A photochemical model of Triton's atmosphere with an uncertainty propagation study

Introduction

Triton is the biggest satellite of Neptune. It was only visited by Voyager 2 in 1989. During this mission, the surface temperature was found to be only 38 K and the surface pressure 16 bar. It has a tenuous nitrogen atmosphere similar to the one of Pluto. This atmosphere was studied through stellar occultations and airglow observations, revealing traces of CH₄ near the surface and also the presence of atomic nitrogen and hydrogen (Broadfoot et al. 1989). Radio observations pointed out the presence of a surprisingly dense ionosphere (Tyler and al. 1989).  Strobel et al. (1990), Stevens et al. (1992) and Krasnopolsky and Cruikshank (1995) showed the consideration of electronic precipitation from Neptune’s magnetosphere was critical to explain the observed electronic number densities. At this distance from the Sun, the interplanetary radiation flux is also not negligible, particularly at Lyman- where it is comparable to the solar one (Broadfoot et al. 1989). Photochemical models of Triton’s atmosphere are few and were published following the Voyager flyby (Krasnopolsky et al. 1993, Krasnopolsky and Cruikshank 1995, Strobel and Summers 1995). Thus, we have developed a new photochemical model of this atmosphere with an up-to-date chemical scheme in order to prepare a potential mission to the Neptunian system.

The photochemical model and methodology

As the atmosphere of Triton is mostly N₂ with traces of CH₄, it recalls the one of Titan. Capitalizing on these similarities, we used a photochemical model of Titan’s atmosphere (Dobrijevic et al. 2016) with the chemical scheme of Hickson et al. (2020) and adapted it to Triton. To do this, we changed the critical input parameters, using data from Strobel and Zhu (2017), and updated the chemical scheme. This led us to add new atmospheric species and consider new chemical reactions. We also added the interplanetary flux and the precipitation of magnetospheric electrons. But as Triton’s atmospheric conditions are extreme, we expected large uncertainties on our results. Thus, we first computed the nominal composition of the atmosphere and then took into account the uncertainties on chemical reaction rates by using a Monte-Carlo procedure. These results were then treated through a sensitivity analysis to see how these uncertainties propagate in the model. We also added a water flux at the top of the atmosphere and used an electron transport code to better model the interaction between Triton and Neptune’s magnetosphere along its orbit.

Results

With the nominal results, we identify critical parameters having a significant influence on the results, such as the eddy diffusion coefficient, magnetospheric electrons or the solar flux. In addition, we highlight the main production and loss processes for the main atmospheric species. The two dominant processes are N₂ ionization and dissociation by solar radiation and magnetospheric electrons, which influence the overall chemistry, and methane photolysis, that governs the chemistry in the lower atmosphere where the absorption of the Lyman- radiation is maximum. Nitrogen chemistry leads to the production of atomic nitrogen, N₂⁺ and N⁺ that appear in several important reactions while methane photolysis is a source of H, H₂, radicals and hydrocarbons. Due to the low temperature near the surface, these hydrocarbons condense and form hazes that were observed by Voyager.

The results of the Monte-Carlo procedure show that we have indeed large uncertainties for most of the main atmospheric species. We also observe epistemic bimodalities in the abundance distribution of some species. These uncertainties rise from the lack of knowledge about reaction rates at temperatures typical of Triton’s atmosphere, which leads to large uncertainty factors on reaction rates. With the sensitivity analysis, we identify key reactions that contribute the most to the model’s uncertainties. These reactions need to be studied in priority in order to decrease the uncertainties on the results and remove any epistemic bimodalities, thus improving the significance of photochemical results.

 

References

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[2] Tyler, G. L., D. N. Sweetnam, J. D. Anderson, S. E. Borutzki, J. K. Campbell, et al. “Voyager Radio Science Observations of Neptune and Triton.” Science 246, no. 4936 (December 15, 1989): 1466–73. https://doi.org/10.1126/science.246.4936.1466.

[3] Strobel, Darrell F., Andrew F. Cheng, Michael E. Summers, and Douglas J. Strickland. “Magnetospheric Interaction with Triton’s Ionosphere.” Geophysical Research Letters 17, no. 10 (1990): 1661–64. https://doi.org/10.1029/GL017i010p01661.

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[5] Krasnopolsky, Vladimir A., and Dale P. Cruikshank. “Photochemistry of Triton’s Atmosphere and Ionosphere.” Journal of Geophysical Research 100, no. E10 (1995): 21271. https://doi.org/10.1029/95JE01904.

[6] Krasnopolsky, V. A., B. R. Sandel, F. Herbert, and R. J. Vervack. “Temperature, N2, and N Density Profiles of Triton’s Atmosphere - Observations and Model.” Journal of Geophysical Research 98 (February 1, 1993): 3065–78. https://doi.org/10.1029/92JE02680.