Scientists have found that molecular oxygen around comet 67P is not produced on its surface, as thought, but may be from its body.
The European Space Agency’s Rosetta spacecraft escorted comet 67P/Churyumov–Gerasimenko on its journey round the sun from August 2014 – September 2016, dropping a probe and eventually crashing onto its surface. When the comet is close enough to the sun the ice on its surface ‘sublimes’ – transforms from solid to gas – forming a gas atmosphere called a coma. Analysis of the coma by instruments on Rosetta revealed that it contained not only water, carbon monoxide and carbon dioxide, as anticipated, but also molecular oxygen.
Lead author Mr Kevin Heritier, from the Department of Physics at Imperial, said: “The first detection of molecular oxygen in 67P’s coma was both very surprising and exciting”.
“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 Rosetta science team originally reported that the oxygen was most likely from the comet’s main body, or nucleus. This meant it was ‘primordial’ – that it was already present when the comet itself formed at the beginning of the Solar System 4.6 billion years ago.
One group of outside researchers however suggested there might be a different source for molecular oxygen at comets. They had discovered a new way to produce molecular oxygen in space triggered by energetic ions – electrically charged molecules. They proposed that reactions with energetic ions on the surface of comet 67P could instead be the source of the detected molecular oxygen. Now, members of the Rosetta team have analysed the data on 67P’s oxygen in light of the new theory.
The new analysis is consistent with team’s original conclusion, that molecular oxygen is most likely primordial. In a paper published today in Nature Communications and led by Imperial College London physicists, they report that the proposed mechanism for producing oxygen on the surface of the comet is not sufficient to explain the observed levels in the coma.