Evolution of the physical properties of dust and cometary dust activity from 67P/Churyumov-Gerasimenko measured in situ by Rosetta/COSIMA

Abstract

The Cometary Secondary Ion Mass Analyzer (COSIMA) collects dust particles in the coma of 67P/Churyumov-Gerasimenko, images them with a resolution of 14 mu m x 14 mu m, and measures their composition via time-of-flight secondary ion mass spectrometry. The particles are collected on targets exposed to the cometary flux for periods ranging from several hours to a week. Images are acquired with the internal camera, the COSISCOPE, before and after each exposure period. This paper focuses on the evolution of the dust flux and of the size distribution of the particles derived from the COSISCOPE images during the two years of the mission. The dust flux reaches its maximum at perihelion. We suggest that the delay of 20 d between the activity measured by COSIMA and the gas activity measured by the other instruments on Rosetta is caused by the presence of a volatile-poor dust layer on the nucleus that is removed around perihelion, uncovering volatile-rich layers that then become active. The difference in morphology between the northern and southern hemispheres observed by OSIRIS, the south being more sintered, is also recorded in the COSIMA data by a change in the size distribution during the southern summer, as the large porous aggregates disappear from the COSIMA collection. The properties of the particles collected during an outburst in early September 2016 indicate that these particles were ejected by a violent event and might originate from regions of low tensile strength.