidence gathered for the study. Image credit: ESA / ATG medialab / Rosetta / NavCam / A. Bieler et al. / CC BY-SA IGO 3.0.
“We weren’t really expecting to detect molecular oxygen at 67P/Churyumov–Gerasimenko because it is so chemically reactive, so it was quite a surprise,” said Prof. Kathrin Altwegg of the University of Bern, one of the scientists on the Rosetta mission and co-author of a paper in the journal Nature.
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.
The composition of the neutral gas comas of most comets is dominated by water, carbon oxide and dioxide, typically comprising as much as 95% of the total gas density. In addition, cometary comas have been found to contain a rich array of other molecules, including sulfuric compounds and complex hydrocarbons.
Despite its detection on other icy bodies such as the moons of Jupiter and Saturn, molecular oxygen had been missing in the inventory of volatile species associated with comets until now.
“It turned out that O2 is the fourth most common gas in the comet’s atmosphere, after water, carbon monoxide and carbon dioxide,” the researchers said.
“It’s also unanticipated because there aren’t very many examples of the detection of interstellar O2,” Prof. Altwegg added. “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.”
“We had never thought that oxygen could survive for billions of years without combining with other substances,” she said.
More than 3000 samples, collected around the comet between September 2014 and March 2015, were analyzed to identify the O2.
High-resolution measurements allowed the scientists to distinguish O2 from other species such as sulfur and methanol.
“We report in situ measurement of O2 in the coma of comet 67P/Churyumov–Gerasimenko, with local abundances ranging from 1% to 10% relative to H2O and with a mean value of 3.8%,” Prof. Altwegg and co-authors wrote in the paper.
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.
“Our observations indicate that the O2/H2O ratio is isotropic in the coma and does not change systematically with heliocentric distance,” the team said.
“This suggests that primordial O2 was incorporated into the nucleus during the comet’s formation, which is unexpected given the low upper limits from remote sensing observations.”
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A. Bieler et al. 2015. Abundant molecular oxygen in the coma of comet 67P/Churyumov–Gerasimenko. Nature 526, 678-681; doi: 10.1038/nature15707
