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

Abstract

An axisymmetric Monte Carlo model of a cometary atmosphere, composed of H<SUB>2</SUB>O and its daughter species H and OH, has been established and applied to comet 1P/Halley at the Giotto encounter. Results are presented for comae modeled with two gas density functions, represented by total water production rates Q<SUB>0</SUB> and water angular distributions f(θ) On the inner boundary (r = 500 km). In case (1), Q<SUB>0</SUB> = 6 x 10<SUP>29</SUP> molecules s<SUP>-1</SUP> and f(θ) = 1 + 0.5 cos θ, and in case (2), Q<SUB>0</SUB> = 5 x 10<SUP>29</SUP> molecules s<SUP>-1</SUP> and f(θ) = 1 + 0.9 cos θ, respectively. <P />Our simulations clearly show that anisotropy of water is maintained at least to a radial distance of 2 x 10<SUP>5</SUP> km, but that the gas density becomes increasingly isotropic at larger radial distances. For daughter species, changes of angular distribution in the coma also depend on the ejection velocity: H becomes more or less isotropic in the outer coma due to its large ejection velocities, while OH preserves the initial angular distribution to a greater degree during its outward expansion due to a small mean ejection velocity of 1 km s<SUP>-1</SUP>. The flattening of angular distributions leads to a reduced asymmetry in gas column density from the intermediate coma (r ∼a few thousand km from the nucleus) to the outer coma and could be partly responsible for reduced asymmetries seen in observed cometary data. <P />We also show that (a) the outward expansion of H<SUB>2</SUB>O, H, and OH is mainly radial, with very small mean lateral velocities throughout the range from the inner boundary (r = 500 km) coma to the outer coma, and (b) the outflow velocities of H<SUB>2</SUB>O and OH increase with increasing local gas production rates, while that of H decreases with increasing local production rates. Furthermore, the gas production rate estimated from fitting the Giotto velocity measurements is a local one and does not represent the true total gas production rate. The retrieval of the latter requires information on the lag angle of outgassing and its angular distribution. Applications to trace volatiles (CH<SUB>3</SUB>OH, H<SUB>2</SUB>S) and to gaseous species having both direct and extended sources (CO, H<SUB>2</SUB>CO, CN) are discussed.