Ionospheres of Ganymede and Europa Observed by Radio Occultation with Juno

NASA’s Juno spacecraft performed close flybys of the Galilean moons Ganymede in June 2021 and Europa in September 2022. During each of these encounters, the Juno spacecraft passed behind the moons as observed from Earth, providing the geometry for a radio occultation experiment to measure the electron densities of the ionospheres of these moons – the first opportunity to do so since the Galileo mission in the 1990s. Electrons encountered along the radio propagation path advance the signal’s phase. These small changes are detectable in the sensitive receivers of the Deep Space Network antennas. Ganymede’s tenuous ionosphere was detected on occultation ingress but no ionosphere was detected on egress. The interaction of the ionosphere with Ganymede’s intrinsic magnetosphere is believed to be the reason for the variability of the ionosphere, since ingress occurred on an open-field line region where electron impact ionization could be higher. At Europa, the occultation probed the Southern mid-latitudes on ingress and near the equatorial region on egress, with results consistent when compared with the six radio occultations of Europa from Galileo. Future occultation science with Juno will occur in 2023 and 2024 with radio occultations of Jupiter’s atmosphere and ionosphere.