Water is denser than ice. This is anomalous — most liquids are denser than their solids, but water's 9% expansion upon freezing is why ice floats, pipes burst, and frost shatters rock. The anomaly is so familiar that its consequences feel exhausted.
They aren't. On small icy moons — Enceladus, Mimas, Miranda — tidal heating from orbital resonances periodically melts the base of the ice shell. The ice becomes liquid water. The liquid occupies less volume than the solid it replaced. On Earth, this pressure change is negligible against the vast column of atmosphere and ocean above. On a 500-kilometer moon with a thin ice shell and weak gravity, it's not.
Rudolph et al. (Nature Astronomy, 2025) show that on the smallest ocean worlds, melting ice reduces the overburden pressure enough to approach water's triple point — the precise conditions where solid, liquid, and vapor coexist. The ocean boils. Not because it was heated past some threshold, but because the act of forming reduced the pressure that kept it stable. The transition that created the ocean is the transition that destabilizes it.
The size of the moon determines the outcome of the same physics. On Titania, gravity is strong enough that the pressure change from melting stays well above the triple point — the ice cracks instead. On Enceladus, the pressure can drop far enough to produce the tiger-stripe fractures and water plumes that Cassini observed. On Miranda, the boiling may explain the coronae — those bizarre ridged formations that look like geological surgery.
The structural point is the relationship between a system's size and its capacity to contain what it produces. Large moons absorb the consequences of their own melting. The pressure shifts, the ice adjusts, the ocean persists. Small moons cannot. Their gravity is insufficient to maintain the conditions under which their own oceans are stable. The product of the transition — liquid water — undermines the parameter — pressure — that permits the product to exist.
This is not the same as optimization-driven fragility, where a system sheds redundancy and becomes brittle. The moons didn't shed anything. They were always this size. The constraint was always present. What changed was the tidal heating that initiated the transition. The system didn't degrade into instability — it was born at the threshold. The first drop of meltwater is already the beginning of the ocean's destruction.
The same physics, three outcomes: melt quietly (large), crack (medium), boil (small). The variable isn't the process — it's whether the system has room to tolerate what the process creates.