A mini outburst from the nightside of comet 67P/Churyumov-Gerasimenko observed by the OSIRIS camera on Rosetta

¹ Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
e-mail: joerg.knollenberg@dlr.de
² Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
³ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
⁵ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astro-physique de Marseille) UMR 7326, 13388 Marseille, France
⁶ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy
⁷ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁸ Department of Mechanical Engineering – University of Padova, via Venezia 1, 35131 Padova, Italy
⁹ University of Padova, Department of Physics and Astronomy, vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁰ LESIA, Obs. de Paris, CNRS, Univ Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon, France
¹¹ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
¹² CNR–IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹³ UNITN, Università di Trento, via Mesiano, 77, 38100 Trento, Italy
¹⁴ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁵ Instituto de Astrofísica de Andalucía – CSIC, 18080 Granada, Spain
¹⁶ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
¹⁷ Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands
¹⁸ ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
¹⁹ Department of Information Engineering – University of Padova, via Gradenigo 6, 35131 Padova, Italy
²⁰ PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
²¹ International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland
²² Centro de Astrobiología, CSIC-INTA, 28850 Torrejón de Ardoz, Madrid, Spain
²³ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Received: 13 November 2015
Accepted: 13 July 2016

Abstract

Context. On 12 March 2015 the OSIRIS WAC camera onboard the ESA Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko observed a small outburst originating from the Imhotep region at the foot of the big lobe of the comet. These measurements are unique since it was the first time that the initial phase of a transient outburst event could be directly observed.

Aims. We investigate the evolution of the dust jet in order to derive clues about the outburst source mechanism and the ejected dust particles, in particular the dust mass, dust-to-gas ratio and the particle size distribution.

Methods. Analysis of the images and of the observation geometry using comet shape models in combination with gasdynamic modeling of the transient dust jet were the main tools used in this study. Synthetic images were computed for comparison with the observations.

Results. Analysis of the geometry revealed that the source region was not illuminated until 1.5 h after the event implying true nightside activity was observed. The outburst lasted for less than one hour and the average dust production rate during the initial four minutes was of the order of 1 kg/s. During this time the outburst dust production rate was approximately constant, no sign for an initial explosion could be detected. For dust grains between 0.01−1 mm a power law size distribution characterized by an index of about 2.6 provides the best fit to the observed radiance profiles. The dust-to-gas ratio of the outburst jet is in the range 0.6−1.8.