A 2 1/2 D thermodynamic model of cometary nuclei I. Application to the activity of comet 29P/Schwassmann-Wachmann 1

Abstract

A2 1 dimensional model of cometary nuclei is pre- sented. 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- Wachmann1.Thiscometisofparticularinterestduetoitsstrong activityanditsunpredictableoutbursts.Weconsiderthattheac- tivity 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 pos- sibletoproduceoutburstsofactivityastheyareobservedforthis comet. The best t 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 main- tain an outburst regime due to crystallisation. No irregularities in its activity were found in runs where the model nucleus con- tained crystalline water ice with carbon monoxide. Thus, this work is an important clue for the presence of amorphous water ice in cometary nuclei.