A P/Wirtanen evolution model

Abstract

Comet P/Wirtanen is the currently selected target for the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus that will be launched in 2003. Presently, little is known about this comet, but the successful design of the ROSETTA mission, and in particular of the on-board scientific instruments, requires some preliminary knowledge of the comet's physical parameters, such as surface temperature, percentage of active surface, intensity of gas and dust fluxes, and so on. These quantities cannot be determined through ground-based observations, so predictive models of the thermal evolution and differentiation of a cometary nucleus are needed. A thermal evolution model is applied to a comet on the orbit of P/Wirtanen with the aim of obtaining some of the needed information. The numerical code solves the heat conduction and the gas diffusion equations throughout a spherical, porous comet nucleus made of amorphous H 2O ice (the dominant constituent), CO 2 ice, CO ice and dust particles of different sizes. The equations are coupled via the source terms, which describe the sublimation and recondensation of ices as latent heat or mass exchanges. Amorphous H 2O ice can undergo an exothermal, irreversible phase transition to crystalline form. The ejection of dust is allowed only if the grains have been liberated from ice, when the drag exerted on them by the outflowing gas is stronger than the gravitational pull of the nucleus. The behaviour is simulated of a “P/Wirtanen”-like object, that starting from the Kuiper belt is injected, through multiple close encounters, in the present orbit of P/Wirtanen. This has been done with the aim of predicting the present characteristics of surface condition and emission of this poorly known comet. From the results of the simulations it can be seen that surface activity starts at 2.1 AU with gas emission and then reaches its maximum at the perihelion, where it is accompanied by a strong dust emission; the range of the computed comet surface temperatures is, at the equator and at the perihelion, between 130 and 200 K. Differences between day and night temperatures on the surface of the nucleus can reach 50 K at perihelion, with a consequent variation in the H 2O and dust emission rate, but the flux of more volatile ice, like CO 2 and CO, is not affected. Such ices can be found at depths varying from a few metres for CO 2 to hundreds of metres for CO.