Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 – 23 September 2022
Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 September – 23 September 2022
EPSC Abstracts
Vol. 16, EPSC2022-105, 2022, updated on 06 Jul 2022
https://doi.org/10.5194/epsc2022-105
Europlanet Science Congress 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Seasonal evolution unveils the internal structure of cometary nuclei

Mauro Ciarniello1, Marco Fulle2, Andrea Raponi1, Gianrico Filacchione1, Fabrizio Capaccioni1, Alessandra Rotundi1,3, Giovanna Rinaldi1, Michelangelo Formisano1, Gianfranco Magni1, Federico Tosi1, Maria Cristina De Sanctis1, Maria Teresa Capria1, Andrea Longobardo1, Pierre Beck4, Sonia Fornasier5,6, David Kappel7,8, Vito Mennella9, Stefano Mottola7, Batiste Rousseau1, and Gabriele Arnold7
Mauro Ciarniello et al.
  • 1IAPS-INAF, Rome, Italy (mauro.ciarniello@inaf.it)
  • 2Osservatorio Astronomico, INAF, Trieste, Italy
  • 3Dipartimento di Scienze e Tecnologie, Universitá degli Studi di Napoli Parthenope, Naples, Italy
  • 4Université Grenoble Alpes, CNRS, IPAG, Grenoble, France
  • 5Institut Universitaire de France (IUF), Paris, France
  • 6LESIA, Université Paris Cité, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Meudon, France
  • 7Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany
  • 8Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany
  • 9Osservatorio Astronomico di Capodimonte, INAF, Naples, Italy

Remote sensing data of comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko (67P hereafter) indicate the occurrence of water-ice-rich spots on the surface of cometary nuclei [1-5]. These spots are up to tens of metres in size and appear brighter and bluer than the average surface at visible wavelengths.

In addition, the extensive observation campaign performed by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS, [6]) and the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS, [7]) during the Rosetta escort phase at 67P revealed a seasonal cycle of the nucleus colour. This is characterised by blueing of the surface while approaching perihelion followed by progressive reddening and restoral of the original colour along the outbound orbit. The temporal evolution of the colour has been interpreted in previous studies as the result of increasing exposure of water ice at smaller heliocentric distances [8, 9], however, an explanation of such seasonal cycle in the context of a quantitative cometary activity model was not yet been provided.

Recently, in [10] we showed that the seasonal colour cycle observed on comet 67P is determined by the occurrence of the above-mentioned water-ice-rich spots (referred to as Blue Patches – BPs –, given their colour). This can be explained in the context of activity models [11, 12] of pebble-made cometary nuclei [13], i.e. in terms of nucleus surface erosion induced by H2O and CO2 ices sublimation, driving the cometary activity.

According to the scenario proposed in [10] (Fig. 1), the presence of the BPs is due to the exposure of subsurface sub-metre-sized Water-ice-Enriched Blocks (WEBs) thanks to surface erosion triggered by CO2 sublimation ejecting decimetre-sized chunks [12]. The WEBs are composed of ice-rich pebbles (dust-to-ice mass ratio δ=2, [14]), embedded in a matrix of drier pebbles (δ>>5) forming most of the nucleus. Once exposed to illumination as BPs, the WEBs are eroded by water-ice sublimation ejecting sub-cm dust [11]. By means of dedicated spectral and thermophysical modelling, we match the nucleus colour temporal evolution measured by the VIRTIS Mapping channel in the 0.55-0.8 µm spectral range. In doing this, we take into account the competing effects of CO2- and H2O-driven erosion that expose and remove the BPs, respectively, and are seasonally modulated by the insolation conditions, primarily depending on the heliocentric distance.

The new nucleus model proposed in [10], implying an uneven distribution of water ice in cometary nuclei, reconciles the compositional dishomogeneities observed on comets (the BPs) at macroscopic (up to tens of metres) scale, with a structurally homogeneous pebble-made nucleus at small (centimetre) scale, and with the processes determining the cometary activity at microscopic (sub-pebble) scales.