HNC is a major product of the dissociative recombination reaction of the im
portant ionospheric ion HCNH+ although to date this neutral has not been id
entified within Titan's atmosphere, nor considered as a component in ionosp
heric or photochemical models, We have studied a simple pseudo-steady-state
model for the formation and removal of HNC, in which the loss processes co
nsidered are HNC protonation by reaction with H-bearing ions; reaction with
H atoms, yielding HCN; reaction with CH3 radicals, to form CH3CN; and reac
tion with a population of unidentified X radicals to yield further hypothes
ized products. Using the ion abundances of C. N. Keller et al. (1998, Plane
t. Space Sci. 46, 1157-1174) and of M. Banaszkiewicz et al. (2000, Icarus 1
47, 386-404), we find that the most important loss processes are the reacti
ons with CH3 and with other unidentified radicals. According to our calcula
tions, the HNC concentration reaches a peak of 10(4)-10(5) molecules per cu
bic centimeter at an altitude of 1000-1100 km (i.e., close to the ionospher
ic peak), but is very much reduced at lower altitudes. We find also that th
e 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. (C) 2001 Academic Press.