Monochromatic brightness variations of comets

A class of comets, of which Comet Kohoutek (1973f) is typical, show total as well as monochromatic brightness asymmetries about perihelion. They are fainter after perihelion than before at the same heliocentric distance. A model of the cometary nucleus consisting of a growing non-volatile dust mantle surrounding a volatile icy core is used to discuss this phenomenon.Numerical results are obtained for Comet Kohoutek (1973f). It is found that dust mantles of thickness in the range of 10–75 cm can be grown by perihelion passage for various values of the thermal conductivity of the dust if there is no substantial dust blow-off by the sublimating volatiles.The thermal conductivity of the dust mantle is quite small and is dominated by radiative conductivity for heliocentric distances 2 AU. Since the radiative conductivity is larger for larger grain size, the thickest mantle corresponds to coarsest matrix.The strong insulation provided by the growing dust layer progressively suppresses the surface temperature of the volatile core below its quasi-equilibrium value had there been no mantle. As a consequence the production rate of the parent-molecules as well as the monochromatic brightness of their daughter products increases less steeply than in the mantleless case, as the comet approaches perihelion. Furthermore, there are significant monochromatic brightness asymmetries about perihelion, which are enhanced if there is a greater dust blow-off before perihelion than after, as is believed to be the case with Comet Kohoutek (1973f). Estimates of this asymmetry of 1 × 2 magnitudes for the OH brightness at a heliocentric distance × 1 AU are consistent with the rather limited observations.