The 'Water Sandwich': How gravity creates exotic ice and hidden oceans
In short: On some celestial bodies, water doesn't just sit on the surface. High pressure can transform it into exotic solid phases, creating a 'water sandwich' where a liquid ocean is trapped between layers of high-pressure ice. These hidden worlds challenge our definition of what a planet can be.
Bodies of liquid water that encircle a globe and remain stable within our solar system are designated as 'ocean worlds'. This distinguishes them from 'ocean planets' or 'water worlds,' which are exoplanets with substantial water mass fractions in their bulk composition. While Earth is the only known body with surface liquid water, subsurface oceans are suspected on moons like Jupiter's Europa and Ganymede, and Saturn's Enceladus and Titan. These bodies are primary targets for astrobiology because they are reachable by space probes, unlike exoplanets light-years away.
The internal structure of these worlds is dictated by a balance of heat and pressure. For smaller satellites like Enceladus, an ocean may sit directly atop a silicate core, potentially providing hydrothermal energy and nutrients for life. However, on larger ice-rich bodies like Ganymede, pressures are so intense that ice at depth transforms into higher-pressure phases. This creates a 'water sandwich' where the liquid ocean is sandwiched between layers of solid ice. Depending on the environment, water can exist in various states, including steam, superfluid, plasma, and exotic forms like ice V or ice VII.
The existence of these oceans depends on internal heating—driven by radioactive decay, tidal flexing, or early luminosity—outpacing the rate of heat removal. For exoplanets, the conditions are even more extreme. A planet with a 300 K surface could host oceans up to 500 km deep. Furthermore, if a planet is too close to its star, water may become supercritical, lacking a well-defined surface. Scientists also study 'Hycean' planets, which have hydrogen-rich atmospheres; however, complex modeling suggests these worlds would only maintain liquid water at specific distances, such as 3.85 AU, to avoid boiling under high atmospheric pressure.
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- Summary by Intellect · source: Ocean world
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