Fig. 9.
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Thermal structure relation of CO2 (cooling process) and X-ray flux (heating process) on the planet, adapted from Johnstone et al. (2021b). The overlaid boundary (solid black line) corresponds to a CO2- and N2-dominated atmosphere, while K2-18b is expected to be H/He-rich with CO2 as a minor species. The X-ray flux is measured in this work, and the CO2 mixing ratio is taken from Madhusudhan et al. (2023, no offset).
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