Numerically improved thermochemical evolution models of comet nuclei

An improved unidimensional model of the heat transport and gas diffusion wi thin a porous cometary nucleus is presented, in which the time-dependent ga s diffusion equation is coupled with the heat diffusion equation to describ e the energy transport due to sublimation and recondensation of volatiles, but is solved independently using a different discrete time step. Also, the erosion of interfaces within the nucleus, due to the sublimation of ices a nd the removal of dust, is now treated by means of a continuous adaptation of the discrete grid to the interfaces positions, removing numerical stabil ity problems associated with the variation of structure and composition of the discrete layers. The results of this model are then compared with those of another unidimensional model which does not make use of the above-menti oned numerical methods, both computed for the same set of physical paramete rs describing comet P/Wirtanen, and the effects of the different modelling assumptions on the results are discussed. A new bidimensional model of the heat transport within a porous comet nucleus is presented, and its results are compared with those obtained from the above-mentioned unidimensional mo del (modified to include the same physics of the bidimensional model). The ability of bidimensional models to better describe the effects of variation s in the local physical conditions on the comet activity is then discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.