Methane nucleation in the atmosphere of Neptune

Observations and modelling of albedo, cloud cover, and aerosols on Neptune (e.g. Lockwood and Thompson, Nature 280, 1979; Irwin et al, JGR Planets 127, 2022; Chavez et al, Icarus 404, 2023) have been ongoing for a long time. A feature that has yet to be fully explained is the (anti)correlation between the albedo and the 11-year solar cycle.

UV light, which varies highly with solar activity, can affect photochemistry by photolysis of methane, which can lead to haze formation (Romani and Atreya, Icarus 74(3), 1988) and under the right conditions methane can form aerosols by itself. Ionization by galactic cosmic rays can enhance aerosol nucleation rates by lowering the Gibbs free energy barrier for stable cluster formation.

In this work we calculate neutral homogeneous nucleation rates for methane as well as the corresponding ion-induced heterogeneous nucleation rates. We do this both for vertical profiles and horizontal maps of the Neptunian atmosphere.

The starting point is the ISO atmospheric profile for a hot, cold, and nominal Neptune atmosphere. This is combined with a simple methane distribution model constrained by observational values in the troposphere and stratosphere. From this the Gibbs free energy for homogeneous nucleation is calculated, giving the neutral nucleation rate.

We then find ionization rates, based on galactic cosmic ray proton flux maps generated from calculations of cut-off rigidities derived from the magnetic field model of Neptune (Connerney et al, ASR 12(8), 1992) and a particle trajectory program (the Geomagnetic Cutoff Rigidity Computer Program by Smart and Shea, 2001, Tech. Rep. No. 20010071975).

From the ionization rates the ion-induced nucleation rate can be found. We then compare the neutral and ion-induced nucleation rates for a vertical profile to identify which altitudes are dominated by which process.

Horizontal maps at altitudes where the nucleation rates are not dominated by neutral nucleation can then be generated. By using latitudinal variations in the methane concentration in combination with the ionization maps we can identify a geographic distribution of where ion-induced nucleation may play a significant role in the generation of aerosols.