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

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 (similar to 11%-similar to 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. Plain Language Summary In the space environments (magnetospheres) of magnetized planets, magnetic fields trap energetic charged particles, including protons and singly charged heavier ionized atoms (ions). These ions can be neutralized by stealing electrons from cold gas atoms within the same environment. They become Energetic Neutral Atoms (ENAs), and no longer confined, travel out of the system similar to photons of light leaving a hot oven. Here we observe energetic oxygen (O) and sulfur (S) ENAs generated by trapped energetic ions interacting with the upper atmosphere of Jupiter's volcanic moon Io. The emission of these ENAs from the atmosphere of Io serves to constrain the densities and distributions of some constituents of Io's atmosphere, and particularly the higher altitude components, including what is referred to as Io's "corona." The new constraints suggest a factors of 2 difference with expectations, but that factor is within the present uncertainties in the analyses.