Photochemical modeling of Triton’s atmosphere: methodology and first results

Introduction 

Triton is the biggest satellite of Neptune. Discovered in 1846 by W. Lassell, it was visited by Voyager 2 in 1989. It was the only spacecraft to study the neptunian system. Very little was known about Triton at the time of the flyby, except its highly inclined retrograde orbit suggesting that it is a Kuiper Belt object captured by Neptune. This idea was also comforted by the similarities between Triton and Pluto. The flyby allowed to take high resolution pictures of the surface, to determine its temperature, pressure and the composition of the surface ices (N₂, CH₄, CO, CO₂ and water ice). Clouds and haze were also observed respectively under 10 and 30km of altitude as well as plumes of organic material propagating up to 8km, pointing out the existence of a troposphere. It appeared that the low atmosphere was at vapor pressure equilibrium with the surface’s ices (Yelle et al. 1995). The atmosphere was also studied by measuring its airglow and by performing stellar occultations (Broadfoot et al. 1989). It revealed that it is mainly composed of N₂, and N with traces of CH₄ near the surface. CO was not detected and so only an upper limit on its abundance had been set. An important ionosphere was also observed with an important peak at 340km (Tyler and al. 1989).  As Triton is far from the Sun, this important ionosphere cannot be explained by solar ionization only. So, it was hypothesized that energy was brought by precipitating electrons from the magnetosphere of Neptune (Strobel et al. 1990b). Another source of power is the interplanetary Lyman-Alpha flux that is not negligible at Triton’s distance from the Sun (Strobel et al. 1990a).

However, since the Voyager 2 mission and its only flyby, Triton remains poorly understood in comparison to Titan which was intensely studied during the Cassini-Huygens mission (despite some observations with ALMA and the VLT, see Lellouch et al. 2010 and Merlin et al. 2018). And as for Titan, it is now supposed that Triton is an ocean world (Fletcher et al. 2020). Sending a new mission to the neptunian system appears as a necessity to increase our knowledge about ice giants and their systems. In order to prepare such a mission, having a theoretical photochemical model for Triton can be useful to have a reference against which we would compare data collected during such a mission.

 The photochemical model

 As a starting point, we used the photochemical model of Titan (see Dobrijevic et al. 2016) and adapted it to Triton. In particular, we used the chemical scheme developed for Titan (with recent updates presented in Hickson et al. 2020), as the two atmospheres are mainly composed of N₂, with presence of CH₄. Our methodology is presented on Figure 1. On Titan, CH₄ is the second most abundant species but only traces were observed on Triton. Strobel et al. (1990a) suggested that this species was destructed by solar and interplanetary Lyman-Alpha radiation (as Triton is at 30 UA from the Sun, the interplanetary Ly-Alpha flux is comparable to the solar one). The atmosphere of Triton is also much less dense as the surface pressure is 14 bar against 1.5 bar on Titan. We use data from Strobel and Zhu (2017) for the initial temperature, pressure, density and eddy coefficient profiles. To obtain a first validation of our model, we compare our results with the Voyager 2 data and with the results presented in the principal articles about the photochemistry of Triton published after the Voyager 2 flyby: Strobel and Summers (1995), Krasnopolsky and Cruikshank (1995). Our model takes the interplanetary Ly-Alpha flux into account as well as the energy input of precipitating electrons from the neptunian magnetosphere by adding reactions of electron impact ionization and dissociation for N₂. The ionization profile was taken from Strobel et al. (1990b). We modified the chemical network to adapt it to Triton’s atmospheric composition in order to get a nominal chemical scheme.  The eddy coefficient profile is constrained to match our CH₄ profile to the one measured by Voyager near the surface, as it was done in Strobel et al. (1990a) and Krasnopolsky and Cruikshank (1995).

Our model takes actually into account 204 species (117 neutrals and 87 ions). Our chemical scheme is composed of 1570 reactions (154 photodissociations, 597 neutral reactions, 31 photoionizations and 788 ionic reactions). We use an altitudinal grid varying in H/5, H being the height scale of the atmosphere, giving 96 levels from 0 to 1026km. 

First results

Our first results confirm that precipitation of magnetospheric electrons is very important to explain the composition of the ionosphere. The solar flux is also a critical parameter of the model since the CH₄ abundance profile near the surface depends on the solar activity (the Voyager 2 flyby occurred near a solar maximum). This abundance profile also depends on the eddy diffusion coefficient. We also observed that the results depend strongly on some reactions. A study of the model’s uncertainties seems mandatory and will allow us to identify these key reactions. Uncertainties may indeed be large due to the low temperature of Triton’s atmosphere.