Europa’s Alkali Exosphere
It has been long debated where sodium and potassium originate on Europa. Are the alkalis coming solely from Io’s volcanism after being carried to Europa via the plasma torus? Or, could Europa have an endogenic source of Na and K that derives from the salty subsurface ocean? The figures above show the modeled vs. measured locations of salt constituents on Europa’s surface. Cassidy et al. (2013) [top] modeled plasma bombardment at Europa’s surface and found that ionized Na and K should preferentially hit the trailing hemisphere, as the plasma torus corotates with the Jovian magnetosphere— much faster than Europa’s orbital velocity. However, Trumbo et al. (2019) measured irradiated NaCl on Europa’s surface with HST and found strong concentrations on the moon’s leading hemisphere, particularly around areas of known over-pressurization and oceanic upwelling through the ice shell. Thus, Europa likely has some endogenic source of alkalis, such as salts from the subsurface ocean.
I use the High Resolution Echelle Spectrograph on the Keck Telescope to measure Na and K in Europa’s extended surface-bound exosphere (a collisionless atmosphere). My data shows that Na is superthermal, and remains consistent with previous measurements despite enhanced volcanism from Io. This work is currently being written up, and will be published soon! In the meantime, enjoy this picture of me presenting my work for the first time at DPS 2023.
