section_path: Physical properties/Possible ocean
section_name: Possible ocean
paragraph_text: Possible ocean
  
  At temperatures exceeding the critical point, liquids and gases stop being different phases and there is no longer a separation between an ocean and the atmosphere. It is unclear whether observations imply that a separate liquid ocean exists on K2-18b, and detecting such an ocean is difficult from the outside; its existence cannot be inferred or ruled out solely from the mass and radius of a planet.
  
  The existence of a liquid water ocean is uncertain. Before the James Webb Space Telescope observations, a supercritical state of the water was believed to be more likely. JWST observations were initially considered to be more consistent with a fluid-gas interface and thus a liquid ocean - trace gases such as hydrocarbons and ammonia can be lost from an atmosphere to an ocean if it exists; their presence may thus imply the absence of an ocean-atmosphere separation. Subsequent work finds that a magma ocean may also be capable of dissolving ammonia and explaining the observation results, but not to explain the observed carbon oxide concentrations. Whether the carbon oxide concentrations can be explained by a mini-Neptune/deep hydrogen atmosphere model is uncertain. Another paper suggests that a liquid water ocean model requires the presence of a biosphere in order to produce sufficient amount of methane.