One of the big surprises in the Rosetta probe's visit to Comet 67P/Churyumov–Gerasimenko was the 2015 announcement that the rock was surrounded by a thin cloud of molecular oxygen.
The discovery, announced in October 2015, set boffins scratching their heads. Since, it was assumed at the time, the oxygen had accompanied the comet for billions of years, how had it survived?
A pair of Caltech boffins have now offered an alternative: the O2 isn't a primordial gas billions of years old. Rather, it's produced by a reaction that pulls oxygen atoms out of the water ice the comet carries.
The pair, Yunxi Yao and Konstantinos Giapis of Caltech's Division of Chemistry and Chemical Engineering, published their work here in Nature Communications.
As they note in the abstract, space scientists have previously considered ways in which oxygen could be produced on the comet: photolysis and radiolysis of water, solar wind–surface interactions, and gas-phase collisions have all been observed in cometary environments.
None of these fitted for 67P, which is one reason the O2 Rosetta spotted was thought to be ancient.
Yao's and Giapis' hypothesis is that the oxygen comes from the interaction between the solar wind and charged water, H2O+.
This, they write, abstracts an oxygen atom from the surface, which forms an “excited precursor state, which dissociates to produce O2−. Subsequent photo-detachment leads to molecular O2”.
Science News explains the mechanism here:
“As the sun evaporates water from the comet’s surface, ultraviolet light could strip an electron from the water, giving it a positive charge. Then, fast-moving particles in the solar wind could shoot the ionized water back toward the comet’s surface, where it could collide with rust or sand particles. Atoms of oxygen from the water could pair with atoms of oxygen from the rust or sand, creating O2.”
The researchers have conducted experiments showing the reaction can work like this, but it will take missions with the right equipment to test whether such things happen out in space. ®