Dust in the Inner Coma of Comet 67P/Churyumov-Gerasimenko

Gerig, Selina-Barbara (2021). Dust in the Inner Coma of Comet 67P/Churyumov-Gerasimenko (Unpublished). (Dissertation, Universität Bern, Philosophisch-naturwissenschaftliche Fakultät)

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The main goal of the work presented in this PhD thesis is to investigate the dust dynamics in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko (67P) and to gain a deeper understanding of the physical processes governing it. In our analyses, we study numerical simulation results of gas and dust comae in combination with image analysis of Rosetta OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) data.
To simulate the gas and dust comae, we first calculate the 3D gas flow field using the Direct Simulation Monte Carlo (DSMC) approach. We use either a simplified spherical nucleus or the complex shape of the nucleus of 67P as a source. Surface temperatures and gas production rates are calculated with a thermal model. We use H2O outgassing in simulations with one gas species only and combine H2O and CO2 in simulation cases with a multispecies gas coma. In a second step, we simulate the motion of dust particles through our simulation domain by solving an equation of motion including gas drag and nucleus gravity. Finally, we generate synthetic images for direct comparison with OSIRI data by calculating column densities and using a light scattering model to compute the image brightness.
We present results from coma simulations corresponding to observations acquired on 11. April 2015 at 67P. In a series of four OSIRIS images we studied the dayside-to-nightside coma brightness ratio (DS/NS ratio) and found it to be surprisingly low at an average 2.49 ± 0.18. Simulated values from purely insolation-driven water outgassing greatly exceed the measured DS/NS ratios. We investigate gravity dominated particles on fall-back trajectories and direct nightside activity as possible explanations and conclude that direct nightside activity is needed to explain the observations. In a follow-up study, we look at a multi-species coma of H2O and CO2 in a model that includes heat conduction into the nucleus interior. We find that such a model can reproduce the correct nightside activity that matches the observations to first order.
We also investigate the effect of the dynamic motion of dust particles on the dust size distribution at different locations in the inner coma. We find that the dust size distribution is uniform above the active dayside of the nucleus but shows deviations towards the nightside coma. In various simulation models we show that the transition from uniform to non-uniform local dust size distributions is dependent on the production rate but happens beyond the terminator region in the coma. We conclude that the results from Rosetta experiments sampling dust were not affected by deviations in the local dust size distribution.

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