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

Abstract

Triton is the largest satellite of Neptune and probably a Kuiper Belt Object that was captured by the planet. It has a tenuous nitrogen atmosphere similar to the one of Pluto and may be an ocean world. The Neptunian system has only been visited by Voyager 2 in 1989. Over the last few years, the demand for a new mission to the Ice Giants and their systems has increased so that a theoretical basis to prepare for such a mission is important. We aim to develop a photochemical model of Triton's atmosphere with an up-to-date chemical scheme, as previous photochemical models date back to the post-flyby years. This is done to better understand the mechanisms governing Triton's atmospheric chemistry and highlight the critical parameters having a significant impact on the atmospheric composition. We also study model uncertainties to find what chemical studies are necessary to improve the modeling of Triton's atmosphere. We adapted a model of Titan's atmosphere to Triton's conditions. We first used Titan's chemical scheme before updating it to better model Triton's atmosphere. Once the nominal results were obtained, we studied model uncertainties with a Monte-Carlo procedure. Then, we performed global sensitivity analyzes to identify the reactions responsible for model uncertainties. With the nominal results, we determined the composition of Triton's atmosphere and studied the main chemical processes. We highlighted key chemical reactions that are the most important for the overall chemistry. We also identified some key parameters having a significant impact on the results. Uncertainties are large for most of the main atmospheric species as the atmospheric temperature is very low. We identified key uncertainty reactions that have the largest impact on the results uncertainties. These reactions must be studied in priority in order to improve the significance of our results.