Evolution of the crystallization front in cometary models
Effect of the net energy released during crystallization
Instituto de Astrofísica de Andalucía-CSIC, PO Box 3004, 18080 Granada, Spain e-mail: email@example.com
Accepted: 9 April 2008
Context. It is plausible that at least part of the water ice of cometary nuclei is initially in an amorphous phase, doped with other volatiles. As the nuclei are heated, the amorphous ice would then transform irreversibly into cubic ice. The net energy liberated in this transformation may be affected by the presence of any impurities because part of the energy liberated during crystallization may be expended in the desorption of dopant elements.
Aims. Our goal is to study the evolution of the crystallization front of the amorphous ice in a simulated nucleus, providing quantitative results. In particular, the influence of the net energy released during crystallization on the thermophysical evolution will be analyzed.
Methods. We use a simplified thermophysical model to simulate a cometary nucleus, where the ice is assumed initially to be in an amorphous phase. The model allows us to estimate the instantaneous rate of crystallization and the time spent in crystallization, for a fixed volume of amorphous ice, as a function of the net energy released. Simulations are performed for different characterizations of the nucleus interior such as dust-to-ice ratio, density, or thermal inertia.
Results. As expected, the evolution of the crystallization front depends strongly on the characteristics of the nucleus interior. If the nucleus interior has, however, a dust-to-ice ratio smaller than 1, and a low thermal inertia, approximately of 20 W K-1 m-2 s1/2, the crystallization front evolves discontinuously, with quasi-periodic increases in the crystallization rate. Those increases have a period that ranges from 1 to 40 days, if the energy released by crystallization is unaffected by impurities. These surges of crystallization could be responsible for the periodic outbursts observed for comet 9P/Tempel 1 shortly before the Deep Impact experiment. The evolution of the crystallization front becomes continuous and almost steady, if the net energy released is half that of the pure, exothermic case, regardless of the characteristics of the nucleus interior. On the other hand, if the dust-to-ice ratio is high (larger than 1) and/or the thermal inertia is high (larger than 100), the crystallization front evolves in a continuous and smooth manner, even for pure, exothermic crystallization. Other quantitative results, including a comparison with plausible erosion rates, are described.
Key words: comets: general / methods: numerical / solar system: general
© ESO, 2008