When the European Space Agency probe Rosetta began to study comet 67P/Churyumov-Gerasimenko, scientists discovered a fair amount of molecular oxygen in its coma, the comet's fuzzy atmosphere. At first, researchers thought it formed on the surface of the object, but a new analysis suggests that it comes from within 67P itself.
The original hypothesis proposed that if particularly energetic ions (atoms with a mismatched number of electrons) were to crash on the surface, they would be capable of liberating oxygen from the molecules that make up the comet. Energetic ions are fairly common in the chaotic coma, so this hypothesis was taken seriously.
As reported in Nature Communications, the team compared the observations from Rosetta against various models and found that the energetic ion mechanism proposed cannot explain what the spacecraft saw. There is too much oxygen around 67P for the number of energetic ions present.
“The first detection of molecular oxygen in 67P’s coma was both very surprising and exciting,” lead author Kevin Heritier, a doctoral researcher at Imperial College London, said in a statement. "We tested the new theory of surface molecular oxygen production using observations of energetic ions, particles which trigger the surface processes which could lead to the production of molecular oxygen. We found that the amount of energetic ions present could not produce enough molecular oxygen to account for the amount of molecular oxygen observed in the coma."
The focus then is back on the primordial oxygen hypothesis. There are models suggesting that the abundant molecular oxygen in the giant clouds from which the Solar System formed, would have frozen on small dust grains that eventually became locked inside the bodies of comets like 67P.
"Surface generation of molecular oxygen may still happen on 67P, but the majority of the molecular oxygen in the coma is not produced through such a process," added co-author Dr Marina Galand, also at Imperial and Science Co-Investigator of the Rosetta Plasma Consortium.
A reply to that study, also in Nature Communication, had evidence supporting a more nuanced explanation and invites for caution. Some of the oxygen was produced by Rosetta itself. Water molecules impacting the surface of the probe would release oxygen molecules, trapping Rosetta in a "bubble" and altering the measurements.
"The oxygen measurement includes contributions from Rosetta itself, in addition to that produced on the nucleus surface. Therefore, the primordial origin of cometary oxygen remains highly speculative," co-author Professor KP Giapis told IFLScience.
67P was studied by Rosetta for two years, from August 2014 until September 2016. It dropped a probe called Philae on the comet, and eventually crash-landed on its surface at the end of its mission. You can see Rosetta's final images here.
UPDATE: We have included the information from the second study which we weren't aware of at the time of publication.