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tokens: are used in the heat transfer model (the simulation domain is shown in Fig. 4 ). At the surface, which is exposed to free space, radiation interaction with the external (solar) irradiation as well as the mutual radiative interaction of the various surface elements is taken into account. For the case that water ice resides on the whole or on parts of the free surface, sublimation causes a cooling effect, which has a feedback on the temperature evolution both on the surface and in the subsurface layers. The surface boundary condition is expressed as where A is the surface albedo (which is assumed negligible on the diffuse surface: A = 0), and F is the local irradiation intensity in [Wm −2 ]. The second term on the right-hand side describes the cooling effect caused by sublimation of water ice (if it is present at a particular position) and the third term is the heat flow towards the interior. The sublimation term is the product of the latent heat of sublimation ( L s = 2.778 × 10 6 J kg −1 ) and the mass-loss term caused by ice sublimation, where α is the accommodation coefficient, assumed to be α = 1 at icy spots where free sublimation prevails. For water ice, the vapour pressure constants have the values a = 3.56 × 10 12 Pa and b = 6141.7 K (Fanale and Salvail 1984 ). For the interior bottom and side boundaries of the computational domain (Fig. 4 ), adiabatic boundary conditions (vanishing heat flux) have been used. Figure 4. Illustration of the Abydos LAMsmall geometry (computation domain). The three surface points ( Ref , Top and Wall ) are indicated together with the vertical lines starting at these positions and directed towards the interior of the comet. The ‘active spot’ where water ice is exposed to the surface in some of our model computations is indicated in blue. In addition to the boundary conditions described above an initial temperature distribution over the whole domain of interest has to be defined, which can serve as a starting point for the time-dependent calculation. To obtain a reasonable estimate for such an initial temperature in the case of Comet 67P, we have consulted old papers published by Squyres, McKay Reynolds ( 1985 ) and by McKay, Squyres Reynolds ( 1986 ). These authors have calculated typical interior temperatures of cometary nuclei, in particular for short period Jupiter family comets. For such objects, they obtained with various spin axis orientations, comet core temperatures between 115 and 129 K (see table 1 of McKay et al. 1986 ). Therefore, in the absence of more detailed information and for reasons of simplicity we have assumed for the calculations presented in this paper, a constant initial temperature of 120 K throughout the domain of interest. 4 NUMERICAL IMPLEMENTATION 4.1 Model geometries and grids For the numerical implementation, we have developed the following procedure that turned out to be realistic w.r.t. the limitations in computer memory and computation time. It uses a combination of