NASA’s Juno Spacecraft Detects Callisto’s Aurora, Completing Jupiter’s Galilean Moons Set

Jupiter's poles blaze with the brightest auroras in the Solar System. Uniquely, each of the planet's four largest moons generates its own faint auroral “footprint” in Jupiter's sky. Until now, scientists had seen footprints for three of these moons – Io, Europa and Ganymede – but Callisto's mark remained elusive. NASA's Juno spacecraft has now captured Callisto's aurora, completing the set. This discovery confirms that all four Galilean moons leave a glowing mark on Jupiter's atmosphere.

Auroral Footprints of the Galilean Moons

According to a paper on the discovery, “In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft”, each of Jupiter's Galilean moons carves a small aurora in the planet's atmosphere when it moves through Jupiter's strong magnetic field. Charged particles channel along magnetic field lines from a moon down into Jupiter's polar atmosphere, lighting up a spot or arc.

These moon-made auroras are called satellite footprints, and they reveal how each moon interacts with its local space environment. For example, Io, Europa and Ganymede all produce distinct auroral spots. By contrast, Callisto's faint footprint had been hidden inside Jupiter's main auroral oval and was not seen until Juno observed it.

Completing the Auroral Family Portrait

The missing Callisto signature appeared in 2019 during a strong solar wind event that pushed Jupiter's main aurora out of the way. During that flyby, Juno's instruments detected the faint glowing arc and even sampled charged particles and magnetic fields around Callisto's field line.

In NASA's words, this observation “completes the family portrait” of the Galilean moon auroras. Now with all four moons accounted for, scientists have a full set of data showing how each moon – from volcanic Io to distant Callisto – influences Jupiter's magnetosphere.