Effects of Planetary Parameters on Disequilibrium Chemistry in Irradiated Planetary Atmospheres: From Gas Giants to Sub-Neptunes

Abstract

A primary goal of characterizing exoplanet atmospheres is to constrain planetary bulk properties, such as their metallicity, C/O ratio, and intrinsic heat. However, there are significant uncertainties in many aspects of atmospheric physics, such as the strength of vertical mixing. Here we use PICASO and the photochem model to explore how atmospheric chemistry is influenced by planetary properties like metallicity, C/O ratio, $T_{\rm int}$, $T_{\rm eq}$, and $K_{\rm zz}$ in hydrogen-dominated atmospheres. We vary the $T_{\rm eq}$ of the planets between 400 K- 1600 K, across ``cold",``warm," and `hot" objects. We also explore an extensive range of $T_{\rm int}$ values between 30-500 K, representing sub-Neptunes to massive gas giants. We find that gases like CO and CO$_2$ show a drastically different dependence on $K_{\rm zz}$ and C/O for planets with cold interiors (e.g., sub-Neptunes) compared to planets with hotter interiors (e.g., Jupiter mass planets), for the same $T_{\rm eq}$. We also find that gases like CS and CS$_2$ can carry a significant portion of the S- inventory in the upper atmosphere near $T_{\rm eq}$ $\le$ 600 K, below which SO$_2$ ceases to be abundant. For solar C/O, we show that the CO/CH$_4$ ratio in the upper atmospheres of planets can become $\le$1 for planets with low $T_{\rm eq}$, but only if their interiors are cold ($T_{\rm int}$$\le$100 K). We find that photochemical haze precursor molecules in the upper atmosphere show very complex dependence on C/O, $K_{\rm zz}$, $T_{\rm eq}$, and $T_{\rm int}$ for planets with cold interiors (e.g., sub-Neptunes). We also briefly explore fully coupling PICASO and photochem to generate self-consistent radiative-convective-photochemical-equilibrium models.