Hydrogen halides on Jupiter and Saturn

The quest to detect gaseous HCl, HBr, and I-IF in the atmospheres of Jupite r and Saturn has led to a tentative detection of 1 ppb HCl near Saturn's cl oud deck. The detection is puzzling because, while these hydrogen halides m ay be present several scale heights below the clouds, they are expected to react with ammonia to form solid ammonium halide salts in the upper troposp here. I show that the loss timescale for condensation of gaseous hydrogen h alides onto particles is similar to 10(3)-10(5) s for realistic cloud densi ties and particle sizes, which is much less than the similar to 10(8) s res idence time of upper tropospheric air. The hydrogen halides can only surviv e transport up to the cloud layer if less than 1 in 10(6) of their collisio ns with particle surfaces leads to condensation, which is unlikely. Even in the absence of foreign particles, homogeneous nucleation would probably pr event supersaturations in excess of a few hundred, which is similar to 10(2 0)-10(40) times too low to explain the observation. These calculations ther efore suggest that hydrogen halides cannot exist at part-per-billion levels in the upper troposphere. The interplanetary source of halogens is also to o low to produce detectable quantities of hydrogen halides except perhaps a t pressures less than 1 mbar. A possible detection of chlorine by the Galil eo probe at pressures exceeding 9 bars on Jupiter may be consistent with th e equilibrium abundance of gaseous HCl or NH4Cl. (C) 2001 Academic Press.