Irradiated Atmospheres IV: Effect of Mixing Heat Flux on Chemistry

TL;DR

This work investigates how vertical mixing-induced heat flux alters exoplanetary atmospheric chemistry by integrating mixing heat flux into radiative transfer via radiative-mixing equilibrium and solving the 1D disequilibrium chemistry with the VULCAN code. The temperature structure is computed with a semi-grey, two-band opacity model and the mixing flux $F_{\rm eddy}$, which depends on the eddy-diffusion coefficient $K_{\rm zz}$ and the temperature gradient $\nabla$, is coupled into the radiative equilibrium equation $\int_0^\infty \kappa_\nu (J_\nu - B_\nu)\,d\nu + \frac{g}{4\pi}\frac{dF_{\rm eddy}}{dP}=0$. The results show that heating in the lower atmosphere shifts thermochemical equilibria, generally lowering $X_{\rm CH_4}$, $X_{\rm H_2O}$, and $X_{\rm NH_3}$ while raising $X_{\rm CO}$, $X_{\rm CO_2}$, and $X_{\rm NO}$ in the lower atmosphere; upper-atmosphere abundances are set by the location of quenching levels relative to the heating region and by transport through the chemical network. Irradiation temperature $T_{\rm irr}$ modulates the sensitivity of these effects by altering chemical timescales, with cooler planets more susceptible to mixing-induced deviations. Overall, the study demonstrates that incorporating mixing heat flux is essential for accurate atmospheric chemistry modeling and retrievals of exoplanet spectra.

Abstract

Vertical mixing disrupts the thermochemical equilibrium and introduces additional heat flux that alters exoplanetary atmospheric temperatures. We investigate how this mixing-induced heat flux affects atmospheric chemistry. Temperature increase in the lower atmosphere by the mixing-induced heat flux alters species abundances there and modifies those in the upper atmosphere through vertical transport. In the lower atmosphere, most species follow thermodynamic equilibrium with temperature changes. In the upper layers, species mixing ratios depend on the positions of quenching levels relative to the regions exhibiting significant mixing-induced temperature variations. When the quenching level resides within such region (e.g. CO, $\rm CH_4$, and $\rm H_2O$ with strong mixing), the mixing ratios in the upper atmosphere are modified due to changes in the quenched ratios affected by the temperature variation in the lower atmosphere. This alters the mixing ratio of other species (e.g. NO and $\rm CO_2$) through the chemical reaction network, whose quenching occurs in the region without much temperature change. The mixing ratios of $\rm CH_4$, $\rm H_2O$, and $\rm NH_3$ decrease in the lower atmosphere with increasing mixing heat flux, similarly reducing these ratios in the upper atmosphere. Conversely, the mixing ratios of CO, $\rm CO_2$, and NO rise in the lower atmosphere, with CO and $\rm CO_2$ also increasing in the upper levels, although NO decreases. Weaker host star irradiation lowers the overall temperature of the planet, allowing a smaller mixing to have a similar effect. We conclude that understanding the vertical mixing heat flux is essential for accurate atmospheric chemistry modeling and retrieval.

Figures (3)

• Figure 1: The temperature profiles for different values of $K_{\rm zz}$ under the fiducial model outlined in § \ref{['RME']}.
• Figure 2: The mixing ratios of $\rm CH_4$, $\rm H_2O$, $\rm CO$, $\rm NH_3$, $\rm NO$ and $\rm CO_2$ across varying intensities of vertical mixing. The dotted line (Case 0) corresponds to the case where there is no vertical mixing in the atmosphere. The solid lines (Case K) represent the scenarios where only the effect of material transport due to vertical mixing is considered, without accounting for the mixing heat flux. The dashed lines (Case KT) consider both the material mixing and the heat flux brought about by vertical mixing. The different colors correspond to different values of $K_{\rm zz}$, with the temperature profile displayed in Figure \ref{['figT0.005']}.
• Figure 3: The first row shows the mixing ratios of CO at 1 mbar across varying intensities of vertical mixing, and the second row presents those at 10 bar. The solid lines (Case K) represent the scenarios where only the effect of material transport due to vertical mixing is considered, without accounting for the mixing heat flux. The dashed lines (Case KT) consider both the material mixing and the heat flux brought about by vertical mixing. The different colors correspond to different values of irradiation temperature $T_{\rm irr}$.