Thermal Structure of Cometary Active Regions: Comet 1P/Halley

We develop a basic thermal model for Comet 1P/Halley aimed at explaining the secular behavior and the diurnal amplitude of the observed water production. Our model incorporates time varying insolation of five active regions as determined for 1P/Halley by the nonprincipal axis spin model of M. J. S. Belton et al. (1991, Icarus93, 183-193). The thermal structure internal to each of the five active regions is governed by the insolation pattern, cooling by reradiation and sublimation, and the nonlinear heat conduction. The model results are compared with the water production rates calculated from the observed OH production in 1P/Halley (D. G. Schleicher et al., 1998, Icarus132, 397-419). The models are sensitive to the thermal inertia of cometary ice; an absolute upper limit to the thermal inertia of 0.016 cal cm ^-2 s ^{- 1/2} K ^-1 is derived based on the model fits to the secular behavior of observational data. This derived thermal inertia of cometary ice is at least a factor of three smaller than that for solid crystalline water ice and therefore consistent with a porous structure for the cometary ice. This observationally based estimate of the thermal inertia of cometary ice is also consistent with the approximate thermal inertia of 0.003 cal cm ^-2 s ^{- 1/2} K ^-1 suggested by P. R. Weissman (1987, Astron. Astrophys.187, 873-878) based on the "turn-on" of water production in Comet 1P/Halley. The models are insensitive to the initial temperature of the nucleus, assumed to be 60 K. We find that the secular behavior of preperihelion model water production, unlike that during the postperihelion, is strongly dependent on the thermal inertia of cometary ice. This underlines the importance of obtaining well sampled accurate preperihelion observations of water (or a daughter species of it such as OH) throughout the entire period when water is the most dominant outgassing species. We obtain approximately 10 ¹² cm ² for the total active area on the nucleus, consistent with that based on spacecraft imagery and other groundbased data.