Time evolution of dust deposits in the Hapi region of comet 67P/Churyumov-Gerasimenko

¹ Center of Studies and Activities for Space (CISAS) “G. Colombo”, University of Padova, Via Venezia 15, 35131 Padova, Italy
² INAF Astronomical Observatory of Trieste, Via Tiepolo 11, 38121 Trieste, Italy
³ INAF Astronomical observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: pamela.cambianica@inaf.it
⁴ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Via Marzolo 8, 35131 Padova, Italy
⁵ CNR-IFN UOS Padova LUXOR, Via Trasea 7, 35131 Padova, Italy
⁶ Department of Geosciences, University of Padova, Via Giovanni Gradenigo 6, 35131 Padova, Italy
⁷ Department of Physics and Astronomy “Galileo Galilei”, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ Physics Department, Allison Laboratory, Auburn University, Auburn AL 36849, USA
⁹ University Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁰ LESIA, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, UPMC Univ. Paris 06, Sorbonne Universités, 5 place Jules Janssen, 92195 Meudon, France
¹¹ Max Planck Institute for Solar System Research, Justusvon-Liebig-Weg 3, 37077 Göttingen, Germany
¹² Instituto de Astrofísica de Andalucía (CSIS), c/Glorieta de la Astronomia s/n, 18008 Granada, Spain
¹³ Insitut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
¹⁴ Deutches Zentrum für Luft- und Raumfahrt (DLR), Institüt für Planetenforschung, Rutherfordstraße 26, 12489 Berlin, Germany
¹⁵ Operations Department, European Space Astronomy Center/ESA, Camino bajo del Castillo s/n, 28692 Villanueva de la Canada (Madrid), Spain
¹⁶ CSFK Konkoly Observatory, H1121 Budapest, Konkoly Thege M. ut 15-17, Hungary

Received: 13 January 2020
Accepted: 13 March 2020

Abstract

Aims. We provide a measurement of the seasonal evolution of the dust deposit erosion and accretion in the Hapi region of comet 67P/Churyumov-Gerasimenko with a vertical accuracy of 0.2–0.9 m.

Methods. We used OSIRIS Narrow Angle Camera images with a spatial scale of lower than 1.30 m px⁻¹ and developed a tool to monitor the time evolution of 22 boulder heights with respect to the surrounding dust deposit. The tool is based on the measurement of the shadow length projected by the boulder on the surrounding pebble deposit. Assuming the position of the boulders does not change during the observational period, boulder height variations provide an indication of how the thickness of the surrounding dust layer varies over time through erosion and accretion phenomena.

Results. We measured an erosion of the dust deposit of 1.7 ± 0.2 m during the inbound orbit until 12 December, 2014. This value nearly balances the fallout from the southern hemisphere during perihelion cometary activity. During the perihelion phase, the dust deposit then increased by 1.4 ± 0.8 m. This is interpreted as a direct measurement of the fallout thickness. By comparing the erosion rate and dust volume loss rate at the Hapi region measured in the coma, the fallout represents ~96% in volume of the ejecta. The amount of the eroded pristine material from the southern hemisphere, together with its subsequent transport and fallout on the nucleus, led us to discuss the pristine water ice abundance in comet 67P. We determine that the refractory-to-ice mass ratio ranges from 6 to 110 in the perihelion-eroded pristine nucleus, providing a pristine ice mass fraction of (8 ± 7)% in mass.