A Search for a Sodium Atmosphere around Ganymede

Long-slit high-resolution spectra have revealed that Ganymede has an atmospheric sodium column density, between a distance of 7800 and 15600 km from the satellite surface, of less than 1 10 8 atoms cm ?2 , a factor of 13 times smaller than the sodium density in the atmosphere of Europa at the same distance. Assuming similar processes creating sodium on both Europa and Ganymede, we estimate that the source rate for sodium production on Ganymede should be within a factor of 2 of that on Europa. Two possibilities for the lack of detectable sodium around Ganymede are that Ganymede's surface is depleted in native sodium compared to Eu-ropa's or that Ganymede has a magnetic eld capable of standing oo impacting energetic particles.