  Europa's tenuous atmosphere results from sputtering of the surface. The trace
element composition of its atmosphere is therefore related to the composition
of Europa's surface. Magnesium salts are often invoked to explain Galileo Near
Infrared Mapping Spectrometer spectra of Europa's surface, thus magnesium may
be present in Europa's atmosphere. We have searched for magnesium emission in
Hubble Space Telescope Faint Object Spectrograph archival spectra of Europa's
atmosphere. Magnesium was not detected and we calculate an upper limit on the
magnesium column abundance. This upper limit indicates that either Europa's
surface is depleted in magnesium relative to sodium and potassium, or magnesium
is not sputtered as efficiently resulting in a relative depletion in its
atmosphere.
