VLT spectropolarimetry of comet 67P: dust environment around the end of its intense southern summer

Abstract

Context. A cornucopia of Rosetta and ground-based observational data sheds light on the evolution of the characteristics of dust particles from comet 67P/Churyumov-Gerasimenko (hereafter 67P) with seasons, implying the different dust environments in the source regions on the surface of the comet. Aims. We aim to constrain the properties of the dust particles of 67P and therefrom diagnose the dust environment of its coma and near-surface layer at around the end of the southern summer of the comet. Methods. We performed spectropolarimetric observations for 67P dust over 4000–9000 Å using the ESO/Very Large Telescope from January–March 2016 (phase angle ranging from ~26° –5°). We examined the optical behaviours of the dust, which, together with Rosetta colour data, were used to search for dust evolution with cometocentric distance. Modelling was also conducted to identify the dust attributes compatible with the results. Results. The spectral dependence of the polarisation degree of 67P dust is flatter than that found in other dynamical groups of comets in similar observing geometry. The depth of its negative polarisation branch appears to be a bit shallower than in long-period comets and might be getting shallower as 67P repeats its apparitions. Its dust colour shows a change in slope around 5500 Å, (17.3 ± 1.4) and (10.9 ± 0.6)% (1000 Å)-1, shortward and longward of the wavelength, respectively, which are slightly redder but broadly consistent with the average of Jupiter-family comets. Conclusions. Observations of 67P dust in this study can be attributed to dust agglomerates of ~100 μm in size detected by Rosetta in early 2016. A porosity of 60% shows the best match with our polarimetric results, yielding a dust density of ~770 kg m-3. The compilation of Rosetta and our data indicates the dust’s reddening with increasing nucleus distance, which may be driven by water-ice sublimation as the dust moves out of the nucleus. We estimate the possible volume fraction of water ice in the initially ejected dust as ~6% (i.e. the refractory-to-ice volume ratio of ~14).