is essentially a naked blob of water, in orbit around a gas giant. It's so cold that the surface freezes, which severely limits the rate at which it can escape to space, and gives Europa a lifetime longer than the solar system (i.e. it is still here now).

You could have an episodic tidal resonance melting the ice on a Europa-class world every million years or so, or even every hundred thousand.

Or Europa in a system with a flare star which could melt the ice in decadal cycles like the sunspot cycle of our sun, or in random events (I'm not sure if flare stars are cyclic or not)

or you just make the blob so big and young that it's still here despite evaporating fast...