Planetary scientists have theorised that under a hard outer shell, Europa has a liquid ocean. Observations made with Hubble have captured a transient excess of ultraviolet light toward the limb of Europa. The explanation offered by the researchers is that a liquid geyser of water erupted from Europa’s surface, sending a plume of water high above the moon’s surface. Then molecules of water were subsequently split by high energy electrons caught in Jupiter’s intense magnetic field, producing the UV emission. See also Phil Plait’s Bad Astronomy article.