I don't know what the rate of mass loss is (I doubt anyone knows), but most objects in the solar systems are not static: they gas out but they also accumulate d

I don't know what the rate of mass loss is (I doubt anyone knows), but most objects in the solar systems are not static: they gas out but they also accumulate dust particles. Most of the gas giant moons have been emitting gas and particles into space since they formed, with most of that matter eventually falling onto the gas giant itself. My guess is the loss of volatiles is so slow that it would take many billions of years until the composition of the moon changes in an appreciable way.