Mapping Europa's Alkali Exosphere During Juno's 2022 Flyby

     Europa’s exospheric sodium (Na) and potassium (K) are thought to have two possible origins: 1) sputtering from the Io plasma torus, and 2) salts from the subsurface ocean in the form of NaCl and KCl. Plasma sputtering is thought to occur solely on Europa’s trailing hemisphere due to the fast corotation speed of the torus with Jupiter. Neutral Na from Io can be ionized, captured in Jupiter’s magnetosphere and painted onto Europa as a steady stream of Na⁺ ions. Implanted Na⁺ would bond with the surface ice and form a Na reservoir in Europa’s regolith. Subsequent bombardment from the Io plasma torus could then sputter off the Na atoms and re-release them into the exosphere. However, Trumbo et al. (2019) mapped irradiated NaCl abundances and found a high concentration on Europa’s leading hemisphere in areas of known “chaos terrain” where freezing and over pressurization occurs. This location suggests an endogenic source of Na on Europa.

     Based on simple orbital mechanics, leading hemisphere sources drift radially inward, while trailing hemisphere ejection drifts outward. East-west asymmetries in the neutral alkali exosphere thus determine where on Europa’s surface Na and K originated: from the trailing hemisphere (Iogenic origins), or from the leading hemisphere (endogenic origins). This work aims to map Na and K column densities in Europa’s exosphere to reveal the primary source of the alkalis.

     Europa was observed on 29 Sept 2022 by Keck/HIRES during the Juno flyby. The 28”-long slit was oriented east-west and north-south relative to Europa’s celestial north on- and off-disk to cover the extended atmosphere. At this time, volcanic activity on Io enhanced the neutral Na population at Europa’s orbit (c.f. Morgenthaler), so the Na measured at this time should exceed previous measurements. From these data, a map of Na column density reveals the fall-off rate of the alkalis with distance from the moon.

     Comparing these data to previous measurements reported in LeBlanc et al. (2005), we find strong agreement in Na abundances despite the enhanced neutral output from Io at the time. This alludes to the fact that Io’s contribution to Europa’s alkali abundance is not as substantial as was originally believed; Europa’s Na may entirely derive from endogenic origins. We report a rough estimate for the Na/K ratio of ~27, consistent with the 25 ± 2 ratio reported by Brown (2001). Despite the intense particle bombardment, the scale heights in these measurements show Na to be superthermal relative to the cold bulk O2 gas that is thermalized with the surface ice. This confirms that Europa’s atmosphere remains a collisionless exosphere, even during times when the plasma bombardment is enhanced.

     This work bolsters our understanding of alkali variability in Europa’s exosphere, which offers insight on the composition and salinity of the subsurface ocean. These are critical factors to understanding the icy satellite’s habitability. This study will inform ESA’s JUICE mission and NASA’s upcoming Europa Clipper mission before its launch in 2024. This work directly relates to the origins and habitability of Europa, as native sources of alkalis on the moon may affect its ability to host life.