Heating and evaporation of Icy particles in the vicinity of comets

Although the detailed structure of the surface layers of a cometary nucleus is still unknown, there are good reasons that favor a porous and fragile ice/dust mixture. Such a nucleus will most probably emit ice particles which are blown off from the surface by sublimating gases. Their sizes may range from meter-sized boulders down to micro- and submicrometer-sized particles. In this paper we present a model which allows to calculate the “history” of ice particles with radii up to 100 μm released from a cometary nucleus. Temperatures and sublimation rates of both pure ice particles and ice particles polluted by a dark, nonvolatile material are computed. It is found that the black body approximation of the solar UV-radiation noticeably shortens the lifetimes of large (≥10 μm) particles. Moreover, the use of different formulae for the vapor pressure of ice found in the literature may affect the lifetime calculations by a factor of 10. The sublimation minimum predicted by Patashnik and Rupprecht (1975) for pure ice particles at a size of about 20 μm is confirmed. However, even small amounts of pollution in the ice tend to smear out this minimum and make it disappear which leads to much shorter particle lifetimes.