Monte Carlo Simulation of Cometary Atmospheres: Application to Comet P/Halley at the Time of the Giotto Spacecraft Encounter. I. Isotropic Model

Abstract

Isotropic and axisymmetric models of a cometary atmosphere, made up of H<SUB>2</SUB>O and its daughter radicals (H, OH, 0, and H<SUB>2</SUB>), have been established using the Monte Carlo particle transport method. Physical parameters of the gases considered were computed for an isotropic cometary coma with a gas production rate of 5 &gt;c 10<SUP>29</SUP> molecules s<SUP>-1</SUP> and heliocentric distances of 0.89 AU, corresponding to comet P/Halley at the Giotto flyby in 1986 March. The simulated velocity profile of water molecules is in good agreement with Giotto Neutral Mass Spectrometer (NM S) measurements over the entire range from the inner coma to the outer coma (800-34,000 km), when realistic semiclassical collision cross sections are used. The successful model for the velocity profile in the outer coma requires the inclusion of rotational cooling of water molecules in this transition region from the optically thick to optically thin. No evidence is found from our simulation for additional heating from the recondensation of icy grains in the inner coma with radial distances larger than 500 km. <P />Our simulation demonstrates that selective photodestruction of slow water molecules in the inner coma contributes significantly to the increase of outflow velocity of parent species at radial distances larger than 20,000 km, at which photochemical heating ceases to be important. <P />The simulation also shows clearly the evolution of velocity distributions for gas particles from being thermal in the inner coma to nonthermal in the outer coma; the critical distance is found to be 6000 km for water molecules (at a gas production rate of 5 x 10<SUP>29</SUP> molecules ). Outside the collision-dominated region (r&gt; 10<SUP>4</SUP> km), hydrogen (H) has three peaks in its velocity distribution, one at 18 km s<SUP>-1</SUP> a second at 8 km s<SUP>-1</SUP>, and a thermal component peaked at about 1-2 km s<SUP>-1</SUP>, consistent with observational results. <P />We show that a gas production rate of 4.5 &gt;c 10<SUP>29</SUP> molecules s<SUP>-1</SUP> gives the best fit to the Giotto NMS velocity measurements.