Our explanation of the outer Solar System has revealed a host of icy moons, many with surface features that suggest a complex geology. In some cases, these features—most notably the geysers of Enceladus—hint at the presence of oceans beneath the icy surfaces. These oceans have been ascribed to gravitational interactions that cause flexing and friction within the moon, creating enough heat to melt the body’s interior.
Something that has received a bit less attention is that some of these orbital interactions are temporary or cyclical. The orbits of any body are not always regular and often have long-term cycles. That’s also true for the other moons that provide the gravitational stress. As a result, the internal oceans may actually come and go, as the interiors of the moons melt and refreeze.
A new study, released today by Nature Astronomy, looks at one of the consequences of the difference in density between liquid water and ice (about 10 percent): the potential for the moon’s interior to shrink as it melts, leaving an area of low pressure immediately below its icy shell. If the moon is small enough, this study suggests, that could cause the surface of the ocean to boil.
Shifting ice
It can be tempting to think of the Solar System’s current configuration as being relatively static. But that’s definitely not the case; there are plenty of hints that the outer planets moved around a bit early in their history. And, even in its present state, the Earth experiences long-term orbital cycles that drive its entry to and exit from ice ages. The moon systems of the outer planets have the potential for even more complex interactions, with many individual bodies of varying sizes sharing space with a giant planet.
So, it’s easy to think that any oceans are the product of constant forces, and so they were always present. Or that the moons started out hot due to their formation and have been gradually cooling since. But the reality is that the tidal heating that drives the formation of these oceans can come and go over time, and that the moons may experience periodic meltings and re-freezings.
