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K. Seiferlin, T. Spohn, A. Hagermann (Institut fuer Planetologie, Muenster)
State of the art models of the thermal evolution of comets take account of heat transport by the pore filling vapor. For higher temperatures, this phenomenon is believed to contribute significantly or even dominate the total heat transport. The pore size has a major influence on the efficiency of this process. In this paper, we adopt the methods used by most authors in modern numerical codes and study the influence of pore size. Two physical limitations that limit the resulting maximal heat transport efficiency are immediately evident: the vapor temperature inside the pores cannot exceed the sublimation temperature of the corresponding volatile ice, and the pressure inside the pore must be small enough to maintain a Knudsen regime inside the pores - a major prerequisite for the validity of the applied equations. As a consequence, even for very large pore radii (>1 cm) the "thermal conductivity" of the pores cannot exceed 1.9 W/mK, as will be shown. When pore size distributions matching typical grain size distributions (power-law type, with exponent around -3) for cometary dust are considered, an equivalent pore radius can be defined such that the heat transport though pores of the equivalent radius matches the heat transport of a matrix with the associated size distributions. Such equivalent pore radii are small compared to the max. pore radii abundant in the distribution. Thus, heat transport by vapor through the pores is small compared to solid state conduction and can contribute only neglibibly to the overall heat transport.
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The author(s) of this abstract have provided an email address for comments about the abstract: mailto:seiferl@uni-muenster.de