Hydrogen Isocyanide, HNC: A Key Species in the Chemistry of Titan's Ionosphere?

HNC is a major product of the dissociative recombination reaction of the important ionospheric ion HCNH⁺, although to date this neutral has not been identified within Titan's atmosphere, nor considered as a component in ionospheric or photochemical models. We have studied a simple pseudo-steady-state model for the formation and removal of HNC, in which the loss processes considered are HNC protonation by reaction with H-bearing ions; reaction with H atoms, yielding HCN; reaction with CH₃ radicals, to form CH₃CN; and reaction with a population of unidentified X radicals to yield further hypothesized products. Using the ion abundances of C. N. Keller et al. (1998, Planet. Space Sci. 46, 1157-1174) and of M. Banaszkiewicz et al. (2000, Icarus 147, 386-404), we find that the most important loss processes are the reactions with CH₃ and with other unidentified radicals. According to our calculations, the HN C concentration reaches a peak of 10⁴-10⁵ molecules per cubic centimeter at an altitude of 1000-1100 km (i.e., close to the ionospheric peak), but is very much reduced at lower altitudes. We find also that the HNC/HCN ratio in Titan's atmosphere may approach unity at the ionization peak altitude and above. We discuss prospects for the detection of HNC, or its reaction products, during the Cassini mission.