Grid of pseudo-2D chemistry models for tidally locked exoplanets -- II. The role of photochemistry

TL;DR

This study explores how photochemistry influences the atmospheric composition of tidally locked exoplanets across various temperatures, revealing significant effects deep in the atmosphere and identifying optimal conditions for haze precursor production.

Contribution

It presents a comprehensive grid of 2D photochemical models for exoplanets, highlighting the impact of photochemistry on atmospheric composition and transport processes.

Findings

Photochemistry significantly alters atmospheric composition up to several bar in cool exoplanets.

Optimal hydrogen cyanide and acetylene production occurs between 800 and 1400 K.

Photochemical products are transported to the night side by horizontal advection.

Abstract

Photochemistry is expected to change the chemical composition of the upper atmospheres of irradiated exoplanets through the dissociation of species, such as methane and ammonia, and the association of others, such as hydrogen cyanide. Although primarily the high altitude day side should be affected by photochemistry, it is still unclear how dynamical processes transport photochemical species throughout the atmosphere, and how these chemical disequilibrium effects scale with different parameters. In this work we investigate the influence of photochemistry in a two-dimensional context, by synthesizing a grid of photochemical models across a large range of temperatures. We find that photochemistry can strongly change the atmospheric composition, even up to depths of several bar in cool exoplanets. We further identify a sweet spot for the photochemical production of hydrogen cyanide and acetylene, two important haze precursors, between effective temperatures of 800 and 1400 K. The night sides of most cool planets (effective temperature < 1800 K) are shown to host photochemistry products, transported from the day side by horizontal advection. Synthetic transmission spectra are only marginally affected by photochemistry, but we suggest that observational studies probing higher altitudes, such as high-resolution spectroscopy, take photochemistry into account.