Photochemistry of forbidden oxygen lines in the inner coma of 67P/Churyumov-Gerasimenko

Cessateur, G.; Keyser, J. De; Maggiolo, R.; Gibbons, A.; Gronoff, G.; Gunell, H.; Dhooghe, F.; Loreau, J.; Vaeck, N.; Altwegg, Kathrin; Bieler, André; Briois, C.; Calmonte, Ursina Maria; Combi, M. R.; Fiethe, B.; Fuselier, S. A.; Gombosi, T. I.; Hässig, Myrtha; Le Roy, Léna; Neefs, E.; ... (2016). Photochemistry of forbidden oxygen lines in the inner coma of 67P/Churyumov-Gerasimenko. Journal of Geophysical Research: Space Physics, 121(1), pp. 804-816. AGU Publications 10.1002/2015ja022013

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Observations of the green and red-doublet emission lines have previously been realized for several comets. We present here a chemistry-emission coupled model to study the production and loss mechanisms of the O(¹S) and O(¹D) states, which are responsible for the emission lines of interest for comet 67P/Churyumov-Gerasimenko. The recent discovery of O₂ in significant abundance relative to water 3.80 ± 0.85% within the coma of 67P has been taken into consideration for the first time in such models. We evaluate the effect of the presence of O₂ on the green to red-doublet emission intensity ratio, which is traditionally used to assess the CO₂ abundance within cometary atmospheres. Model simulations, solving the continuity equation with transport, show that not taking O₂ into account leads to an underestimation of the CO₂ abundance within 67P, with a relative error of about 25%. This strongly suggests that the green to red-doublet emission intensity ratio alone is not a proper tool for determining the CO₂ abundance, as previously suggested. Indeed, there is no compelling reason why O₂ would not be a common cometary volatile, making revision of earlier assessments regarding the CO₂ abundance in cometary atmospheres necessary. The large uncertainties of the CO₂ photodissociation cross section imply that more studies are required in order to better constrain the O(¹S) and O(¹D) production through this mechanism. Space weather phenomena, such as powerful solar flares, could be used as tools for doing so, providing additional information on a good estimation of the O₂ abundance within cometary atmospheres.

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