A study investigates the atmospheric chemistry of the mini-Neptune exoplanet K2-18b, focusing on the presence of CO₂ and the absence of NH₃ in its atmosphere. The findings suggest that these observations could be explained by a magma ocean beneath a hydrogen-dominated atmosphere, rather than a liquid water ocean. The high solubility of nitrogen in magma under reducing conditions leads to the depletion of NH₃ in the atmosphere, consistent with the observed data. The study models the atmospheric composition and chemistry of K2-18b under a magma ocean scenario, showing that the resulting transmission spectrum matches the observed JWST spectrum within 3σ. This suggests that the magma ocean scenario is as credible as the liquid water ocean hypothesis. The study also highlights that deeper observations in the >4μm region could help distinguish between a water ocean and a magma ocean by analyzing the CO₂/CO ratio. The results indicate that the magma ocean scenario is a plausible explanation for the observed atmospheric chemistry of K2-18b, and further research is needed to confirm or refute this hypothesis.A study investigates the atmospheric chemistry of the mini-Neptune exoplanet K2-18b, focusing on the presence of CO₂ and the absence of NH₃ in its atmosphere. The findings suggest that these observations could be explained by a magma ocean beneath a hydrogen-dominated atmosphere, rather than a liquid water ocean. The high solubility of nitrogen in magma under reducing conditions leads to the depletion of NH₃ in the atmosphere, consistent with the observed data. The study models the atmospheric composition and chemistry of K2-18b under a magma ocean scenario, showing that the resulting transmission spectrum matches the observed JWST spectrum within 3σ. This suggests that the magma ocean scenario is as credible as the liquid water ocean hypothesis. The study also highlights that deeper observations in the >4μm region could help distinguish between a water ocean and a magma ocean by analyzing the CO₂/CO ratio. The results indicate that the magma ocean scenario is a plausible explanation for the observed atmospheric chemistry of K2-18b, and further research is needed to confirm or refute this hypothesis.