Thermodynamic Constraints on the Lower Atmosphere of Venus

Abstract

The lower Venusian atmosphere is the region from the surface to the cloud deck or about 0–50 km. Early modeling studies of the atmosphere were primarily based on thermodynamics; more recent modeling studies are based on kinetics of the elementary reactions. In this paper, we take the accepted nominal composition of near-surface gases at ∼42 km and show some of the constituents are indeed at thermodynamic equilibrium. We impose a small oxygen gradient and use a thermodynamic free energy minimization code to describe the vertical gradients of mixing ratios for the primary gases in the lower atmosphere. The oxygen gradient is within the measurement errors on oxygen and thus maintains mass conservation. Reasonable agreement is found between our calculations and the vertical profiles of H2O, H2SO4, OCS, H2S, and S n (n = 1–8). We then did a kinetic analysis of kinetic expressions for the formation of these species. Consistent with other investigators, we find that very few if any reactions should be at equilibrium in the lower atmosphere. Yet our equilibrium calculations do show some agreement with observations. We conclude that the available kinetic expressions likely need improvement and factors such as catalysis must be included to reflect actual Venus conditions.

Global thermodynamic, transport-property and dynamic characteristics of the Venus lower atmosphere below the cloud layer

“…rain). This information is only valid for 50 H km (Jacobson et al, 2017); at 50 H km, where sulfuric acid has been detected, leading to a sulfuric acid haze in the layer immediately below, these results no longer hold. (Table 4) at constant composition, and its projections onto the ( , ) pT and ( , ) pv planes.…”

“…The information provided in Jacobson et al, 2017 summarizes data from several Venus missions and indicates that in the lower atmosphere, the most preponderant chemical species are those listed in Table 1, where their molar fraction, X  (  labels a chemical species), thermodynamic critical pressure, c p , and thermodynamic critical temperature, c T , and the acentric factor,   , are provided. At the Venus surface, defined by the altitude =0 H m, the nominal pressure is = p 92 bar and the nominal temperature is = T 735 K (Blumenthal et al, 2012).…”

“…The lack of knowledge regarding the mole fraction of all noble gases combined with their lack of reactivity induced us to relegate their consideration to a future study. Table 1: 7-species model for the Venus lower atmosphere (Jacobson et al, 2017). The critical pressure and temperature conditions for the species are listed, as well as their acentric factor (Poling et al, 2001).…”

“…In the present context, to inquire about the sensitivity of the results to the assumed composition (e.g. influence of measurement errors (Jacobson et al, 2017)), the stability analysis is conducted for a variety of mixtures. The mixtures used in computations are selected according to data collected from several Venus observations and missions (Jacobson et al, 2017), and are listed in Table 4.…”

“…Indeed, and despite the information obtained from several missions conducted to explore the planet, several aspects of its atmosphere remain poorly understood (e.g. Johnson and de Oliveira, 2019, Jacobson et al, 2017, Esposito et al, 2013. For example, the origin of the variety of the chemical species detected close to the Venus surfacewithin 50 km of altitudeis still unknown as these chemical species could emanate from the Venus solid surface, be the product of local chemical reactions or be transported from upper layers of the atmosphere, particularly from the cloud tops (e.g.…”