[phyujin]

Real magnetospheres are created by an active core, which Venus, Mars and Europa don't have. Venus and Europa have induced magnetospheres, which are basically just the result of their ionospheres (the ionized upper atmosphere) interacting with the magnetism surrounding them (from the sun and jupiter respectively).

Mars is actually an interesting case because it used to have an active core (and hence a true magnetosphere) a long time ago but it has since died out. The latent magnetism still remaining in its iron rich crust is what protects it from the solar wind now and if you look at a 3d map from orbiting magnetometers it's all lumpy and not symmetrical in the least.

But yeah, anything below the ice in europa would be fine. Ice is an electric and thermal insulator so it doesn't even need an atmosphere to protect from dangerous radiation.