A quasi-3D model for the evolution of shape and temperature distribution of comet nuclei - Application to Comet 46P/Wirtanen

We introduce a quasi 3-D thermal evolution model for a spherical comet nucleus, which takes into account the diurnal and latitudinal variation of the solar flux, but neglects lateral heat conduction, which is shown to be insignificant. Comparing this model with the 'fast-rotator', evenly heated nucleus model, we show that, for long-term calculations of the evolution of the interior, the latter provides a good approximation. The 3-D model is applied to comet 46P/Wirtanen: yielding the distribution of temperature and water production rate over the nucleus surface. A very good agreement with observations is obtained for the integrated production rate. The model is also used in order to derive the change of shape due to uneven erosion. Two cases are considered for different inclination angles of the spin axis; it is concluded that the evolved shape depends strongly on the obliquity of the comet's spin axis.