The paper investigates the possibility of a magma ocean on the mini-Neptune K2-18b, a planet with a H₂-dominated atmosphere and a low atmospheric NH₃ abundance. The authors propose that the depletion of NH₃ and the presence of CO₂ in K2-18b's atmosphere can be explained by the high solubility of nitrogen in magma under reducing conditions, where a thick hydrogen envelope is in contact with a molten planetary surface. They demonstrate that this model reproduces the JWST observations of K2-18b to within 3σ, suggesting it is a credible explanation for the planet's atmospheric chemistry. The study also discusses the potential for distinguishing between a magma ocean and a liquid water ocean on mini-Neptunes through deeper observations in the >4 μm region, where the CO₂/CO ratio can provide additional constraints. The findings highlight the importance of considering volatile solubility in predicting the atmospheric composition of planets with magma oceans and suggest that the magma ocean scenario may be more consistent with planet formation and climate evolution models.The paper investigates the possibility of a magma ocean on the mini-Neptune K2-18b, a planet with a H₂-dominated atmosphere and a low atmospheric NH₃ abundance. The authors propose that the depletion of NH₃ and the presence of CO₂ in K2-18b's atmosphere can be explained by the high solubility of nitrogen in magma under reducing conditions, where a thick hydrogen envelope is in contact with a molten planetary surface. They demonstrate that this model reproduces the JWST observations of K2-18b to within 3σ, suggesting it is a credible explanation for the planet's atmospheric chemistry. The study also discusses the potential for distinguishing between a magma ocean and a liquid water ocean on mini-Neptunes through deeper observations in the >4 μm region, where the CO₂/CO ratio can provide additional constraints. The findings highlight the importance of considering volatile solubility in predicting the atmospheric composition of planets with magma oceans and suggest that the magma ocean scenario may be more consistent with planet formation and climate evolution models.