On deviations from free-radial outflow in the inner coma of comet 67P/Churyumov–Gerasimenko

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2018-09, Vol.311, p.1-22
• Main Authors: Gerig, S.-B., Marschall, R., Thomas, N., Bertini, I., Bodewits, D., Davidsson, B., Fulle, M., Ip, W.-H., Keller, H.U., Küppers, M., Preusker, F., Scholten, F., Su, C.C., Toth, I., Tubiana, C., Wu, J.-S., Sierks, H., Barbieri, C., Lamy, P.L., Rodrigo, R., Koschny, D., Rickman, H., Agarwal, J., Barucci, M.A., Bertaux, J.-L., Cremonese, G., Da Deppo, V., Debei, S., De Cecco, M., Deller, J., Fornasier, S., Groussin, O., Gutierrez, P.J., Güttler, C., Hviid, S.F., Jorda, L., Knollenberg, J., Kramm, J.-R., Kührt, E., Lara, L.M., Lazzarin, M., Lopez Moreno, J.J., Marzari, F., Mottola, S., Naletto, G., Oklay, N., Vincent, J.-B.
• Format: Article
• Language: English
• Subjects: 67P/Churyumov–Gerasimenko; Astrophysics; Dust; Inner coma; OSIRIS; Rosetta; Sciences of the Universe
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2018.03.010

•Dust outflow from comet 67P follows a 1/r law at distances >11.9 km from the nucleus.•We discuss processes leading to deviations from 1/r close to the nucleus.•Acceleration and effects of non-point source geometry can explain the observables.•Comparisons to model calculations support our results.•As a by-product, Afρ can be deduced and is given as a function of time. 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/bv+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 processe