H2O-activity of comet Hale-Bopp

Abstract

The outstanding brightnesss of comet Hale-Bopp allowed observers to measure reliable water production rates over a wide range of heliocentric distances (up to 5 AU). Different methods have been applied, including radio observations, IR- and UV-measurements. The H2O-production of a comet is closely connected with the energy balance and the composition of its surface. By comparing measured and calculated rates one can try to derive properties of the nucleus. Therefore, a one-dimensitonal thermal model has been applied to calculate the energy transport at each point on Hale-Bopp's surface. The balance of solar input, re-radiation, latent heat of sublimation and heat conduction provides the upper boundary condition of the problem.A spherical nucleus with randomly distributed active areas is assumed. The strong obliquity of the spin axis that causes pronounced seasonal effects and the rotation of the nucleus are included. The H2O-activity has been calculated as a function of heliocentric distance by an integreation of the local flux over the entire surface of the comet. The influence of the heat conduction and of the nucleus' rotational properties is discussed and the results are compared with experimental data. It is shown that the derivation of nuclear properties from coma measurements is a quite complex problem.