The effect of local topography and self-heating on the sublimation rate of cometary nuclei q A.V. Ivanova, L.M. Shulman * The Main Astronomical Observatory of the National Academy of Sciences of Ukraine, 27 Zabolotny St, Kyiv-22, 03680, Ukraine Received 6 October 2005; received in revised form 1 May 2006; accepted 2 May 2006 Abstract This work presents the results of calculation of the temperature regime and total flux of sublimation for a new physical model of local active areas on a cometary nucleus. As it was shown before a conical hole in the surface dust layer of a cometary nucleus can work as a concentrator of solar energy and, therefore, intensifies sublimation. The temperature of ice on the bottom and that of dust on the side of the crater is calculated for different geometrical parameters of the crater and different conditions of rotation of the nucleus. The depen- dence of the effect on the location of the active region as well as the effect of changing the solar zenith angle is studied. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Cometary nucleus; Active region on a cometary nucleus; A meteoroid impact; A temperature regime of a cometary nucleus; A model of cometary nucleus 1. Introduction Since VEGA-GIOTTO mission’s Sagdeev, 1986 com- etary nuclei are surely considered as non-spherical bodies with inhomogeneous surfaces. Since the measured value of albedo was equal 0.04 one can conclude that bare ice occupied only 10–15% of the surface. This esti- mation corresponds to zero albedo of the dust mantle and the dirty ice albedo 0.4. It would be unreasonable to persist on the similarity of every cometary nucleus to that of P/Halley, but it is impossible to ignore the fact that outgassing of volatiles must be concentrated in a small number of active zones Sagdeev, 1986. Krasnopol- sky (1986) measured during the VEGA mission the col- umn density in the atmosphere of Comet Halley. It gave the gasproductivity that was three times higher than that estimated by the area of the open insolated naked ice surface Shulman, 1987. The mentioned discrepancy between the gas productivity and the albedo of Comet Halley was obtained using some assumption on the properties of the nucleus. One can see that we tried to solve one equation with two or even three unknowns: the fraction of bare ice surface, the ice albedo A i , and the albedo of dust A d if A d 6¼ 0. It is possible to increase the icy part of the nucleus surface taking a lower albedo of ice but in this case we should meet another difficulties. The very dirty ice vaporization is not describing by the simple Hetz–Knudsen equation. This complicated problem considered by Skorov (2001). One can expect a rather fast mantling of such areas. It means that the model of jet formation due to an icy cavity on the homogeneous surface calculated by Crifo and Rodionov (1998) does not fit the real sit- uation in Comet Halley because it corresponds to the nuclear albedo 0.4 that is ten times more than observed. The other missions (Stardust to comet 81P/ Wild-2, Deep Space to comet 19P/Borrelly and Deep impact to comet 9P/Tempel-1) to comets showed that crater structure are typical for cometary nuclei. An attempt to consider the role of topography was made 0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.05.003 q This investigation was supported by a grant of the State Foundation for Basic Research of the Ministry of Education and Science of Ukraine. * Corresponding author. Tel.: +38044 2664769; fax: +38044 5262147. E-mail addresses: sandra@mao.kiev.ua (A.V. Ivanova), shulman@ mao.kiev.ua (L.M. Shulman). www.elsevier.com/locate/asr Advances in Space Research 38 (2006) 1932–1939