Jupiter is well-known for the massive aurorae that occur near the planet’s polar regions, the brightest and most powerful in the Solar System. Much like aurora here on Earth, these shimmering lights are the result of interaction between the planet’s magnetic field and solar wind. Unlike Earth’s, though, Jupiter’s largest moons – Io, Europa, and Ganymede (aka. the Galileans) – leave their own auroral signatures in the planet’s atmosphere. These induced aurorae are known as “satellite footprints” and track how each moon interacts with Jupiter and the local space environment.
Whereas scientists have observed how Io, Europa, and Ganymede create satellite footprints in Jupiter’s atmosphere, Callisto has remained a bit of an outlier. Despite multiple attempts using the Hubble Space Telescope (HST), signatures caused by Callisto remained elusive. Thanks to NASA’s Juno mission, which achieved orbit around Jupiter in 2016, Callisto’s satellite footprint has finally been found. In a recent study, an international team of scientists presented evidence of these polar light signatures in Jupiter’s atmosphere for the first time.
The team was led by Jonas Rabia of the Institut de Recherche en Astrophysique et Planétologie (IRAP). He was joined by researchers from the
Institut Origines at the Laboratoire d’Astrophysique de Marseille (LAM), the Laboratoire d’Instrumentation et de Recherche en Astrophysique (LIRA), Institut Supérieur de l’Aéronautique et de l’Espace (ISAE), the Laboratory for Planetary and Atmospheric Physics (LPAP), the Minnesota Institute for Astrophysics (MIfA), the Southwest Research Institute (SwRI), and NASA’s Goddard Space Flight Center.
While Juno has provided close-up views of Jupiter’s aurorae, capturing Callisto’s footprint presented a major challenge. In addition to being faint, Callisto’s auroral signature typically resides above the brighter region where Jupiter’s aurorae are displayed (the main auroral oval). To measure Callisto’s footprint, the science team needed to image Jupiter’s polar region only after the main auroral oval moved aside. The spacecraft also needed to cross the magnetic field line linking Callisto and Jupiter for its instruments to detect the auroral signature.
This included its Magnetometer(MAG), Jovian Auroral Distributions Experiment (JADE), and Jovian Energetic Particle Detector Instrument (JEDI). As luck would have it, both of these events occurred in September 2019 when a massive high-density solar stream hit Jupiter’s magnetosphere, causing it to shift to lower latitudes. This is similar to how solar storms often push the northern lights to more southern latitudes. This revealed Callisto’s auroral footprint and provided vital data on the energetic particles, electromagnetic waves, and magnetic fields associated with the interaction.
This confirms that all four Galilean moons leave lasting indications of their passage through Jupiter’s magnetic field. The paper on the discovery, “In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft,” was published in the journal Nature Communications on Sept. 1st, 2025.
Further Reading: NASA