22 October: Namrah Habib

Speaker: Namrah Habib (Oxford University)

Date: Wednesday 22 October 2025

Time: 15:00

Location: Physics 3.21

Understanding Vertical Mixing in Hydrogen Rich Atmospheres: Implications for Sub-Neptunes and the Solar System Giant Planets

Sub-Neptune planets are among the most common exoplanets, but our understanding of their atmospheric structure and dynamics remains limited. Atmospheric convection behaves differently in hydrogen rich atmospheres compared to high mean molecular weight atmospheres, like Earth. In hydrogen-rich atmospheres, any condensable or atmospheric tracer is denser than the surrounding atmosphere, which can strongly supress vertical mixing, and impact tracer distribution and cloud formation within these atmospheres. Previous studies have suggested that when the abundance of a condensible exceeds a critical threshold (“Guillot” threshold), convective mixing can be entirely suppressed within these atmospheres. Additionally, recent modelling studies of hydrogen-rich atmospheres suggest that episodic storms and convective activity occur when the mean molecular weight is below 6 g/mol. Further, modelling studies of Jupiter, Uranus, and Neptune along with observations of Saturn have suggested that convective storms may occur periodically within their hydrogen-rich atmospheres. Episodic storms would significantly affect the cloudiness, distribution (and detectability) of condensibles, and the atmospheric temperature profile.

In this talk, I will present an overview of convection and vertical mixing in hydrogen rich atmospheres. I will first discuss the implications for heavier tracers in hydrogen rich atmospheres and then will present a 3D convection-resolving parameter study. We investigate the parameter space of episodic storms and vertical mixing in hydrogen-rich atmospheres to better understand the impact of episodic mixing on  clouds and tracer distribution. We use 3D convection-resolving simulations to examine a range of initial surface temperatures (250–800 K) and condensable abundances both above and below the Guillot critical threshold. Our findings aim to inform the atmospheric structure and cloudiness of sub-Neptune atmospheres and the solar system giant planets.