Question

Three models are proposed to explain the thermal energy source and fracturing mechanism responsible for the cryovolcanic plumes observed at the south pole of Saturn's moon, Enceladus.

Model 1
The parallel fractures (tiger stripes) are open conduits connected to a localized subsurface reservoir of liquid water. Saturn's gravitational pull exerts varying tidal forces on Enceladus along its eccentric orbit. This tidal flexing causes the walls of the fractures to rub against one another. Frictional heating along these sliding faults melts the surrounding ice, generating the heat that keeps the vents open and drives the vapor plumes.

Model 2
The thermal energy source is radiogenic decay within the silicate core, which maintains a global subsurface ocean. As the moon slowly cools, the outer ice shell thickens. Because ice is less dense than liquid water, this freezing process expands the shell, generating intense hydrostatic pressure within the underlying ocean. Once the pressure exceeds the tensile strength of the ice shell, fracturing occurs, violently venting pressurized water into space.

Model 3
Cold water from the subsurface ocean migrates downward, circulating through a porous, fractured silicate core. An exothermic chemical reaction known as serpentinization occurs between the water and olivine-rich rocks in the core, raising the water temperature. This reaction also releases gases, primarily $H_2$. The resulting warm, buoyant, gas-rich fluids rise rapidly, melting conduits through the overlying ice shell to erupt as plumes.

Based on the models provided, match each key hypothesis regarding the primary energy source or fracturing mechanism on Enceladus to the corresponding model.