This could have serious consequences for the stresses experienced by the icy shells of these satellites. Water is much denser than ice. Thus, when the Moon's ocean freezes, its interior will expand, creating external forces that counteract the gravity holding the Moon together. The potential of this transition to shape the surface geology of a number of moons, including Europa and Enceladus, has already been studied. So the researchers behind the new work decided to look at the opposite question: what happens when the interior starts to melt?
Instead of focusing on a specific moon, the team created a general model of the ice-covered ocean. This model treated the ice shell as an elastic surface, meaning it couldn't easily break, and placed viscous ice underneath it. Below was a liquid ocean and eventually a rocky core. As the ice melted and the ocean expanded, the researchers monitored the stress in the ice shell and pressure changes occurring at the ice-ocean interface. They also tracked the spread of thermal energy through the ice shell.
Pressure drop
Clearly, there are limits to how much the Moon's outer shell can bend to accommodate the shrinkage of the Moon's melting interior. This creates an area of low pressure under the shell. The consequences of this depend on the size of the Moon. For larger moons—which includes most of the moons the team studied, including Europa—there were two options. For some, gravity is strong enough to maintain pressure at the point where the water at the interface remains liquid. In other cases, gravity was enough to cause even an elastic surface to fail, causing the surface to collapse.
However, this will not work for smaller satellites; the pressure becomes so low that water boils even at ambient temperature (just above the freezing point of water). In addition, low pressure is likely to release gases dissolved in the water. As a result, 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 needed to balance [crustal] pressure.”
