NASA’s Juno mission has detected the long-sought auroral footprint of Callisto, confirming that all four of Jupiter’s Galilean moons imprint distinct signatures in the planet’s atmosphere. The finding, revealed using Juno’s ultraviolet observations and in-situ measurements, completes a decades-long search and refines understanding of how Jupiter’s magnetosphere couples to its largest satellites. The results are detailed in a study published Sept. 1, 2025, in Nature Communications and summarized by NASA (source).
Why Callisto’s signature proved elusive
Io, Europa, and Ganymede footprints were documented prior to Juno, but Callisto’s signal remained hidden. Its footprint is intrinsically faint and typically overlaps Jupiter’s bright main auroral oval, masking the moon-driven glow against a more intense background. As a result, repeated attempts from Earth-orbiting observatories struggled to isolate the feature.
How Juno captured the footprint
Two conditions aligned during Juno’s 22nd orbit in September 2019. First, a high-density solar stream compressed Jupiter’s magnetosphere, shifting the main auroral oval closer to the equator and clearing the polar field of view. Second, Juno’s trajectory crossed the magnetic field line that links Callisto to Jupiter, enabling simultaneous remote ultraviolet imaging of the polar region and in-situ sampling of particles, electromagnetic waves, and magnetic fields associated with the interaction. This conjunction revealed a faint but distinct footprint tied to Callisto.
Because Juno has orbited Jupiter since 2016 with repeated close passes over the poles, it can capture transient auroral configurations while directly probing the surrounding plasma environment. The 2019 event provided both the geometric alignment and space-weather forcing needed to expose Callisto’s subtle signature.
What the detection means for Jupiter system science
Confirming Callisto’s footprint completes the Galilean set and underscores that each large moon participates in current systems that channel energy along Jupiter’s magnetic field lines into the upper atmosphere. The observation offers new constraints on the strength and variability of Callisto’s electrodynamic coupling, the properties of the local plasma, and how solar-wind dynamics modulate auroral morphology at a giant planet.
These insights improve models of moon–magnetosphere interactions, inform comparative studies across the Galilean satellites, and help quantify how external drivers, such as solar wind compressions, reorganize Jupiter’s auroral regions on short timescales.
Key details
- Discovery context: Juno’s ultraviolet and in-situ measurements during orbit 22 (September 2019) identified Callisto’s footprint when the main oval shifted equatorward.
- Drivers: A high-density solar stream buffeted Jupiter’s magnetosphere, enabling a clear polar view while Juno crossed the Callisto-connected field line.
- Completes set: All four Galilean moons—Io, Europa, Ganymede, and Callisto—are now confirmed to generate auroral footprints in Jupiter’s atmosphere.
- Publication: Findings reported in Nature Communications on Sept. 1, 2025, by an international team led by researchers in Toulouse, France.
As Jupiter’s magnetosphere responds to the solar wind, conditions can expose or obscure moon-driven emissions. Juno’s vantage point and comprehensive instrument suite continue to capture these evolving dynamics, offering a fuller picture of energy transport from the planet’s vast magnetic environment into its upper atmosphere and the role its major moons play in shaping those processes.
For NASA’s summary of the result, see the mission update (NASA).
