ICARUS 84, 118-153 (1990) A Time-Dependent Dusty Gas Dynamic Model of Axisymmetric Cometary Jets A. KOROSMEZEY 1 AND T. I. GOMBOSI Space Physics Research Laboratory, Department of Atmospherw, Oceant~ and Spaee Scwnces, University of Michtgan, Ann Arbor, Mwhtgan 48109 Received December 5, 1988, revised June 21, 1989 The first results of a new time-dependent, axisymmetric dusty gas dynamical model of inner cometary atmospheres are presented. The model solves the coupled, time-depen- dent continuity, momentum, and energy equations for a gas-dust mixture between the nucleus surface and 100 km using a 40 x 40 axisymmetric grid structure. The time- dependent multidimensional partial differential equation system was solved with a new numerical technique employing a second-order accurate Godunov-type scheme with di- mensional splitting. It is found that narrow axisymmetric jets generate a subsolar dust spike and a jet cone, where a significant amount of the jet ejecta is accumulated. This subsolar dust spike has not been predicted on earlier calculations. The opening angle of the jet cone depends on the jet strength and it also varies during the time-dependent phase of the jet. For weak jets the steady-state half-opening angle is about 50 ° . In the case of the strong jets the jet cone extends to the nightside in good agreement with the Giotto imaging results. © 1990Academic Press,Inc. 1. INTRODUCTION Our present, postencounter understand- ing of cometary nuclei is based on Whip- pie's (1950) "dirty iceball" idea, which vis- ualizes them as chunks of ice, rock, and dust with negligible surface gravity. Whip- ple's hypothesis quickly replaced the cen- tury-long series of "sandbank" models, wherein the nucleus was thought of as a diffuse cloud of small particles traveling to- gether. As comets approach the Sun, water vapor and other volatile gases sublimate from the surface layers, generating a rap- idly expanding dusty atmosphere. The sub- limated gas molecules (often called parent molecules) undergo collisions and various fast photochemical processes in the near nucleus region, thus producing a whole chain of daughter atoms and molecules. There is growing evidence that delayed gas Permanent address" Central Research Institute for Physics, Hungarian Academy of Sciences, Budapest, Hungary. 0019-1035/90 $3.00 Copyright © 1990by Academic Press. lnc All rights of reproduction in any form reserved emission from dust particles, dust grain photochemistry, as well as gas-dust chemi- cal reactions also contribute to the mainte- nance of cometary atmospheres. In the vicinity of the nucleus the gas and dust flows are strongly coupled: frequent gas-dust collisions accelerate small grains to velocities up to several hundreds of me- ters per second and inject them into the ex- tensive cometary exosphere, where the gas and dust are decoupled. The expanding gas eventually converts most of its original in- ternal energy to bulk motion, while it also loses momentum and energy to the dust flow. It was recognized as early as the mid- 1930s that gas outflow plays an important role in cometary dust production. In early treatments of the gas-dust interaction it was assumed that the gas drag coefficient was independent of the gas parameters and that the gas velocity was constant in the dust acceleration region. In the late 1960s this very naive picture was replaced by a 118