Europa's Variable Alkali Exosphere After the Juno 2022 Flyby

     A thin atmosphere of sodium and potassium traces the behaviors of Europa’s neutral gases. Brown & Hill (1996) first discovered Europa’s sodium exosphere with the assumption that it was entirely sourced exogenically: ionized Na from Io corotates with the plasma torus and implants in Europa’s ice surface where subsequent sputtering could then liberate it in neutral form. However, different ratios of sodium to potassium at Io and Europa offered the first evidence that Europa contributes to its own alkali exosphere (Brown 2001). Surface spectroscopy with HST revealed concentrations of irradiated sodium chloride around regions of known chaos terrain, suggesting that subsurface ocean water may upwell through the ice shell and become available to sputtering from incoming plasma bombardment at the surface (Trumbo et al. 2019). Together, these prompt the question: how much does Io contribute to Europa’s alkali exosphere versus what is endogenically sourced? To answer this, we must better understand the roles of Europa’s centrifugal latitude and orbital longitude around Jupiter, and whether it reacts to Io’s stochastic volcanism.

     Observations from Brown (2001), 3D Monte Carlo simulations from Leblanc et al. (2002), and laboratory measurements from Johnson et al. (2002) work in tandem to support a scenario where Europa’s alkali gases are sputtered from compounds that derive from its saltwater ocean. This sputtering process varies with the Jovian radiation environment. The ~10° offset of Jupiter’s magnetic axis from its rotational axis and the corotation of the plasma torus leads to the sinusoidal relationship of the Galilean moons’ centrifugal latitude. This geometry modulates plasma precipitation and north-south asymmetries can be seen in Europa’s oxygen aurorae, which alternate with an 11.1-hour period (Roth et al. 2016). Leblanc et al. (2005) accounted for this variation and compared their model to existing observations to show the effects of centrifugal latitude, as well as photon stimulated desorption at various orbital longitudes and contributions from Io.

Figure 1. Map of Europa’s sodium (left) and potassium (right) atmospheric column densities on 2022 Sept 29 during Juno’s close flyby. The 28″-long slit was used with a sodium filter and blocking filter for Na and K, respectively. Alkali emission within 3RE is overwhelmed by reflected sunlight.

     Europa’s sodium and potassium exospheres were mapped with Keck/HIRES during the 2022 Sept 29 Juno flyby (Lovett et al. submitted). The 28″-long slit was used to collect maximal spatial information around Europa’s disk and was offset 10 and 20 Europa radii (R_E) in all directions, as well as along the Juno flyby trajectory for the Na data. At the time of observations, Europa was within 1″ of western elongation from Jupiter and was centered in the plasma sheet. Io was transiting Jupiter and in Europa’s wake, resulting in minimal contamination. Measurements close to Europa’s disk are overwhelmed by reflected sunlight, rendering altitudes within 3R_E unusable. The resultant maps in Figure 1 suggest an oval-shaped sodium cloud elongated east-west with remarkable east-west and north-south symmetry. When fit to a power law, the radial profile of Na falls off close to R^-1, indicative of purely escaping gas. Much of the potassium data was discarded due to high noise and imperfect pointing of the north-south slit positions, and the resulting map falls off as R^-0.66. Potassium outweighs lighter sodium, so the more extended K exosphere comes as a surprise.

     The Na map in Figure 1 during Juno’s flyby was obtained just after a plasma sheet crossing and during a unique time where Io’s sodium escape was enhanced several-fold (Morgenthaler et al. 2024). To help place these data in context, several additional measurements were made with the associated orbital and magnetic coordinates seen in Figure 2. These data could illuminate the neutral cloud’s dependence on Europa’s geometry, and in principle permit detection of highly debated transient plume eruptions (Roth et al. 2014; Jia et al. 2018; Paganini et al. 2020).

Figure 2. Observation geometries of Europa when observed by Keck/HIRES from 2022-2024. Panel a: Europa’s orbital longitude, with eclipse measurements at 0° and western elongation at 270°. Panel b: Europa’s magnetic longitude in System III coordinates, where the magnetic and orbital equators align at 291° and 111°. The solid line at 5.9RE represents Io’s orbit and dotted line at 9.4RE represents Europa’s orbit. Panel c: Europa’s centrifugal latitude as calculated from Phipps & Bagenal (2021) and magnetic longitude at the time of each observation.

     Europa’s orbital geometry during Juno’s flyby was nearly replicated on 5 Feb 2024, providing a valuable comparison in Figure 3. Despite Europa’s opposite magnetic latitude, and influx from Io that is plausibly significantly lesser, a very similar distribution is seen between the two measurements. These results support the interpretation that Europa is a net source of Na, and that this source rate significantly dominates influx from Io. Further analysis of this dataset will characterize how orbital and magnetic geometry modulate Europa’s alkali exosphere, and if further differences between sodium and potassium are evident.

Figure 3. Map of Europa’s sodium atmospheric column density on 5 Feb 2024. Europa’s angular diameter was ~0.86″ and the Io and Europa orbital geometry is nearly identical to Fig. 1.