Rock vapour is opaque: implications for dynamics and observations of lava planets

TL;DR

Lava planets host rock-vapour atmospheres whose spectra encode mantle and interior processes. The authors present SonicVapour, a multi-species, steady-state hydrodynamics and radiative-transfer framework that integrates LavAtmos magma-outgassing chemistry with ExoMol opacity data to simulate K2-141b and TOI-431b. A key finding is that thick, optically opaque atmospheres yield near-blackbody top-of-atmosphere emission with muted spectral features, while thinner atmospheres with intermediate opacity produce detectable atmospheric signatures, making cooler lava planets potentially easier to characterize. The work highlights observation strategies that favor moderately cool lava planets and provides an open-source tool to enable future interior-atmosphere coupling studies.

Abstract

Extreme instellation on lava planets causes the rocky surface to melt and vaporize. Because the rock vapour composition is intrinsically tied to the mantle, atmospheric characterization of lava planets can hold valuable insight into the interior processes of rocky planets. To help interpret current data and strategize for future observations, we develop the model SonicVapour to simulate the dynamics of chemically complex secondary atmosphere of lava planets. We find that for planets with surface temperatures exceeding 2700 K, the rock vapour outgassed is optically thick, making the atmosphere vertically isothermal thus suppressing convection and severely limiting atmospheric detection via emission spectroscopy. In contrast, cooler planets with surfaces between 2300 K - 2700 K have an atmospheric opacity close to 50% and produce distinct spectral features. Counter-intuitively, therefore, cooler lava planet atmospheres are easier to detect. Our results ultimately emphasize the importance of considering atmospheric "detectability" in tandem with signal-to-noise for future observation programs.

Figures (6)

• Figure 1: Saturated vapour pressure of nine most abundant species outgassed from Bulk Silicate Earth magma as predicted by LavAtmosvan2023lavatmos. The two most dominant species are Na which dominates at low temperatures, and SiO which dominates at high temperatures (> 2800 K).
• Figure 2: Absorption cross-section per molecule extracted from the ExoMol database. For this plot, we set the temperature to 2800 K and pressure to 7.7 kPa as thermal and pressure broadening parameters. Note that MgO has very broad peaks spanning the visible and IR and will play a large role in the atmosphere's radiative balance.
• Figure 3: Atmospheric pressure (top), surface and atmospheric temperature (middle), and wind speeds (bottom) of K2-141b and TOI-431b at an altitude of half a scale-height. Because the surface temperature of K2-141b is hundreds of Kelvin hotter, the atmosphere of K2-141b is an order of magnitude thicker than that of TOI-431b. For K2-141b, the atmosphere shares the same temperature as the surface for much of the dayside due to the atmosphere's optical thickness. TOI-431b has an optically thinner atmosphere so there is an appreciable difference in temperature between the atmosphere and the surface. Surprisingly, both planets have similar wind speeds despite having wildly different atmospheric conditions.
• Figure 4: Total opacity (top) and synthetic eclipse spectra of K2-141b and TOI-431b (bottom). Total opacity is calculated as the fraction of stellar absorption and total bolometric flux. Because most of K2-141b's opacity is roughly unity, the atmosphere radiates like a perfect blackbody resulting in severely muted spectral features. TOI-431b, on the other hand, has significantly lower opacity for its entire atmosphere making atmospheric signals more distinguishable in its eclipse spectrum.
• Figure 5: Opacity contribution of individual gas species on K2-141b (top) and TOI-431b (bottom). For thin atmospheres like TOI-431b with substellar surface pressures of $\sim10^3$ Pa, Na and K are the main contributor of stellar flux absorption. Given a thick enough atmosphere like K2-141b's, there is enough MgO ($\sim 10^2$ Pa) to absorb all incoming stellar radiation.