Existence of a water-dominated layer between the envelope and rocky interior

Determine whether K2-18 b possesses a distinct water-dominated layer separating its volatile envelope from its rocky interior, as opposed to a well-mixed envelope without a discrete water layer, to clarify the planet’s internal architecture and its implications for atmospheric–interior coupling.

Background

The refined characterization roadmap discusses multiple internal configurations for temperate sub-Neptunes, including small atmospheres overlying liquid-water oceans and massive envelopes that may be well mixed with underlying volatiles.

Establishing the presence or absence of a distinct water-dominated layer would help break degeneracies in interpreting atmospheric chemistry and guide the use of diagnostics such as CO2/CO and NH3 to infer interior structure.