Scientists have for the first time detected abundant oxygen in the atmosphere of a comet which streaked past the Sun in August, a surprise finding that may change our understanding of the evolution of our solar system.
The finding suggests the oxygen molecules were incorporated into the Comet 67P/Churyumov-Gerasimenko during its formation.
European Space Agency (ESA)'s Rosetta spacecraft has been studying the comet 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 noble gases also recorded.
"We weren't really expecting to detect oxygen at the comet - and in such high abundance - because it is so chemically reactive, so it was quite a surprise," said 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 oxygen. 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," Altwegg said.
The team analysed more than 3,000 samples collected around the comet between September 2014 and March 2015 to identify the oxygen molecule.
They determined an abundance of 1-10 per cent relative to water, with an average value of 3.80 per cent, 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 molecular oxygen seen was poorly correlated with carbon monoxide and molecular nitrogen, even though they have a similar volatility to oxygen molecule. 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 kilometre from the nucleus.
The comet made its closest approach to the Sun on August 13, 2015.
Despite the decreasing distance to the Sun, the oxygen/water ratio remained constant over time, and it also did not change with Rosetta's longitude or latitude over the comet.
In more detail, the oxygen/water ratio was seen to decrease for high water abundances, an observation that might be influenced by surface water ice produced in the observed daily sublimation-condensation process.
"This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of Solar System evolution," said Matt Taylor, ESA's Rosetta project scientist.
