Monte Carlo modeling of neutral gas and dust in the coma of Comet 1P/Halley Martin Rubin a,⇑ , Valeriy M. Tenishev a , Michael R. Combi a , Kenneth C. Hansen a , Tamas I. Gombosi a , Kathrin Altwegg b , Hans Balsiger b a Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109, USA b Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland article info Article history: Received 29 September 2010 Revised 24 March 2011 Accepted 7 April 2011 Available online 20 April 2011 Keywords: Comet Halley Comets, Coma Comets, Composition Comets, Dust Comets, Dynamics Photochemistry abstract The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spec- trometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and var- ious ground based observations. The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659–677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H 2 O, CO, H 2 CO, CO 2 , CH 3 OH, C 2 H 6 ,C 2 H 4 ,C 2 H 2 , HCN, NH 3 , and CH 4 ) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H 2 , O, OH, C, CH, CH 2 , CH 3 , N, NH, NH 2 ,C 2 ,C 2 H, C 2 H 5 , CN, and HCO) from the comet’s surface all the way out to 10 6 km. As a result we are able to further constrain cometary the gas production rates of CO (13%), CO 2 (2.5%), and H 2 CO (1.5%) relative to water without invoking unknown extended sources. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Neutral gas, either produced by the comet or through dissociation of these cometary parent species, moves along ballistic trajectories until it is ionized, dissociates, or interacts with other particles nearby. The length scale over which these processes take place spans over several orders of magnitude: especially for a well-developed cometary atmosphere the collisional mean free path close to the nucleus is on the order of a fraction of a meter and increases with cometocentric distance to almost infinity. On March 14, 1986 the European Space Agency’s Giotto space- craft encountered Comet 1P/Halley for a thorough investigation of the comet’s environment. The obtained observations greatly im- proved our knowledge of comets. In this work we re-analyze the data obtained by Giotto’s Neutral Mass Spectrometer (NMS) using a Direct Simulation Monte Carlo (DSMC) model including neutral gas and dust. We further compare our results to other relevant observations obtained during the same period. In the past several studies have been carried out to broaden our understanding of the comet’s neutral gas and dust environment. Krankowsky et al. (1986) calculated gas production rates from the measurements performed with the NMS using a neutral gas bulk velocity of 900 ± 200 m s 1 . Lämmerzahl et al. (1987) then also derived the radial gradient of the neutral gas expansion veloc- ity. Combi (1989) modeled these outflow velocities using a dusty- gas-dynamic/Monte Carlo model see also Hodges (1990). Gombosi et al. (1986) used a fluid approach to model the dust and gas inter- action in the inner atmosphere of comets. Combi et al. (1993) used observations of the OH radical performed by the International Ultraviolet Explorer (IUE) to deduce water production rates. Combi (1996) also used a time-dependent Monte Carlo technique to ac- count for Halley’s variable gas production rate. Eberhardt (1999) and Altwegg et al. (1999) presented elemental and molecular abundances derived from measurements performed by the NMS and the Ion Mass Spectrometer (IMS) including extended sources for CO and H 2 CO (Eberhardt et al., 1987; Meier et al., 1993), respec- tively. Our DSMC approach includes a larger set of gas species and a fully coupled dust phase and therefore allows us to further con- strain production rates for some of the species. The above extended sources have been broadly discussed (Cottin and Fray, 2008) and their existence is still subject to controversy. Candidates such as POM as the source have been investigated by e.g. Fray et al. (2006). In this work we will show that the measurements 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.04.006 ⇑ Corresponding author. Fax: +1 734 763 0437. E-mail address: rubinmar@umich.edu (M. Rubin). Icarus 213 (2011) 655–677 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus