An orbital water-ice cycle on comet 67P from colour changes

Abstract

Solar heating of a cometary surface provides the energy necessary to sustain gaseous activity, through which dust is removed1,2. In this dynamical environment, both the coma3,4 and the nucleus5,6 evolve during the orbit, changing their physical and compositional properties. The environment around an active nucleus is populated by dust grains with complex and variegated shapes7, lifted and diffused by gases freed from the sublimation of surface ices8,9. The visible colour of dust particles is highly variable: carbonaceous organic material-rich grains10 appear red while magnesium silicate-rich11,12 and water-ice-rich13,14 grains appear blue, with some dependence on grain size distribution, viewing geometry, activity level and comet family type. We know that local colour changes are associated with grain size variations, such as in the bluer jets made of submicrometre grains on comet Hale–Bopp15 or in the fragmented grains in the coma16 of C/1999 S4 (LINEAR). Apart from grain size, composition also influences the coma’s colour response, because transparent volatiles can introduce a substantial blueing in scattered light, as observed in the dust particles ejected after the collision of the Deep Impact probe with comet 9P/Tempel 117. Here we report observations of two opposite seasonal colour cycles in the coma and on the surface of comet 67P/Churyumov–Gerasimenko through its perihelion passage18. Spectral analysis indicates an enrichment of submicrometre grains made of organic material and amorphous carbon in the coma, causing reddening during the passage. At the same time, the progressive removal of dust from the nucleus causes the exposure of more pristine and bluish icy layers on the surface. Far from the Sun, we find that the abundance of water ice on the nucleus is reduced owing to redeposition of dust and dehydration of the surface layer while the coma becomes less red.

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Fig. 1: Time series of the spectral properties of 67P coma dust.
Fig. 2: Time evolution of 67P nucleus colour measured through the 0.5–0.8 μm spectral slope above 12 control areas.
Fig. 3: Simulations of the λmax as a function of the scattering angle for spherical particles of different composition.
Fig. 4: Simulations of the 0.5–0.8 μm spectral slope as a function of the scattering angle for spherical particles of different composition.

Data availability

The VIRTIS calibrated data are publicly available through the European Space Agency’s Planetary Science Archive website (https://archives.esac.esa.int/psa/).

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Acknowledgements

The authors thank the following institutions and agencies that supported this work: Italian Space Agency (ASI-Italy), Centre National d'Etudes Spatiales (CNES-France), Deutsches Zentrum für Luft-und Raumfahrt (DLR-Germany). VIRTIS was built by a consortium from Italy, France and Germany, under the scientific responsibility of IAPS, Istituto di Astrofisica e Planetologia Spaziali of INAF, Rome, which lead also 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 Leonardo Company, in Campi Bisenzio, Florence, Italy. This research has made use of NASA’s Astrophysics Data System.

Author information

The paper is a collective effort by the VIRTIS dust working group. F.G. as the main author of the paper calibrated, processed and interpreted VIRTIS data. F.C. as the VIRTIS principal investigator managed the experiment; M.C., A.R. and G.R. supported the Mie scattering calculations and data processing; F.C. and D.B.-M. planned 67P coma observations by VIRTIS; F.C. and G.F. planned the 67P nucleus observations by VIRTIS; S.E. provided geometry files for coma and nucleus observations; all authors, including M.C.D.C., G.A., V.M., M.F., A.L. and S.M., have contributed to the discussion of the results.

Correspondence to Gianrico Filacchione.

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Competing interests

The authors declare no competing interests.

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Peer review information Nature thanks Evgenij S. Zubko and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Example of a typical VIRTIS-M observation of 67P nucleus and coma.

Top left: visible colours RGB = (0.7, 0.55, 0.44 μm) image, stretched to saturate the nucleus and enhance the visibility of jets in the coma. Top right: tangent altitude image where the green mask corresponds to the annulus containing all pixels with a tangent altitude between 1 and 2.5 km from the limb. Bottom left: average radiance spectra as derived from official pipeline (black curve) and after correction with Vega data (red curve). The fourth-degree polynomial fit to the corrected radiance is shown (cyan curve). The retrieved maximum emission wavelength on the fit is indicated by the magenta dashed line. Bottom right: corresponding I/F spectra normalized at 0.5 μm. The best-fitting slopes in the 0.4–0.5 and 0.5–0.8 μm ranges are indicated by blue and green dashed lines, respectively.

Extended Data Fig. 2 Vega star signal and derived VIRTIS responsivity.

Top left: average spectrum of Vega, in DN, from observation V1_00402035638.QUB (red curve) and spectrum corrected for dark current, despiked and filter notch removed (green curve). Note the correlation of negative spikes in the Vega and average sky spectrum (black curve). Owing to the instrumental point spread function and spectral tilt, the signal is an average taken from 18 pixels. Top right: average spectrum of Vega derived from the four observations listed in Extended Data Table 2 after having applied a processing similar to the one shown in the previous plot. The curve is averaged with a two-point running boxcar filter. The Vega flux36 is shown in relative units (blue curve). Bottom left: VIRTIS responsivity derived from Vega signal averaged with a two-point running boxcar filter (red curve) and nine points (blue curve) after normalization at 0.635 μm above the standard responsivity value. Bottom right: comparison between standard pipeline (black curve) and Vega responsivities with a nine-point running boxcar filter (red curve). The ratio between the two responses is the blue curve.

Extended Data Fig. 3 Rosetta spacecraft three-dimensional trajectory and solar phase angle variations with time.

Top: Rosetta trajectory in the 67P XYZ reference frame. Points along the X axis are shown starting from Rosetta’s position at 2015-01-13T23:28:53 (MTP012) with an increment of 1 km every 20 min to improve visualization. The Y axis is oriented towards the Sun and the Z axis is perpendicular to the orbital plane. The red line indicates the position of the nucleus along the X axis. The integrated radiance as measured on each observation is reported according to the colour scale. Bottom: variation of the solar phase angle (Sun–nucleus centre–Rosetta) during the mission.

Extended Data Table 1 Rosetta’s calendar MTP periods dates and heliocentric distance
Extended Data Table 2 List of VIRTIS observations of the Vega star
Extended Data Table 3 Summary of spectral indicator compatibility

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Filacchione, G., Capaccioni, F., Ciarniello, M. et al. An orbital water-ice cycle on comet 67P from colour changes. Nature 578, 49–52 (2020). https://doi.org/10.1038/s41586-020-1960-2

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