Models of P Wirtanen nucleus: active regions versus non-active regions

From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation. The evolution of the comet depends on the sublimation of ices and the relea se of different gases and dust grains: the formation of a dust crust, the s urface erosion and the development of the coma are related to the gas fluxe s escaping from the nucleus. New observations, laboratory experiments and n umerical simulations suggest that the gas and dust emissions are locally ge nerated, in the so-called 'active regions'. This localized activity is prob ably superimposed to the global nucleus activity. The differences between a ctive and inactive regions can be attributed to differences in texture and refractory material content of the different areas. In this paper we present the results of numerical models of cometary nucleu s evolution, developed in order to understand which are the processes leadi ng to the formation of active and non-active regions on the cometary surfac e. The used numerical code solves the equations of heat transport and gas d iffusion within a porous nucleus composed of different ices-such as water ( the dominant constituent), CO2, CO- and of dust grains embedded in the ice matrix. By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas ca n be followed. Comet P/Wirtanen is the target of the international ROSETTA mission, the co rnerstone ESA mission to a cometary nucleus. The successful design of ROSET TA requires some knowledge of comet status and activity: surface temperatur es, amount of active and inactive surface areas, gas production rate and du st flux. (C) 1999 Elsevier Science Ltd. All rights reserved.