Simulating nightside activity at comet 67P/Churyumov-Gerasimenko inferred from Rosetta/OSIRIS image analysis
Authors: Gerig Selina-Barbara , Pinzón-Rodríguez Olga , Marschall Raphael , Thomas Nicolas 
Published: October 2020

OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System, [1]) was the scientific camera system on board the Rosetta spacecraft and acquired a huge image dataset of the nucleus and the inner dust coma of comet 67P/Churyumov-Gerasimenko (hereafter 67P). The 2D-projections of the 3D dust coma contain information about the surface source distribution and physical processes governing the dust outflow in the innermost tens of kilometres above the nucleus surface. Numerical simulation has become an important tool in the attempt to disentangle and study these different processes at work. Here, we study the dust outflow behaviour with the help of &#8220;azimuthal average&#8221; [2, 3] profiles in Rosetta/OSIRIS data and artificial simulation images. Further, we look at the dayside-to-nightside brightness ratio (DS:NS) to characterise the relative difference in brightness from the dayside to the nightside coma [4]. A low DS:NS ratio indicates that the nightside coma is not much brighter than the dayside coma. This is not expected for a coma where outgassing is only originating from directly illuminated surface areas (as we show in our simulation results), but it is observed in OSIRIS images of the coma of 67P.We show in our simulation results that direct activity from the non-illuminated nightside of 67P is needed to explain the low ratio of brightness from the dayside to the nightside coma observed in OSIRIS images. The DS:NS ratio in a series of 4 images acquired at 90&#176; phase angle during one comet rotation on 11. April 2015 was determined to be 2.49 &#177; 0.18 on average. In our simulation results, such low DS:NS values can only be reached if the non-illuminated areas on the nucleus are outgassing in addition to the illuminated dayside regions. With only the directly illuminated surface outgassing the dayside-to-nightside asymmetry is factors of 4-10 too high. With additional outgassing from the nightside on the level of ~10% of the total gas production rate, the correct level for dayside-to-nightside brightness can be reproduced in artificial simulation images. Furthermore, the dust outflow behaviour in our simulation results matches the observed dust outflow behaviour better if nightside activity is present in the model. We additionally show that the DS:NS ratio increases when the comet moves towards perihelion (Fig. 1). We suggest that outgassing CO2 or CO rather than water is driving the direct activity from the nightside [5] and that the increasing DS:NS ratio towards perihelion is a sign of the increasing domination of H2O outgassing over outgassing of the more volatile gas species.Figure 1: DS:NS ratio values as a function of days to perihelion.For our simulations we use a two-step approach to simulate the 3D dust coma in the innermost 10 km around the nucleus of 67P. In a first step, the 3D gas field is calculated using a Direct Simulation Monte Carlo (DSMC) approach. In this study we use water as the only gas species in our simulations. Nightside activity is artificially introduced by letting the non-illuminated surface facets be active, so that their cumulative gas production rate is ~10% of the total gas production rate. The 3D dust field is then calculated by propagating particles through the gas field by solving an equation of motion taking into account gas drag and nucleus gravity. We simulate the dust fields of particles in 40 discrete size bins ranging from 8 nm to 0.3 mm separately. A density integration along the camera line of sight through the coma gives the column densities that are translated into brightness values via a Mie scattering model. The results from the different dust size bins are weighted according to a dust size distribution function and combined into an artificial image that can be directly compared to OSIRIS images. The dust size distribution follows a power law function of the form n(r)~r-b, with n(r) being the number density, r the dust particle radius and b the power law index determining the steepness of the size distribution. For more details on the simulation model see also [2,6].&#160;AcknowledgementsThe team from the University of Bern is supported through the Swiss National Science Foundation under the grant 200020-178847 and through the NCCR PlanetS.Calculations were performed on UBELIX (http://www.id.unibe.ch/hpc), the HPC cluster at the University of Bern.OSIRIS was built by a consortium of the Max- Planck-Institut f&#252;r Sonnensystemforschung, G&#246;ttingen, Germany; CISAS&#8211;University of Padova, Italy, the Laboratoire d&#8217; Astrophysique de Marseille, France; the Instituto de Astrof&#237;sica de Andalucia, CSIC, Granada, Spain; the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands; the Instituto Nacional de Tecnica Aeroespacial, Madrid, Spain; the Universidad Politechnica de Madrid, Spain; the Department of Physics and Astronomy of Uppsala University, Sweden; and the Institut f&#252;r Datentechnik und Kommunikationsnetze der Technischen Universit&#228;t Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged.&#160;References[1] H. U. Keller, C. Barbieri, P. Lamy et al., 2007, Space Science Reviews 128, p. 433-506[2] N. Thomas and H. Keller, 1990, Annales Geophysicae 8, p. 147-166[3] S.-B. Gerig, R. Marschall, N. Thomas et al., 2018, Icarus 311, p. 1-22[4] S.-B. Gerig, O. Pinz&#243;n-Rodr&#237;guez, R. Marschall et al., 2020 , Icarus (submitted)[5] D. Bockel&#233;e-Morvan, V. Debout, S. Erard, 2015, A&amp;A 583, A6[6] R. Marschall, C.C. Su, Y. Liao et al., 2016, A&amp;A 589, A9