Comet 46P/Wirtanen: Evolution of the subsurface layer

Cometary nuclei are composed mostly of water ice and mineral grains, When a comet passes near the Sun, volatiles sublimate. This process leads to the formation of a dust layer covering the nucleus. Various other metamorphic processes are also predicted to modify the subsurface layer of the nucleus, Consequently, it should become stratified and more cohesive than initially, At present, because of preparations for the Rosetta mission to Comet 46P/Wirtanen, it is especially important to understand the processes responsible for the evolution of a cometary nucleus, This can be of key importance to a successful landing on the nucleus and further interpretation of the results of the mission. This work is intended primarily to estimate how quickly the process of grain sintering can modify the outer part of the nucleus, Other effects, such as crystallization of water ice, are, however, also taken into account. The simulation is performed for the period 1944 to 2016 taking into account the orbit of the comet perturbed by planets and by nongravitational effects. The numerical procedure used to simulate the evolution of the material texture and temperature distribution below the dust layer is based on the self-consistent model of the thermal and structural evolution of an initially homogeneous ice-dust mixture, The original model is, however, significantly extended. The present analysis includes the possible presence of amorphous water ice and other, nonwater volatiles, They are assumed to be initially trapped in amorphous water ice. It was found that a cohesive layer from about 1 m to about 10 m thick can be formed, depending on the size of the ice grains and the properties of the dust mantle. (C) 1999 Academic Press.