Horizontal transport as a source of disequilibrium chemistry on the nightside of a hot exoplanet

TL;DR

This study provides observational evidence that horizontal atmospheric transport causes disequilibrium chemistry on the nightside of a hot exoplanet, affecting chemical species distribution beyond equilibrium predictions.

Contribution

First direct observational evidence of horizontal transport-induced disequilibrium chemistry on an exoplanet's nightside using JWST/NIRSpec data.

Findings

H2O and CO have similar abundances on both sides of the planet.

Nightside is depleted in CH4 compared to equilibrium models.

Horizontal chemical quenching is responsible for observed disequilibrium.

Abstract

Hot Jupiters have temperature gradients of several hundreds of degrees between their permanent day and nightsides. In equilibrium, the primary carbon reservoir is expected to transition from CO on the dayside to CH4 on the nightside. Theory predicts that the atmospheric circulation, characterised by km/s winds, can advect chemical species from the dayside to the nightside faster than the time needed for the CO-to-CH4 chemical reaction to reach equilibrium. However direct evidence of this process has, so far, remained elusive, partly because it is often degenerate with other processes, such as vertical mixing or non-stellar elemental abundances. Here, we present observational evidence for such day-to-night transport of chemical species by observing both the dayside and the nightside of the hot Jupiter NGTS-10A b with the JWST/NIRSpec instrument. We constrain the presence of H2O and CO with similar abundances on both the dayside and nightside. Our observations are compatible with a solar-composition atmosphere at chemical equilibrium on the dayside, but indicative of disequilibrium chemistry for the nightside as it is significantly depleted in CH4 compared to equilibrium chemistry predictions. We further show that the lack of CH4 on the planet's nightside cannot be attributed to non-solar elemental abundances or to vertical mixing mechanisms and must therefore be due to horizontal chemical quenching. Our study shows the fundamental role atmospheric transport plays in shaping the distribution of chemical species on exoplanet atmospheres.