When people think of auroras, they usually imagine the ethereal green and purple curtains of light dancing across Earth’s polar skies. These northern and southern lights inspire awe and wonder, but they are only a small piece of the cosmic puzzle. Travel hundreds of millions of miles outward, past Mars and the asteroid belt, and you arrive at Jupiter—a world so immense it could swallow more than a thousand Earths. Here, auroras blaze brighter and larger than anywhere else in the solar system.
These Jovian auroras are not just spectacular to the eye; they are powerful laboratories of physics and cosmic connection. They reveal how the giant planet interacts with the solar wind streaming from the Sun, as well as with its fleet of moons. And within this symphony of light lies a unique mystery: the elusive auroral footprint of Callisto, the most distant of Jupiter’s Galilean moons.
The Mystery of Moon-Made Auroras
On Earth, auroras form when charged particles from the solar wind crash into our atmosphere near the poles, guided by Earth’s magnetic field. Jupiter’s auroras are also shaped by the solar wind, but the story is far more complex. The gas giant’s magnetic field is enormous—the largest structure of its kind in the solar system—and it envelops many of its moons.
Three of Jupiter’s largest moons—Io, Europa, and Ganymede—are known to generate their own auroral signatures. These moon-made lights are called satellite footprints. Unlike anything seen on Earth, they appear as glowing spots or trails in Jupiter’s atmosphere, created when the moons disturb the planet’s powerful magnetic environment. Imagine each moon dragging its own invisible “fingerprint” across Jupiter’s glowing polar skies.
But one moon remained silent. Callisto, the fourth Galilean moon, seemed to leave no trace. For decades, astronomers searched with ground-based telescopes and with the Hubble Space Telescope, hoping to find Callisto’s footprint. Again and again, the trail went cold. Was Callisto different from its siblings? Or had its footprint simply been hidden in plain sight?
Juno Arrives at the Scene
The arrival of NASA’s Juno spacecraft in 2016 transformed our understanding of Jupiter. Unlike previous missions, Juno orbits close to the giant planet in long, looping paths that carry it over the poles. From this vantage point, the spacecraft has delivered the most detailed observations ever made of Jupiter’s auroras.
To catch Callisto’s elusive footprint, however, the conditions had to be perfect. The glowing oval of Jupiter’s main aurora—the planet’s constant ring of light near the poles—needed to shift out of the way. At the same time, Juno’s orbit had to cross the invisible magnetic field line that directly links Callisto to Jupiter.
This cosmic alignment seemed almost impossible, but in September 2019, during Juno’s 22nd orbit of the planet, it happened. The spacecraft’s cameras and instruments were watching at exactly the right moment. For the first time, scientists glimpsed Callisto’s faint signature etched into Jupiter’s atmosphere.
A Solar Storm’s Helping Hand
The timing of Juno’s discovery was not just lucky—it was influenced by the Sun itself. At the same time Juno was poised to make its observations, a massive stream of dense solar wind slammed into Jupiter’s magnetic field. The storm compressed and shifted the auroral oval closer to Jupiter’s equator, leaving just enough space near the pole for Callisto’s footprint to appear.
Like a curtain lifting on a hidden stage, the solar storm revealed what astronomers had been seeking for decades: Callisto’s mark upon Jupiter’s polar lights. Faint but distinct, it confirmed that all four Galilean moons carve their own unique auroral trails.
Completing the Family Portrait
The discovery of Callisto’s footprint is more than just a scientific curiosity. It completes the picture of how Jupiter’s largest moons interact with their parent planet. Each Galilean moon—Io, Europa, Ganymede, and Callisto—now has a known auroral signature, a glowing testament to the invisible electrical currents and magnetic forces binding them to Jupiter.
For Io, the most volcanically active world in the solar system, the interaction is fierce. Charged particles spewed from its volcanoes are captured by Jupiter’s magnetic field, creating intense auroras. Europa, with its hidden ocean beneath an icy crust, leaves a subtler footprint, but one that sparks hope for habitability. Ganymede, the only moon known to have its own magnetic field, produces complex and shifting signatures. And now Callisto, once thought to be the quiet sibling, has spoken at last, joining the chorus with its own delicate imprint.
A Window Into Invisible Worlds
Why does this matter? Because auroras are more than beautiful light shows. They are windows into invisible realms. By studying the footprints of moons, scientists can learn how plasma, magnetic fields, and charged particles move between worlds. These connections reveal not only the hidden physics of Jupiter’s system but also how similar processes might unfold around other giant planets and even distant exoplanets.
In Callisto’s case, its footprint shows that even a distant, icy moon with no atmosphere to speak of is still deeply tied to Jupiter’s immense magnetosphere. The discovery reminds us that in space, distance does not always mean disconnection. Gravity, magnetism, and plasma link these worlds together in ways that challenge the imagination.
The Continuing Journey
The paper confirming Callisto’s footprint, led by Jonas Rabia and an international team of scientists, appeared in Nature Communications. It represents not just a scientific milestone but a human triumph of patience, technology, and curiosity. After years of uncertainty, after countless attempts with Hubble and ground telescopes, Juno’s persistence and the Sun’s storm conspired to unveil the truth.
Yet, as always in science, one answer leads to new questions. How stable is Callisto’s footprint? Does it vary with solar activity? What does it reveal about the moon’s interaction with Jupiter’s magnetic field? And what might this teach us about similar processes elsewhere in the universe?
The Beauty of Connection
In the end, the discovery of Callisto’s footprint is not only about Jupiter’s auroras. It is about the beauty of connection. Moons orbiting a planet, particles streaming from the Sun, magnetic fields weaving invisible threads—all come together to create shimmering light shows at the edge of the solar system.
When we look at Jupiter’s auroras, we are not only witnessing a spectacle of color; we are glimpsing the hidden architecture of a cosmic family. And now, with Callisto’s delicate signature revealed, the family portrait is complete. The lights above Jupiter’s poles remind us that even in the vastness of space, worlds are never alone—they are bound together by forces both powerful and beautiful, painting the heavens with their shared story.
More information: J. Rabia et al, In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft, Nature Communications (2025). DOI: 10.1038/s41467-025-62520-4
