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Europa’s Alkali Exosphere During the 2022 Juno Flyby

Presentation #314.04 in the session Icy Satellites, Fields and Particles (Poster + Lightning Talk)

Published onOct 23, 2023
Europa’s Alkali Exosphere During the 2022 Juno Flyby

Exospheric sodium (Na) and potassium (K) are thought to originate from salts in Europa’s subsurface oceans. These species were mapped by Keck/HIRES during the 29 Sept 2022 Juno flyby, allowing contemporaneous measurements on Europa’s local plasma environment. The scale heights in these data show alkalis to be superthermal relative to the cold bulk O2 gas in equilibrium with the satellite’s icy surface. Because Io’s atmospheric escape was strongly enhanced at this time (cf. Io presentations by Morgenthaler, Schmidt), this confirms that Europa’s atmosphere remains collisionally thin during times of intense plasma bombardment.

Keck’s measurements were timed within one arcsecond of Europa’s maximum elongation from Jupiter, and the observed cloud was measurably asymmetric. Trumbo et al. (2019; 2021) mapped irradiated NaCl surface abundances and found the highest concentration on Europa’s leading hemisphere, particularly surrounding chaos terrain. Their measurements support the understanding that Na in Europa’s exosphere is endogenic, since Na and K influx from Io precipitates solely on Europa’s trailing side. Our constructed maps of Na and K column density around Europa show an enhancement in the anti-Jovian direction. This may suggest that Na and K originated preferentially from Europa’s leading hemisphere, since particles escaping in this direction have orbital velocities greater than Europa, causing them to drift radially outward. This suggests Na and K in the exosphere are sourced from regions where freezing and over-pressurization occurs and are thus endogenic to Europa’s subsurface oceans.

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