For one of the research papers I was doing for my class, I came across a journal article that suggested if Ganymede gets affected by planetary migration, with tidal heating from Jupiter and radiation from the Sun, the subsurface water on Ganymede would make its way to the surface through geysers and cryovolcanoes. Combined with the magnetosphere, an atmosphere could form, eventually starting a hydrological cycle similar to that on Earth and Titan.