Global Chemical Transport on Hot Jupiters: Insights from 2D VULCAN photochemical model

TL;DR

This paper introduces VULCAN 2D, a new 2D photochemical transport model for hot Jupiter atmospheres, revealing how horizontal and vertical mixing influence atmospheric composition and observational signatures.

Contribution

The paper presents VULCAN 2D, an advanced 2D photochemical model that accounts for non-uniform zonal winds, improving upon previous pseudo-2D approaches for hot Jupiter atmospheric studies.

Findings

Horizontal transport dominates below 0.1 mbar, vertical mixing above.

HD 209458 b shows limb asymmetry in C2H4 due to horizontal transport.

Horizontal transport significantly affects transmission spectra and limb observations.

Abstract

The atmospheric dynamics of tidally-locked hot Jupiters is characterized by strong equatorial winds. Understanding the interaction between global circulation and chemistry is crucial in atmospheric studies and interpreting observations. Two-dimensional (2D) photochemical transport models shed light on how the atmospheric composition depends on circulation. In this paper, we introduce the 2D photochemical (horizontal and vertical) transport model, VULCAN 2D, which improves on the pseudo-2D approaches by allowing for non-uniform zonal winds. We extensively validate our VULCAN 2D with analytical solutions and benchmark comparisons. Applications to HD 189733 b and HD 209458 b reveal a transition in mixing regimes: horizontal transport predominates below $\sim$0.1 mbar while vertical mixing is more important at higher altitudes above 0.1 mbar. Motivated by the previously inferred carbon-rich atmosphere, we find that HD 209458 b with super-solar carbon-to-oxygen ratio (C/O) exhibits pronounced C$_2$H$_4$ absorption on the morning limb but not on the evening limb, owing to horizontal transport from the nightside. We discuss when a pseudo-2D approach is a valid assumption and its inherent limitations. Finally, we demonstrate the effect of horizontal transport in transmission observations and its impact on the morning-evening limb asymmetry with synthetic spectra, highlighting the need to consider global transport when interpreting exoplanet atmospheres.