Impact of Clouds on the Atmosphere–Mantle Interface of Sub-Neptunes

Abstract Sub-Neptunes are among the most common type of close-in planets found in our Galaxy, yet their bulk composition remains largely uncertain; H-rich envelopes overlaying rocky cores, volatile-rich planets, and carbon-rich interiors all remain viable configurations for members of this population. Atmospheric characterization has been proposed as a means of distinguishing between these scenarios, but growing evidence suggests that sub-Neptunes may host molten atmosphere–mantle interfaces that could alter the composition of their atmosphere. We use the PICASO 1D climate model, coupled to interior-structure and magma–atmosphere chemistry frameworks to quantify how clouds alter the atmospheric and interior structure of sub-Neptunes. For temperate sub-Neptunes like TOI-270 d, we find that clouds can lead to ≥1000 K heating at depth (∼10 4 bar) and ∼600 K cooling at shallow pressures (∼1 bar). This heating is very sensitive to the cloud sedimentation efficiency and, to a lesser extent, to metallicity. Most sub-Neptunes in our sample should have a molten atmosphere–mantle interface, except TOI-1231 b and GJ 1214 b. For these two planets, cloudy models have a molten interface whereas clear models can allow for a solid boundary. Clouds can heat the atmosphere–mantle interfaces by a temperature difference between ∼1400 and 2600 K for most sub-Neptunes in our sample. Such cloud-driven heating can substantially change the composition of the interface with abundances of O 2 , SiH 4 , and SiO showing a ≥36% increase between cloudy and clear models of TOI-270 d. We discuss the implications of our results for the thermal evolution and measurements of intrinsic heat flux for this population.