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Macro and micro structures of pebble-made cometary nuclei reconciled by seasonal evolution


Comets evolve due to sublimation of ices embedded inside porous dust, triggering dust emission (that is, erosion) followed by mass loss, mass redistribution and surface modifications. Surface changes were revealed by the Deep Impact and Stardust NExT missions for comet 9P/Tempel 1 (ref. 1), and a full inventory of the processes modifying cometary nuclei was provided by Rosetta while it escorted comet 67P/Churyumov–Gerasimenko for approximately two years2,3,4. Such observations also showed puzzling water-ice-rich spots that stood out as patches optically brighter and spectrally bluer than the average cometary surface5,6,7,8,9. These are up to tens of metres large and indicate macroscopic compositional dishomogeneities apparently in contrast with the structural homogeneity above centimetre scales of pebble-made nuclei10. Here we show that the occurrence of blue patches determines the seasonal variability of the nucleus colour4,11,12 and gives insight into the internal structure of comets. We define a new model that links the centimetre-sized pebbles composing the nucleus10 and driving cometary activity13,14 to metre-sized water-ice-enriched blocks embedded in a drier matrix. The emergence of blue patches is due to the matrix erosion driven by CO2-ice sublimation that exposes the water-ice-enriched blocks, which in turn are eroded by water-ice sublimation when exposed to sunlight. Our model explains the observed seasonal evolution of the nucleus and reconciles the available data at micro (sub-centimetre) and macro (metre) scales.

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Fig. 1: VIS spectral-slope (SVIS) maps of comet 67P/Churyumov–Gerasimenko’s surface at different orbital phases.
Fig. 2: Spectral-slope (SVIS) temporal evolution from August 2014 to September 2016, for different areas on 67P’s surface.
Fig. 3: Temporal evolution of the BP areal fraction in the ROI 1.
Fig. 4: 67P/Churyumov–Gerasimenko surface blueing at perihelion is due to the progressive exposure to sunlight of subsurface WEBs.

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Data availability

The VIRTIS calibrated data are publicly available through the European Space Agency’s Planetary Science Archive website ( Source data are provided with this paper.

Code availability

The computer code used to produce VIS spectral-slope maps of 67P is a direct implementation of a published method11. The computer codes used to perform spectral modelling and simulations of the BP fraction temporal evolution are direct implementations of the models described in the present paper.


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We thank the Italian Space Agency (ASI, Italy; ASI-INAF agreements I/032/05/0 and I/024/12/0), Centre National d’Etudes Spatiales (CNES, France) and Deutsches Zentrum für Luft und Raumfahrt (DLR, Germany) for supporting this work. VIRTIS was built by a consortium from Italy, France and Germany, under the scientific responsibility of Istituto di Astrofisica e Planetologia Spaziali (IAPS) of INAF, Rome, which also led the scientific operations. The VIRTIS instrument development for ESA has been funded and managed by ASI (Italy), with contributions from Observatoire de Meudon (France) financed by CNES and from DLR (Germany). The VIRTIS instrument industrial prime contractor was former Officine Galileo, now the Leonardo Company, in Campi Bisenzio, Florence, Italy. Part of this research was supported by the ESA Express Procurement (EXPRO) request for proposal for IPL-PSS/JD/190.2016. D.K. acknowledges DFG grant no. KA 3757/2-1. This work was supported by the International Space Science Institute (ISSI) through the ISSI International Team ‘Characterization of cometary activity of 67P/Churyumov-Gerasimenko comet’. This research has made use of NASA’s Astrophysics Data System.

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Authors and Affiliations



M.C. wrote the manuscript, performed data analysis, modelling and interpretation, and contributed to VIRTIS data calibration. M. Fulle contributed to model conceptualization, data interpretation and manuscript drafting. A. Raponi, G.F., F.C., A. Rotundi and G.R. contributed to data interpretation. A. Raponi supported spectral modelling and contributed to VIRTIS data calibration. G.F. provided VIRTIS data calibration. F.C. managed the VIRTIS experiment. A. Rotundi managed the GIADA experiment. F.T. provided geometric files for VIRTIS nucleus observations. All authors, including M. Formisano, G.M., M.C.D.S., M.T.C., A.L., P.B., S.F., D.K., V.M., S.M., B.R. and G.A., contributed to the discussion of the results and helped with the manuscript preparation.

Corresponding author

Correspondence to Mauro Ciarniello.

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Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Tables 1–3, Figs. 1–12, Sections 1–7 and references.

Source data

Source Data Fig. 2

Data points (including errors) for the different curves.

Source Data Fig. 3

Data points for the different curves (including upper and lower bound values when appropriate).

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Ciarniello, M., Fulle, M., Raponi, A. et al. Macro and micro structures of pebble-made cometary nuclei reconciled by seasonal evolution. Nat Astron 6, 546–553 (2022).

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