MONTE-CARLO SIMULATION OF COMETARY ATMOSPHERES - APPLICATION TO COMETP HALLEY AT THE TIME OF THE GIOTTO SPACECRAFT ENCOUNTER .1. ISOTROPICMODEL/

Isotropic and axisymmetric models of a cometary atmosphere, made up of H2O and its daughter radicals (H, OH, O, and H-2), have been establis hed using the Monte Carlo particle transport method, Physical paramete rs of the gases considered were computed for an isotropic cometary com a with a gas production rate of 5 x 10(29) molecules s(-1) and helioce ntric distances of 0.89 AU, corresponding to comet P/Halley at the Gio tto flyby in 1986 March. The simulated velocity profile of water molec ules is in good agreement with Giotto Neutral Mass Spectrometer (NMS) measurements over the entire range from the inner coma to the outer co ma (800-34,000 km), when realistic semiclassical collision cross secti ons are used. The successful model for the velocity profile in the out er coma requires the inclusion of rotational cooling of water molecule s 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 distan ces larger than 500 km. Our simulation demonstrates that selective pho todestruction of slow water molecules in the inner coma contributes si gnificantly to the increase of outflow velocity of parent species at r adial distances larger than 20,000 km, at which photochemical heating ceases to be important. The simulation also shows clearly the evolutio n of velocity distributions for gas particles from being thermal in th e 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(29) molecules s(-1)). Outside the collision-dominated region (r > 10(4) km), hydrogen (H) has three peaks in its velocity distributio n, one at 18 km s(-1), a second at 8 km s(-1), and a thermal component peaked at about 1-2 km s(-1), consistent with observational results. We show that a gas production rate of 4.5 x 10(29) molecules s(-1) giv es the best fit to the Giotto NMS velocity measurements.