Olivier Groussin, Björn Davidsson, Giampiero Naletto, S. Fornasier, Cecilia Tubiana, Horst Uwe Keller, P. L. Lamy, Michael Küppers, J.F. Wu, Hans Rickman, M. A. Barucci, W-H. Ip, Ekkehard Kührt, Stubbe F. Hviid, Selina-Barbara Gerig, L. M. Lara, C. C. Su, Marco Fulle, Nilda Oklay, R. Rodrigo, Monica Lazzarin, Stefano Mottola, Jean-Baptiste Vincent, V. Da Deppo, Laurent Jorda, Jörg Knollenberg, P. J. Gutierrez, Stefano Debei, Frank Scholten, G. Cremonese, M. De Cecco, Holger Sierks, Frank Preusker, J. L. Bertaux, Cesare Barbieri, Jakob Deller, Raphael Marschall, Dennis Bodewits, J. J. Lopez Moreno, Ivano Bertini, Detlef Koschny, Nicolas Thomas, Carsten Güttler, Jessica Agarwal, Imre Tóth, Francesco Marzari, J.-R. Kramm, Swiss National Science Foundation, European Commission, Physikalisches Institut [Bern], Universität Bern [Bern] (UNIBE), Centro di Ateneo di Studi e Attività Spaziali 'Giuseppe Colombo' (CISAS), Università degli Studi di Padova = University of Padua (Unipd), Department of Astronomy [College Park], University of Maryland [College Park], University of Maryland System-University of Maryland System, Jet Propulsion Laboratory (JPL), NASA-California Institute of Technology (CALTECH), INAF - Osservatorio Astronomico di Trieste (OAT), Istituto Nazionale di Astrofisica (INAF), Institute of Astronomy [Taiwan] (IANCU), National Central University [Taiwan] (NCU), Institut für Geophysik und Extraterrestrische Physik [Braunschweig] (IGEP), Technische Universität Braunschweig = Technical University of Braunschweig [Braunschweig], Operations Department (ESAC), European Space Astronomy Centre (ESAC), Agence Spatiale Européenne = European Space Agency (ESA)-Agence Spatiale Européenne = European Space Agency (ESA), DLR Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt [Berlin] (DLR), Department of Mechanical Engineering [NCTU], National Chiao Tung University (NCTU), Konkoly Observatory, Research Centre for Astronomy and Earth Sciences [Budapest], Hungarian Academy of Sciences (MTA)-Hungarian Academy of Sciences (MTA), Laboratoire d'Astrophysique de Marseille (LAM), Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS), Max-Planck-Institut für Sonnensystemforschung = Max Planck Institute for Solar System Research (MPS), Max-Planck-Gesellschaft, Dipartimento di Fisica e Astronomia 'Galileo Galilei', PLANETO - LATMOS, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Centro de Astrobiologia [Madrid] (CAB), Instituto Nacional de Técnica Aeroespacial (INTA)-Consejo Superior de Investigaciones Científicas [Madrid] (CSIC), International Space Science Institute [Bern] (ISSI), European Space Research and Technology Centre (ESTEC), Agence Spatiale Européenne = European Space Agency (ESA), Space Research Centre of Polish Academy of Sciences (CBK), Polska Akademia Nauk = Polish Academy of Sciences (PAN), Department of Physics and Astronomy [Uppsala], Uppsala University, Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA (UMR_8109)), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), INAF - Osservatorio Astronomico di Padova (OAPD), Laboratory for Ultraviolet and X-ray Optical Research [Padova] (LUXOR), CNR Institute for Photonics and Nanotechnologies (IFN), National Research Council of Italy | Consiglio Nazionale delle Ricerche (CNR)-National Research Council of Italy | Consiglio Nazionale delle Ricerche (CNR), University of Trento [Trento], Instituto de Astrofísica de Andalucía (IAA), Consejo Superior de Investigaciones Científicas [Madrid] (CSIC), Universität Bern [Bern], Institute of Space Science [Taiwan], Max-Planck-Institut für Sonnensystemforschung (MPS), Max Planck Institute for Solar System Research (MPS), DLR Institute of Planetary Research, German Aerospace Center (DLR), University of Bern (UBERN), International Space Science Institute (ISSI), Centro Interdipartimentale di Studi e Attività Spaziali 'Giuseppe Colombo', Università di Padova (CISAS), Department of Astronomy, University of Maryland, Jet Propulsion Laboratory, California Institute of Technology (JPL), INAF - Osservatorio Astronomico, Via Tiepolo 11, 34014 Trieste, Italy, Graduate Institute of Astronomy, Institute for Geophysics and Extraterrestrial Physics, Technische Universität Braunschweig, Science Operations Department, European Space Astronomy Centre/ESA, P.O. Box 78, 28691 Villanueva de la Canada, Madrid, Spain, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Department of Physics, National Tsing Hua University, Konkoly Observatory, Research Center for Astronomy and Earth Sciences, Max-Planck-Institut fur Sonnensystemforschung, Justus-von-Liebig-Weg, 3, 37077 Gottingen, Germany, Department of Physics and Astronomy, University of Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy, Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), European Space Agency (ESA), Space Research Center, Polish Academy of Sciences (Warsaw), Observatoire de Paris, Université Paris sciences et lettres (PSL), CNR-IFN UOS Padova LUXOR, Università di Trento (UNITN), Instituto de Astrofisica de Andalucia, CSIC (IAA), Dipartimento di Fisica, Università di Padova, Universita degli Studi di Padova, California Institute of Technology (CALTECH)-NASA, European Space Agency (ESA)-European Space Agency (ESA), Sorbonne Université (SU)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS), Consiglio Nazionale delle Ricerche [Roma] (CNR)-Consiglio Nazionale delle Ricerche [Roma] (CNR), Technische Universität Braunschweig [Braunschweig], Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Aix Marseille Université (AMU)-Centre National d'Études Spatiales [Toulouse] (CNES), IMPEC - LATMOS, Consejo Superior de Investigaciones Científicas [Spain] (CSIC)-Instituto Nacional de Técnica Aeroespacial (INTA), Polska Akademia Nauk (PAN), PSL Research University (PSL)-PSL Research University (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), and Consejo Superior de Investigaciones Científicas [Spain] (CSIC)
This article is available under the Creative Commons CC-BY-NC-ND license and permits non-commercial use of the work as published, without adaptation or alteration provided the work is fully attributed. For commercial reuse, permission must be requested below., The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) onboard the European Space Agency's Rosetta spacecraft acquired images of comet 67P/Churyumov–Gerasimenko (67P) and its surrounding dust coma starting from May 2014 until September 2016. In this paper we present methods and results from analysis of OSIRIS images regarding the dust outflow in the innermost coma of 67P. The aim is to determine the global dust outflow behaviour and place constraints on physical processes affecting particles in the inner coma. We study the coma region right above the nucleus surface, spanning from the nucleus centre out to a distance of about 50 km comet centric distance (approximately 25 average comet radii). We primarily adopt an approach used by Thomas and Keller (1990) to study the dust outflow. We present the effects on azimuthally-averaged values of the dust reflectance of non-radial flow and non-point-source geometry, acceleration of dust particles, sublimation of icy dust particles after ejection from the surface, dust particle fragmentation, optical depth effects and the influence of gravitationally bound particles. All of these physical processes could modify the observed distribution of light scattered by the dust coma. In the image analysis, profiles of azimuthally averaged dust brightness as a function of impact parameter b (azimuthal average, “Ā-curve”) were fitted with a simple function that best fits the shape of our profile curves (f(b;u,v,w,z)=u/b+wb+z). The analytical fit parameters (u, v, w, z), which hold the key information about the dust outflow behaviour, were saved in a comprehensive database. Through statistical analysis of these information, we show that the spatial distribution of dust follows free-radial outflow behaviour (i.e. force-free radial outflow with constant velocity) beyond distances larger than ∼11.9 km from the comet centre, which corresponds to a relative distance of about 6 average comet radii from the comet centre. Hence, we conclude that beyond this distance, and on average, fragmentation and gravitationally bound particles are negligible processes in determining the optically scattered light distribution in the innermost coma. Closer to the nucleus we observe dust outflow behaviour that deviates from free-radial outflow. A comparison of our result profiles with numerical models using a Direct Simulation Monte Carlo (DSMC) approach with dust particle distributions calculated using a test particle approach has been used to demonstrate the influence of a complex shape and particle acceleration on the azimuthal average profiles. We demonstrate that, while other effects such as fragmentation or sublimation of dust particles cannot be ruled out, acceleration of the dust particles and effects arising from the shape of the irregular nucleus (non-point source geometry) are sufficient to explain the observed dust outflow behaviour from image data analysis. As a by-product of this work, we have calculated “Afρ” values for the 1/r regime. We found a peak in the coma activity in terms of Afρ (normalised to a phase angle of 90°) of ∼210 cm 20 days after perihelion. Furthermore, based on simplified models of particle motion within bound orbits, it is shown that limits on the total cross-sectional area of bound particles might be derived through further analysis. An example is given. © 2018 The Authors, The team from the University of Bern is supported through the Swiss National Science Foundation and through the NCCR PlanetS. The project has also received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 686709. This work was supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number 16.0008-2. The opinions expressed and arguments employed herein do not necessarily reflect the official views of the Swiss Government.