The vertical Distribution and Origin of HCN in Neptune's Atmosphere

Abstract

Measurements and modeling of the (3-2) rotational line of hydrogen cyanide at 265.9 GHz in Neptune's atmosphere are presented. High signal-to-noise observations, performed at IRAM in October 1991 and April 1992, provide information on the HCN vertical distribution in Neptune's stratosphere. The HCN mixing ratio is found to be nearly uniform with height above the condensation level (3 mbar). Best fits occur for HCN distributions that have a slight increase with altitude (HCN scale height approximately 5 × the atmospheric scale height). A least-squares analysis yields a mixing ratio of (3.2 ± 0.8) 10 -10 at 2 mbar and a mean mixing ratio scale height of 250 +750 +110 km in the 0.1-3 mbar region. To interpret these results, we developed a photochemical model of HCN, based in part upon the hydrocarbon model of Romani et al. (Icarus, 106, 442-463 (1993)). HCN formation is initiated by the reaction between CH 3 radicals, produced from methane photochemistry, and N atoms. The primary sink for HCN is condensation, with minor contributions from photolysis and chemical losses. Two possible sources of N atoms are investigated: (1) infall of N escaped from Triton's upper atmosphere, and (2) galactic cosmic ray (GCR) impact on internal N 2. For the Triton source, a total N flux of 5 × 10 24 atoms sec -1 is necessary to match the HCN abundance, and the observed HCN profile suggests an eddy diffusion profile with a rapid transition from slow mixing at p ≥ 2 mbar to rapid mixing at lower pressures. The required N flux is approximately 50% of the estimated N escape from Triton's upper atmosphere. For the GCR source, a simple model of ionization and dissociation of N 2 by cosmic rays was developed. In this case, both smooth and discontinuous eddy K profiles satisfy the observations. The tropospheric N 2 mixing ratio required to fit the HCN observations is in the range 0.3-1.7% if our GCR interaction model is correct. The range results from uncertainties in the chemistry of the NH radical and in the eddy K profile. Our GCR model, however, disagrees with earlier models by Capone et al. ( Icarus 55, 73-82 (1983)) and, if the latter are correct, the required N 2 mixing ratios are a factor of 6 larger. The GCR source leads to a higher formation rate of HCN ice than the Triton source, but one not large enough for HCN ice to be detected in the Voyager IRIS spectra. Additionally, the GCR source implies the possible formation of methylenimine (CH 2NH) in amounts comparable to HCN. Given the uncertainties on (i) the transport and possible ionization of N in Neptune's magnetosphere, and the fate of N + reaching Neptune's upper atmosphere and (ii) the N 2 mixing ratio in Neptune's deep atmosphere, we suggest that both sources of N atoms may significantly contribute to the formation of HCN.