Triton's Ionosphere: Chemistry and Composition

The ionosphere of Triton was observed by the Voyager spacecraft in 1989 to have a remarkably high electron density of 40,000/cc at its peak altitude. Delitsky et al. (1990) modeled this ionosphere using only N₂ and CH₄, the constituents of the atmosphere known at that time, and found that, at the extremely cold temperatures in the Triton atmosphere, cluster ions would form. These clusters are created when N⁺ or N₂⁺ resulting from photolysis or radiolysis accrete neutral N₂ molecules and form ions such as (N₂⁺(N₂)_n). In these clusters, n can be very high, around 50-100, depending on temperature. Cluster ions easily sweep up electrons at the low altitudes where they form (keeping the e^- content low) which leads to dissociative recombination. This neutralizes the cluster ions and releases the N₂ molecules back into the atmosphere.

In 1991, CO and CO₂ were observed on Triton (Cruikshank et al. 1991). At Tritonian temperatures, CO will have a very high vapor pressure and could constitute up to 6% of the Triton atmosphere. Any N⁺ or N₂⁺ will charge exchange with CO (and NO from chemistry) to yield CO⁺, NO⁺ and C⁺. These then become the core ions to the clusters (CO⁺(N₂)n), (NO⁺(N₂)n), or (C⁺(N₂)n). (Delitsky et al. 1992, Delitsky, 1995). Clusters cannot form at higher altitudes and lower pressures and so at the peak altitude, the ionosphere is comprised almost totally of C⁺ ions. From modeling, CO + hv -> C⁺ (+ O) does not appear to be an important source of the C⁺ . Rather, the charge exchange reaction, CO⁺ + C -> C⁺ + CO produces the C⁺ which charge balances the electrons in the ionosphere.

Ref: Cruikshank et al., BAAS, 23,1208 (1991);.. Delitsky et al. GRL, 17, 1725 (1990); ..Delitsky et al. Neptune conf, 1992; ..Delitsky, BAAS, 27, 1100 (1995)