Biochemistry & Organic Chemistry

A 3D Picture Of Moist-convection Inhibition In Hydrogen-rich Atmospheres: Implications For K2-18 b

By Keith Cowing
Status Report
January 16, 2024
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A 3D Picture Of Moist-convection Inhibition In Hydrogen-rich Atmospheres: Implications For K2-18 b
Atmospheric structure of the baseline simulation. From left to right: Temperature, Water vapor specific concentration, vertical velocity (in m/s), and relative humidity. The two first panels show horizontal and temporal averages. The black, dash-dotted line in the first panel show the standard moist-adiabat profile for the same conditions. In the second panel, the black dotted and dashed lines show the value of the critical inhibition vapor concentration (Eq. (1)) and the saturation concentration respectively. The two last panels show snapshots along vertical slices that go through a moist convective plume. From bottom to top, the atmosphere exhibits a dry troposphere, a stable layer where vertical motions are strongly suppressed, a moist troposphere, and a stratosphere. Horizontal dashed lines are plotted at the boundaries between these zones to allow an easier comparison of the altitudes of the various features. The rising moist plume (with maximal velocities of around 8 m/s) is mirrored by a descending cold plume in the dry region (-15 m/s) caused by the reevaporation of rains at the bottom of the stable layer. — astro-ph.EP

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.

Jérémy Leconte, Aymeric Spiga, Noé Clément, Sandrine Guerlet, Franck Selsis, Gwenaël Milcareck, Thibault Cavalié, Raphaël Moreno, Emmanuel Lellouch, Óscar Carrión-González, Benjamin Charnay, Maxence Lefèvre

Comments: Accepted for publication in Astronomy and Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Fluid Dynamics (physics.flu-dyn)
Cite as: arXiv:2401.06608 [astro-ph.EP] (or arXiv:2401.06608v1 [astro-ph.EP] for this version)
Submission history
From: Jeremy Leconte
[v1] Fri, 12 Jan 2024 14:54:55 UTC (662 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻