Titan’s Modeled Thermospheric Composition Sensitivity to Solar Cycle and Cross Section Resolution

Abstract:

Saturn’s largest moon, Titan has been in the spotlight of planetary research for seven decades now. This unique satellite is the only other planetary body besides the Earth to possess a substantial N₂ dominated atmosphere, and stable bodies of liquids on its surface. The atmosphere and the surface are strongly coupled, and the liquids on the surface are a consequence of chemistry in the atmosphere. Titan’s dense atmosphere is the site of rich organic chemistry, which begins with the photolytic destruction of the two most abundant atmospheric constituents: N₂ and CH₄. Photochemical modeling is an important tool in understanding the details of higher-order hydrocarbon and nitrile formation and loss at various altitudes in Titan’s atmosphere, which also affect the rate of deposition onto Titan’s surface. However, significant discrepancies exist among modeled (and measured) atmospheric densities of minor species, which warrant the evaluation of influencing factors in photochemical modeling. Here, we address the role of the choice of N₂ photoabsorption cross-section resolution and variations in solar flux with solar cycle on the vertical profiles of various minor species in Titan’s thermosphere. Special attention is placed on the changes in production and loss rates of C₂H₆ and HCN in response to the varying model parameters. C₂H₆ is the most important sink for CH₄, and is a major component of the surface hydrocarbon lakes. HCN production is the first step in the incorporation of nitrogen into tholins, and is relevant to potential amino acid formation. Furthermore, HCN is the main coolant in Titan’s upper atmosphere. Thus, these two minor species are particularly important in the grand scheme of Titan’s coupled surface-atmosphere system.