Europa’s Sodium and Potassium Exosphere during Juno’s Flyby

Abstract Jupiter’s icy moon Europa is enveloped in gaseous sodium and potassium that resonantly scatter sunlight as optical emission lines. The High Resolution Echelle Spectrograph on the Keck I telescope has spatially mapped these alkalis contemporaneously with Juno’s PJ45 flyby. Beyond 3 Europa radii, where emission becomes separable from bright surface reflectance, their extended radial profiles indicate atmospheric escape. This suggests that alkalis are ejected from the ice by a high-energy process—likely ion sputtering—into an exosphere where initial energies are not appreciably dampened through collisions with the cold O 2 atmosphere and icy surface. The neutral sodium cloud exhibits remarkable symmetry east–west and north–south, a sign that incident plasma has access to the majority of Europa’s surface, and the effects of centrifugal latitude may be negligible at the time of observations. However, sodium is more extended east–west, suggesting an oval-shaped cloud. The Na column density is a few × 10 10 atoms cm −2 , and the Na/K ratio is 28 ± 8 at 10 Europa radii, consistent with prior estimates. However, this ratio decreases with distance despite sodium’s lower mass and lifetime against ionization. Doppler broadening in the resolved Na line profiles grows with tangent altitude from a few thousand kelvins to >15,000 K by 25 Europa radii. Considering the more than twofold increase in Io’s Na loss at this time, brightness levels in good agreement with past data reinforce an interpretation that Europa is a net source of alkali atoms that ultimately derive from its subsurface brines or saline ocean.