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The changing temperature of the nucleus of comet 67P induced by morphological and seasonal effects

Abstract

Knowledge of the surface temperature distribution on a comet’s nucleus and its temporal evolution at different timescales is key to constraining its thermophysical properties and understanding the physical processes that take place at and below the surface. Here we report on time-resolved maps of comet 67P/Churyumov–Gerasimenko retrieved on the basis of infrared data acquired by the Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta orbiter in 2014, over a roughly two-month period in the pre-perihelion phase at heliocentric distances between 3.62 and 3.31 au from the Sun. We find that at a spatial resolution ≤15 m per pixel, the measured temperatures point out the major effect that self-heating, due to the complex shape of the nucleus, has on the diurnal temperature variation. The bilobate nucleus of comet 67P also induces daytime shadowing effects, which result in large thermal gradients. Over longer periods, VIRTIS-derived temperature values reveal seasonal changes driven by decreasing heliocentric distance combined with an increasing abundance of ice within the uppermost centimetre-thick layer, which implies the possibility of having a largely pristine nucleus interior already in the shallow subsurface.

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Fig. 1: VIRTIS-derived global temperature map of comet 67P.
Fig. 2: Temperature versus solar illumination in several morphological regions.
Fig. 3: Surface temperature versus albedo.
Fig. 4: Measured and modelled surface temperature.
Fig. 5: Spatial and temporal thermal gradients.
Fig. 6: Seasonal evolution of surface temperature in Imhotep.

Data availability

The VIRTIS calibrated data are publicly available through the ESA’s Planetary Science Archive (PSA) website (https://archives.esac.esa.int/psa/) and NASA’s Planetary Data System (https://pds.nasa.gov/) in accordance with the schedule established by the Rosetta project. Other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Readers are welcome to comment on the online version of the paper.

Code availability

The code used to retrieve surface temperature values from VIRTIS-M infrared data is a direct implementation of a published method7. The code used to derive synthetic thermal images of the nucleus of comet 67P is a direct implementation of published models18,44,45,46,51. The code used to derive theoretical temperature profiles for specific locations of the nucleus of comet 67P is a direct implementation of a published model28,29.

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Acknowledgements

The authors thank the following institutions and agencies, which supported this work: Italian Space Agency (ASI-Italy), Centre National d’Etudes Spatiales (CNES-France), Deutsches Zentrum für Luft- und Raumfahrt (DLR-Germany), National Aeronautic and Space Administration (NASA-USA). 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, Italy, which led also the scientific operations. The VIRTIS instrument development for ESA has been funded and managed by ASI, with contributions from Observatoire de Meudon financed by CNES and from DLR. The VIRTIS instrument industrial prime contractor was former Officine Galileo, now Leonardo SpA in Campi Bisenzio, Florence, Italy. The authors thank the Rosetta Liaison Scientists, the Rosetta Science Ground Segment and the Rosetta Mission Operations Centre for their support in planning the VIRTIS observations. We also thank the MIRO science and MIRO archiving teams for making MIRO data available to us before their public release. This research has made use of NASA’s Astrophysics Data System. D.K. acknowledges DFG-grant KA 3757/2-1. This work is dedicated to Angioletta Coradini (1946–2011), conceiver of the VIRTIS instrument, and to Sergio Fonti (1945–2018), co-author of this Article and active contributor in the development of VIRTIS. The first author dedicates this work also to Luca Malagutti (1965–2017), who was a brilliant researcher at the University of Milan.

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F.T. carried out the surface temperature retrieval from VIRTIS-M infrared data, derived geometric information for those data, and led the analysis of VIRTIS-measured temperature data, writing major sections of the main text and Methods. F.C. is the principal investigator of the VIRTIS instrument; he designed the overall study and wrote part of the main text. S.M., M.T.C. and M.F. carried out the thermophysical modelling and wrote part of the Methods. M.C. led the spectrophotometric analysis and derived the single scattering albedo values. G.F. was responsible for the VIRTIS calibration pipeline and flagged the saturated spectral pixels. M.H. is the principal investigator of the MIRO instrument; he granted MIRO data calibrated with the latest responsivity function, and provided key information for their proper interpretation. F.C., G.F., S.E., D.B.-M. and C.L. planned VIRTIS observations. S.E. and G.A. are respectively the French and German group leaders within the VIRTIS Science Team. The other authors are all VIRTIS co-investigators and associates who participated in the study and/or reviewed the manuscript, providing edits, comments and suggestions that led to substantial improvement of the paper.

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Correspondence to F. Tosi.

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Journal peer review information: Nature Astronomy thanks Ben Rozitis and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Tosi, F., Capaccioni, F., Capria, M.T. et al. The changing temperature of the nucleus of comet 67P induced by morphological and seasonal effects. Nat Astron 3, 649–658 (2019). https://doi.org/10.1038/s41550-019-0740-0

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