Diagnosing the Atmosphere of Jupiter's Volcanic Moon Io Using Energetic Neutral Atoms (ENAs)

Abstract

During a Juno spacecraft encounter with Jupiter's volcanic moon Io (30 December 2023), the Jupiter Energetic Particle Detector Instrument (JEDI) observed Energetic Neutral Atoms (ENAs) of >150 keV oxygen and sulfur (O + S) coming from Io's mostly nightside atmosphere. JEDI lines-of-sight approached closest to Io between 1.31 and 1.49 Io radii (R_Io), above the predominantly SO₂ core atmosphere but imbedded within the extended atmosphere and corona of SO₂, SO, O, and S. The ratio of O + S ENA intensities and O + S ion intensities estimated to be interacting with Io's atmosphere along JEDI lines-of-sight are 4%–7%. Those estimates are below expectations (∼11%–∼14%), but presently within uncertainties, based on integrations along the JEDI lines-of-sight through a prevailing but uncertain, local-time-symmetric model of Io's extended atmosphere. Expected ENA production rates will remain uncertain until detailed modeling of ion/atmosphere interactions are performed, and knowledge of charge exchange cross sections is verified.