A thermal modelling of cometary activity with a crystallized water ice nucleus

Arguments are presented to suggest that surface layers of the nuclei of periodic comets consist of crystallized rather than amorphous water ice and thermal modelling of such nuclei is presented. The rate of sublimation of water from a rotating nucleus is found to be greater than that from a uniformly heated nucleus. When the model is applied to P/Halley, the sublimation rate at perihelion is found to be 8.1 × 10²⁹ mol s⁻¹ for a nucleus rotating with a period of 50 hours and 7.6 × 10²⁹ for a uniformly heated nucleus on the premise that the effective radius of the nucleus is 2.5 km. The total sublimation of water per revolution is 5.38 × 10³⁶ molecules for P/Halley and 3.91 × 10³⁶ molecules per P/Crommelin. The result so obtained is discussed in relation to the observational data.