An improved thermal model for cometary nuclei

Abstract

We have modified our earlier thermal model of cometary nuclei by incorporating a more accurate calculation of the opacity of the cometary dust coma. The new model uses a modified Sekanina‐Miller dust particle size distribution and a Delsemme‐Miller gas outflow dynamics model. The improved model has been applied to the case of the 1986 apparition of Halley's Comet, also incorporating a revised radius and albedo for the Halley nucleus. The estimated opacity of the Halley coma is reduced from that of the earlier model. The revised opacity is 0.12 at 1.0 AU and 0.43 at perihelion, 0.587 AU. This results in a decrease in the energy supplied to the nucleus from the coma by multiply scattered radiation and thermal emission, and brings the model into better agreement with the results of others. The coma feedback mechanism enhances nucleus gas production by a factor of 1.4 at 1.0 AU over that for a bare ice nucleus with no dust coma, and by a factor of 2.3 at perihelion. Predicted gas production at 0.9 AU postperihelion, the expected heliocentric encounter distance for the Giotto mission, is 4.4×1029 molecules per second. This agrees well with a recent estimate of 3.9×1029 molecules s−1 for the Halley gas production rate at 0.9 AU during the 1910 apparition.