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How does tidal heating create liquid water beneath a moonâs surface?
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Habitable zone
The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.
Tidal heating is a critical mechanism in astrobiology that can maintain liquid water beneath the icy crusts of moons, potentially supporting extraterrestrial life.1
This process occurs primarily through the following steps:
- Orbital Resonance: Moons often exist in stable orbital resonances with other nearby bodies. These periodic gravitational interactions force the moon into an eccentric (elliptical) orbit rather than a perfectly circular one.2
- Dynamic Deformation: As a moon travels along its eccentric path, its distance from its parent planet constantly changes. This variation subjects the moon to fluctuating gravitational forces, causing the body to undergo cyclic physical deformations.2
- Internal Friction: These continuous, rhythmic changes in shape generate heat within the moonâs interior through internal friction. Even if a moon is in synchronous rotation, the eccentricity forced by resonance ensures that the tidal deformation remains dynamic, providing a persistent source of thermal energy.2
This internal heat can be sufficient to melt subsurface ice, creating a global liquid water ocean beneath a protective icy crust, as is theorized for moons like Europa and Enceladus.31