Astron. Astrophys. 319, 995-1006 (1997)

A 2 1/2 D thermodynamic model of cometary nuclei

I. Application to the activity of comet 29P/Schwassmann-Wachmann 1

A. Enzian , H. Cabot and J. Klinger

Laboratoire de Glaciologie et Géophysique de l'Environnement du CNRS, Université Joseph Fourier, Grenoble I, B.P. 96, 54 Rue Molière, F-38402 St Martin d'Hères, France

Received 27 June 1996 / Accepted 5 September 1996

Abstract

A dimensional model of cometary nuclei is presented. The comet is considered as a porous ice-dust medium. Heat and gas diffusion between the surface and the inner part of the nucleus is considered. Going beyond models published so far, this work takes into account nucleus rotation and a two dimensional resolution of the diffusion equations, including a gas diffusion theory derived from the Boltzmann equation. The icy constituent is considered to be initially amorphous water ice containing solid carbon monoxide. The crystallisation of the amorphous ice follows an activation law found by means of laboratory studies.

The model is applied to comet 29P/Schwassmann-Wachmann 1. This comet is of particular interest due to its strong activity and its unpredictable outbursts. We consider that the activity is driven by a sublimation process taking place below the nucleus surface. Special attention is given to the obliquity of the rotation axis. The model results are in good agreement with observed gas and dust production rates. It was in particular possible to produce outbursts of activity as they are observed for this comet. The best fit of the observation is obtained for a nucleus containing initially amorphous ice and having a tilted rotation axis. The surface erosion, considered to be very small so far for comet 29P/Schwassmann-Wachmann 1, is necessary to maintain an outburst regime due to crystallisation. No irregularities in its activity were found in runs where the model nucleus contained crystalline water ice with carbon monoxide. Thus, this work is an important clue for the presence of amorphous water ice in cometary nuclei.

Key words: comets: 29P/Schwassmann-Wachmann 1 conduction diffusion comets: general methods: numerical

Send offprint requests to: A. Enzian

Contents

• 1. Introduction
• 2. Model description
  • 2.1. Composition
  • 2.2. Physico-chemical reactions
  • 2.3. Diffusion equations
  • 2.4. Boundary conditions and singularities
• 3. Discretisation and numerical resolution
• 4. Physical and numerical parameters
• 5. Parametrisation of surface erosion
• 6. Results and discussion
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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