Dayside-to-nightside dust coma brightness asymmetry and its implications for nightside activity at Comet 67P/Churyumov–Gerasimenko

Gerig, S.-B.; Pinzón-Rodríguez, O.; Marschall, R.; Wu, J.-S.; Thomas, N. (2020). Dayside-to-nightside dust coma brightness asymmetry and its implications for nightside activity at Comet 67P/Churyumov–Gerasimenko. Icarus, 351, p. 113968. Elsevier 10.1016/j.icarus.2020.113968

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We have determined the dust coma brightness ratio between the dayside and the nightside (DS:NS) in OSIRISimages of comet 67P/Churyumov–Gerasimenko and compared them to results from numerical dust comasimulations to learn more about the dynamic processes that are involved in coma formation. The primaryfocus of this paper lies in the analysis of a subset of OSIRIS images acquired during one comet rotation on11. April 2015 when the spacecraft was at a phase angle of 90◦and therefore directly above the terminator.The DS:NS ratio was found to be 2.49±0.18 on average - a very low value if insolation-driven sublimation ofwater dominates dust emission. We investigated two possible hypotheses: First, the influence of direct activityfrom non-illuminated (nightside) areas of the comet and second, the brightness contribution of large gravity-dominated particles in the innermost coma. For our numerical simulations, we used a combination of DSMCgas dynamics simulation and particle propagation by an equation of motion to simulate the dust coma. Oursimulations show that direct activity from the nightside is preferred, contributing≈10% of the total emission.We show that intensity profiles, used to quantify dust outflow behaviour, fit the observations better whennightside activity is present and we suggest that nightside gas emission by CO2or CO is responsible for theobserved dust flux. With the help of a simplified Keplerian modelling approach we exclude large particles ongravitationally bound or ballistic orbits from being the major contributor to the observed dust coma brightness.Additionally, we show the DS:NS ratio as a function of days to perihelion and observe that it is on a similarlevel as in the April OSIRIS time series from February to mid-June 2015, but increases towards a maximum of≥4.07±0.49 shortly after perihelion passage. We suggest that this is correlated to the increasing importanceof H2O production when approaching perihelion

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