Effects of irregular shape and topography in thermophysical models of heterogeneous cometary nuclei

Several improvements in the thermophysical model by Gutierrez et al. (2000) have been included in a new code to specifically deal with fully irregular cometary nuclei. Also, the new code allows for the inclusion of regions with different ice to dust ratios, regions of different albedos and regions of different emissivity. The new model has been applied to groups of irregular bodies characterized by 3 statistical parameters, the so-called Gaussian random shapes. In simulations, these bodies rotate steadily around their maximum inertia moment axes. The results of the runs show that the main conclusions of Gutierrez et al. (2000) still hold, and some new features are observed: 1) In general, very irregular objects have higher water production rates than spheres of the same radius for most of the orbital period. The fact that an irregular object has a larger area than the sphere cannot explain the differences in water production. The main differences appear to be a consequence of its topographic features. Also, topography can diminish the pre- and post-perihelion asymmetries in the lightcurves. Concerning the results for plausible albedo and icy fraction area distributions, 2) the mean water production of a comet with an albedo distribution on the surface is equal to the water production of a homogeneous comet with an albedo equal to the mean albedo of the distribution. The same result is obtained for icy fraction area distributions. 3) Close to perihelion, objects with icy fraction area distributions have nearly the same productions as fully water ice objects. 4) The largest diurnal oscillations in the synthetic lightcurves result from the irregular shape, whereas albedo and icy fraction area inhomogeneities induce oscillations of only a few percent.