Atmospheres as windows into sub-Neptune interiors: coupled chemistry and structure of hydrogen-silane-water envelopes

Abstract

Sub-Neptune exoplanets are commonly hypothesized to consist of a silicate-rich magma ocean topped by a hydrogen-rich atmosphere. Previous work studying the outgassing of silicate material has demonstrated that such atmosphere-interior interactions can affect the atmosphere's overall structure and extent. But these models only considered SiO in an atmosphere of hydrogen gas, without considering chemical reactions between them. Here we couple calculations of the chemical equilibrium between H, Si, and O species with an atmospheric structure model. We find that substantial amounts of silane, SiH${}_4$, and water, H${}_2$O, are produced by the interaction between the silicate-rich interior and hydrogen-rich atmosphere. These species extend high into the atmosphere, though their abundance is greatest at the hottest, deepest regions. For example, for a 4 $M_{\oplus }$ planet with an equilibrium temperature of 1000 K, a base temperature of 5000 K, and a 0.1 $M_{\oplus }$ hydrogen envelope, silicon species and water can comprise 30 percent of the atmosphere by number at the bottom of the atmosphere. Due to this abundance enhancement, we find that convection is inhibited at temperatures $\gtrsim 2500$ K. This temperature is lower, implying that the resultant non-convective region is thicker, than was found in previous models which did not account for atmospheric chemistry. Our findings show that significant endogenous water is produced by magma-hydrogen interactions alone, without the need to accrete ice-rich material. We discuss the observability of the signatures of atmosphere-interior interaction and directions for future work, including condensate lofting and more complex chemical networks.

13 pages, 12 figures, accepted for publication in MNRAS

References in corpus (22)

• Array Programming with NumPy
• Growth Model Interpretation of Planet Size Distribution
• Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core
• Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H
• A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
• A generalized bayesian inference method for constraining the interiors of super Earths and sub-Neptunes
• Identification of carbon dioxide in an exoplanet atmosphere
• Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• Beyond Equilibrium Temperature: How the Atmosphere/Interior Connection Affects the Onset of Methane, Ammonia, and Clouds in Warm Transiting Giant Planets
• Earth shaped by primordial H${}_2$ atmospheres
• To cool is to keep: Residual H/He atmospheres of super-Earths and sub-Neptunes
• The runaway greenhouse on subNeptune waterworlds
• The importance of silicate vapor in determining the structure, radii, and envelope mass fractions of sub-Neptunes
• ExoMol line lists -- XXII. The rotation-vibration spectrum of silane up to 1200 K
• Properties of hydrogen, helium, and silicon dioxide mixtures in giant planet interiors
• A Universal Cloud Composition on the Nightsides of Hot Jupiters
• Convective inhibition with an atmosphere, I: super-critical cores on sub-Neptune/super-Earths
• Helium Enhanced Planets Along the Upper Edge of the Radius Valley
• Observability of silicates in volatile atmospheres of super-Earths and sub-Neptunes

Cited by in corpus (20)

• The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets
• Role of magma oceans in controlling carbon and oxygen of sub-Neptune atmospheres
• Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited
• Planetary evolution with atmospheric photoevaporation II: Fitting the slope of the radius valley by combining boil-off and XUV-driven escape
• Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds
• Magma ocean interactions can explain JWST observations of the sub-Neptune TOI-270 d
• Constraining exoplanet interiors using observations of their atmospheres
• How planets form by pebble accretion V. Silicate rainout delays contraction of sub-Neptunes
• Building Wet Planets through High-Pressure Magma-Hydrogen Reactions
• Diversity of low-mass planet atmospheres in the C-H-O-N-S-Cl system with interior dissolution, nonideality, and condensation: Application to TRAPPIST-1e and sub-Neptunes
• Redefining interiors and envelopes: hydrogen-silicate miscibility and its consequences for the structure and evolution of sub-Neptunes
• Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d
• Blowin' in the non-isothermal wind: core-powered mass loss with hydrodynamic radiative transfer
• Carbon-rich Sub-Neptune Interiors Are Compatible with JWST Observations
• Not All Sub-Neptune Exoplanets Have Magma Oceans
• The Effects of Non-ideal Mixing in Planetary Magma Oceans and Atmospheres
• Characterizing the bolometric-photoevaporative transition in young sub-Neptunes with radiation-hydrodynamic simulations
• Sibling Sub-Neptunes Around Sibling M Dwarfs: TOI-521 and TOI-912
• Using observations of escaping H/He to constrain the atmospheric composition of sub-Neptunes
• Impact of Clouds on the Atmosphere-Mantle Interface of Sub-Neptunes