section_path: Atmospheric models/Composition models
section_name: Composition models
paragraph_text: Composition models
  
  There are challenges in examining an exoplanetary surface and its atmosphere, as cloud coverage influences the atmospheric temperature, structure as well as the observability of spectral features. However, planets composed of large quantities of water that reside in the habitable zone (HZ) are expected to have distinct geophysics and geochemistry of their surface and atmosphere. For example, in the case of exoplanets Kepler-62e and -62f, they could possess a liquid ocean outer surface, a steam atmosphere, or a full cover of surface Ice I, depending on their orbit within the HZ and the magnitude of their greenhouse effect. Several other surface and interior processes affect the atmospheric composition, including but not limited to the ocean fraction for dissolution of CO and for atmospheric relative humidity, redox state of the planetary surface and interior, acidity levels of the oceans, planetary albedo, and surface gravity.
  
  The atmospheric structure, as well as the resulting HZ limits, depend on the density of a planet's atmosphere, shifting the HZ outward for lower mass and inward for higher mass planets. Theory, as well as computer models suggest that atmospheric composition for water planets in the habitable zone (HZ) should not differ substantially from those of land-ocean planets. For modeling purposes, it is assumed that the initial composition of icy planetesimals that assemble into water planets is similar to that of comets: mostly water (H O), and some ammonia (NH), and carbon dioxide (CO). An initial composition of ice similar to that of comets leads to an atmospheric model composition of 90% H O, 5% NH, and 5% CO.
  
  Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 bar and 5 bar of CO are needed to warm the surface temperature above freezing, leading to a scaled surface pressure of 0.56–1.32 times Earth's.