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A homogeneous nucleus for comet 67P/Churyumov–Gerasimenko from its gravity field

Abstract

Cometary nuclei consist mostly of dust and water ice1. Previous observations have found nuclei to be low-density and highly porous bodies2,3,4, but have only moderately constrained the range of allowed densities because of the measurement uncertainties. Here we report the precise mass, bulk density, porosity and internal structure of the nucleus of comet 67P/Churyumov–Gerasimenko on the basis of its gravity field. The mass and gravity field are derived from measured spacecraft velocity perturbations at fly-by distances between 10 and 100 kilometres. The gravitational point mass is GM = 666.2 ± 0.2 cubic metres per second squared, giving a mass M = (9,982 ± 3) × 109 kilograms. Together with the current estimate of the volume of the nucleus5, the average bulk density of the nucleus is 533 ± 6 kilograms per cubic metre. The nucleus appears to be a low-density, highly porous (72–74 per cent) dusty body, similar to that of comet 9P/Tempel 12,3. The most likely composition mix has approximately four times more dust than ice by mass and two times more dust than ice by volume. We conclude that the interior of the nucleus is homogeneous and constant in density on a global scale without large voids. The high porosity seems to be an inherent property of the nucleus material.

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Figure 1: The porosity of the nucleus.

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Acknowledgements

Rosetta is an ESA mission with contributions from its member states and NASA. The Rosetta RSI experiment is funded by the Bundesministerium für Wirtschaft BMWi, Berlin, via the German Space Agency DLR, Bonn, under grants 50QM1401 (RIU-PF) and 50QM1002 (UniBw). J.-P.B. is supported by CNES, Paris. Support for the Multimission Radio Science Support Team is provided by NASA/JPL. We thank everyone involved with the Rosetta mission at ESTEC, ESOC, ESAC and JPL. The RSI team expresses deep appreciation for the critical support provided by the Rosetta SGS at ESAC during the planning and in particular by the ESTRACK and DSN ground station networks during the data acquisition periods. We dedicate this work to the late Claudia Alexander, for her support of RSI over many years.

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Authors and Affiliations

Authors

Contributions

T.A., M.H., J.-P.B., K.P. and S.T. processed the RSI data, S.W.A. was responsible for the data recording at the DSN antennas, H.S., L.J., R.G., F.P. and F.S. provided the nucleus shape models based on OSIRIS images, M.P. is the RSI Principal Investigator and interpreted the data and wrote the paper, B.H. and S.T. are the technical and experiment managers, respectively, M.K.B., B.H., E.G., P.R.W. and all other authors interpreted and discussed the results and commented on the manuscript.

Corresponding author

Correspondence to M. Pätzold.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Gravitational mass solutions.

a, GM solutions after each iteration step for two different starting values estimated from pre-arrival considerations. For the circle symbols, the initial value is 800 m3 s−2; for the triangle symbols it is 500 m3 s−2. The final solutions converge to an average GM of 666.2 ± 0.2 m3 s−2 for both runs (tracking data for distances larger than 30 km). b, As for a, but with a limited ordinate and not showing the values of the initial values. The error bars are one standard deviation (1σ).

Extended Data Figure 2 Gravity coefficients power spectrum.

The power spectrum of the computed gravity coefficients from the shape models for degree l = 2 to 10. The degree-2 power observed by RSI is the open data point with error bar (1σ), which agrees very well with degree 2 from the shape models.

Extended Data Table 1 Translation of the preliminary coordinate systems to the common coordinate system
Extended Data Table 2 SPICE kernels used for gravity field computation
Extended Data Table 3 Correlation matrix of the fit parameters GM and the gravity coefficients up to order and degree 2

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Pätzold, M., Andert, T., Hahn, M. et al. A homogeneous nucleus for comet 67P/Churyumov–Gerasimenko from its gravity field. Nature 530, 63–65 (2016). https://doi.org/10.1038/nature16535

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