Trace element abundances in the sputtered extended atmospheres of the Galilean satellites reflect the surface abundances of these trace elements and can thus serve as signatures of surface processes on these satellites. We have measured sodium and potassium abundances in the extended atmospheres of Europa and Io, and derive sodium-to-potassium ratios of 25±2 and 10±3. The large difference between the values of Na/K in these atmospheres is difficult to explain if the dominant source of sodium for Europa's surface is implantation of Io sodium, as suggested earlier. The three remaining likely sources for trace elements on the surface of Europa—metals brought in from meteoritic impact, trace elements native to the surface ices, and salts deposited in liquid water resurfacing events—are all broadly consistent with the measured sodium-to-potassium ratio at Europa. Observations of additional trace elements are required to further discriminate among these possible sources.