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Examination of CN/OH as a proxy for CO2/H2O in cometary comae

Presentation #210.04 in the session “Comets”. Cross-listed as presentation #102.06.

Published onOct 03, 2021
Examination of CN/OH as a proxy for CO2/H2O in cometary comae

Comets are cold, primitive bodies that can help us understand the composition of the early solar system. However, comets are not pristine, and their surfaces can evolve, especially when warmed by the Sun. This point was underscored by the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko. Rosetta returned a wealth of information on that comet’s nucleus and coma properties, and their temporal and spatial variations. Of foremost relevance is the discovery that the surface of 67P's nucleus possesses a dichotomy in terrain and composition. Smooth terrain, primarily found at northern latitudes, appears to be depleted in CO2 with respect to the rough terrain, which is primarily found at southern latitudes. This scenario led to the hypothesis that the nucleus contains two volatile ice phases, one water-based, the other carbon-dioxide-based, each with their own correlated minor species (e.g., Gasc et al. 2017, MNRAS 469, S108). We seek to find the signatures of 67P's terrain dichotomy in telescopic observations. The basis of this study is the correlation between CO2 and HCN found at 67P. HCN is a primary candidate to be the parent molecule of CN, which is produced in the coma and commonly observed at near-UV and optical wavelengths. By combining the telescopic observations of Schleicher (2006, Icarus 181, 442) with the Rosetta/ROSINA monthly averages presented by Henry et al. (2021, P&SS 200, 105194), we identify a strong positive correlation between the CN-to-OH ratio and the CO2-to-H2O ratio. The correlation is expected given that HCN produces CN and H2O produces OH in the coma. This finding suggests that rotational or seasonal variations in the CN-to-OH ratio of a given comet might be used to infer the heterogeneity of the nuclear surface, allowing us to find other comets with surface dichotomies similar to 67P. We examine the polar orientations of several comets with this goal in mind.

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