This can have important consequences on the stresses experienced by the icy shells of these moons. Water is considerably denser than ice. So as a moon’s ocean freezes, its interior will expand, creating external forces that oppose the gravity that holds the moon together. The potential for this transition to shape the surface geology of a number of moons, including Europa and Enceladus, has already been explored. So the researchers behind this new work decided to examine the opposite question: What happens when the inside starts to melt?
Rather than focusing on a specific moon, the team made a general model of an ice-covered ocean. This model treated the ice shell as an elastic surface, meaning it would not break, and placed viscous ice underneath. Further down there was a liquid ocean and eventually a rocky core. As the ice melted and the ocean expanded, researchers tracked stresses on the ice shell and pressure changes at the ice-ocean interface. They also tracked the propagation of thermal energy through the ice shell.
Pressure drop
Clearly, there are limits to how much the outer shell can bend to accommodate the shrinking of the melting interior parts of the moon. This creates a low pressure zone under the hull. The consequences depend on the size of the moon. For larger moons – and this includes most of the moons the team examined, including Europa – there were two options. For some, gravity is strong enough to maintain pressure at a point where the water at the interface remains liquid. In others, gravity was sufficient to cause even an elastic surface to rupture, leading to surface collapse.
However, for small moons this doesn’t work; the pressure becomes low enough that water boils even at room temperature (just above the freezing point of water). Additionally, the low pressure will likely cause the dissolved gases in the water to be released. The result is that gas bubbles should form at the ice-water interface. “Boiling is possible on these bodies – and not on others – because they are small and have relatively low gravitational acceleration,” the researchers conclude. “Therefore, less ocean underpressure is required to counterbalance the [crustal] pressure.”
