Effects of Bulk Composition on The Atmospheric Dynamics on Close-in Exoplanets

TL;DR

This study uses a 3D general circulation model to analyze how atmospheric composition, especially molecular weight, influences temperature, wind patterns, and jet structures on tidally locked sub-Jupiter exoplanets, providing theoretical explanations.

Contribution

It systematically investigates the impact of bulk atmospheric composition on atmospheric dynamics, highlighting the dominant role of molecular weight and offering analytical theories for observed trends.

Findings

Higher molecular weight increases day-night temperature contrast.

Larger molecular weight results in narrower equatorial jets.

Molar heat capacity effects are small unless the atmosphere is near adiabatic.

Abstract

Super Earths and mini Neptunes likely have a wide range of atmospheric compositions, ranging from low-molecular mass atmospheres of H2 to higher molecular atmospheres of water, CO2, N2, or other species. Here, we systematically investigate the effects of atmospheric bulk compositions on temperature and wind distributions for tidally locked sub-Jupiter-sized planets, using an idealized 3D general circulation model (GCM). The bulk composition effects are characterized in the framework of two independent variables: molecular weight and molar heat capacity. The effect of molecular weight dominates. As the molecular weight increases, the atmosphere tends to have a larger day-night temperature contrast, a smaller eastward phase shift in the thermal phase curve and a smaller zonal wind speed. The width of the equatorial super-rotating jet also becomes narrower and the "jet core" region, where the zonal-mean jet speed maximizes, moves to a greater pressure level. The zonal-mean zonal wind is more prone to exhibit a latitudinally alternating pattern in a higher-molecular-weight atmosphere. We also present analytical theories that quantitatively explain the above trends and shed light on the underlying dynamical mechanisms. Those trends might be used to indirectly determine the atmospheric compositions on tidally locked sub-Jupiter-sized planets. The effects of the molar heat capacity are generally small. But if the vertical temperature profile is close to adiabatic, molar heat capacity will play a significant role in controlling the transition from a divergent flow in the upper atmosphere to a jet-dominated flow in the lower atmosphere.