Potential interior compositions for K2-18 b given our retrieval analysis. The green shaded region on the ternary diagram shows an MCMC posterior of compositions consistent with K2-18 b’s mass, radius, host star flux, atmosphere metallicity (∼30% CH₄), and system age. We group methane with water as volatile species for analysis in terms of ice-to-rock ratios, but is physically located in the H/He layer in our model. The three cases discussed in Section 5.2 are circled and labeled. The inset shows the posterior histogram of the log(H/He) mass fraction, with the amount permitting a liquid water ocean (shaded region) containing only 0.58% of our posterior. Above that, the water layer is a supercritical-fluid.