A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b (2401.06608v1)

Abstract: While small, Neptune-like planets are among the most abundant exoplanets, our understanding of their atmospheric structure and dynamics remains sparse. In particular, many unknowns remain on the way moist convection works in these atmospheres where condensable species are heavier than the non-condensable background gas. While it has been predicted that moist convection could shut-down above some threshold abundance of these condensable species, this prediction is based on simple linear analysis and relies on strong assumptions on the saturation of the atmosphere. To investigate this issue, we develop a 3D cloud resolving model for H2 atmospheres with large amounts of condensable species and apply this model to a prototypical temperate Neptune-like planet -- K2-18b. Our model confirms the shut-down of moist convection and the onset of a stably stratified layer in the atmosphere, leading to much hotter deep atmospheres and interiors. Our 3D simulations further provide quantitative estimates of the turbulent mixing in this stable layer, which is a key driver of the cycling of condensables in the atmosphere. This allows us to build a very simple, yet realistic 1D model that captures the most salient features of the structure of Neptune-like atmospheres. Our qualitative findings on the behavior of moist convection in hydrogen atmospheres go beyond temperate planets and should also apply to the regions where iron and silicates condense in the deep interior of H2-dominated planets. Finally, we use our model to investigate the likelihood of a liquid ocean beneath a H2 dominated atmosphere on K2-18b. We find that the planet would need to have a very high albedo (>0.5-0.6) to sustain a liquid ocean. However, due to the spectral type of the star, the amount of aerosol scattering that would be needed to provide such a high albedo is inconsistent with the latest observational data.

• The paper confirms that moist convection is inhibited in hydrogen-rich atmospheres when condensable species exceed a critical threshold, leading to stable thermal stratification.
• It employs advanced 3D cloud-resolving simulations to demonstrate the formation of a hot, stable layer between dry and moist convective regions, altering the deep atmospheric temperature.
• The study quantifies latent heat fluxes and turbulent mixing, providing key observational markers to distinguish atmospheric regimes on Neptune-like exoplanets.

Atmospheric Dynamics and Structure of Temperate Neptune-Like Exoplanets

The paper "A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b" by Leconte et al. explores the atmospheric dynamics of temperate Neptune-like exoplanets, with a particular focus on the planet K2-18b. The paper employs a three-dimensional (3D) cloud-resolving model to investigate the inhibition of moist convection in atmospheres dominated by hydrogen, where condensable species like water vapor are heavier than the surrounding gas. This research addresses the complex thermodynamic and dynamic processes involved in these atmospheres, providing substantial insights into their thermal structure and implications for habitability.

Key Findings

1. Inhibition of Moist Convection: The paper confirms the hypothesis that moist convection in hydrogen-rich atmospheres can be inhibited when the abundance of condensable species surpasses a critical threshold. This results in the formation of a stable, stratified layer, which significantly alters the thermal and chemical profile of the atmosphere.
2. Atmospheric Structure: Through 3D simulations, the paper demonstrates that a stable layer forms between a lower dry convective region and an upper moist convective region in the atmosphere of K2-18b. The temperature in the stable layer is substantially higher than previously assumed, which results in a hotter deep atmosphere and interior.
3. Energy and Mixing: The 3D simulations provide new insights into the turbulent mixing within the stable layer, which is crucial for the cycling of condensable species within the atmosphere. The work quantifies energy fluxes, showing that latent heat transported by water vapor plays a significant role in the atmospheric energy budget.
4. Model Development: A simplified 1D model is developed to encapsulate the critical features identified by the 3D simulations. This model allows for broader exploration of parameter space, providing a tool for predicting atmospheric and thermal structures under varying conditions.
5. Observational Implications: The presence of a stable stratified layer impacts the observable characteristics of such exoplanets. The differences in chemical abundances, particularly of carbon-bearing species, can serve as potential observational markers to differentiate atmospheric regimes and verify the existence of convection inhibition.

Implications for Exoplanetary Studies

The research has profound implications for the understanding of exoplanetary atmospheres, especially those around Neptune-like planets. By refining the theoretical models and providing a methodology for predicting atmospheric behaviors under different conditions, the paper offers a framework for interpreting observational data from current and future telescopes. The work suggests that atmospheric convection dynamics can influence the potential habitability and climate stability of these worlds by affecting surface and atmospheric temperatures.

Future Directions

Future research may focus on exploring how these findings apply to other exoplanets in similar conditions, particularly those where different heavy condensable species might play a role. Additionally, further integration of observational data with models like those developed in this paper can enhance predictive capabilities and aid in the search for extraterrestrial biosignatures.

Leconte et al.'s work is instrumental in constructing a more nuanced understanding of atmospheric processes on Neptune-like exoplanets. By integrating comprehensive 3D modelling approaches with plausible physical and chemical feedbacks, the paper charts a path forward for both the theoretical and observational exploration of these enigmatic worlds.