The Mystery of Saturn's Missing X-Ray Auroras

By the end of this article, you will understand what a planetary magnetotail is, what happens when Saturn gets swallowed by Jupiter’s magnetic field, and why scientists are struggling to find X-rays on the ringed planet.

In November 2020, scientists aimed the Chandra X-ray Observatory at Saturn. They were waiting for a Surprise: a rare event that happens only once every 19 years. Jupiter has a massive magnetic tail blown back by the solar wind. Because Jupiter is so huge, its tail stretches past Saturn’s orbit. For a few days, Saturn actually passes right through it! Inside this tail, the solar wind drops to almost zero, causing Saturn’s own magnetic field to expand. When Saturn pops back out into the normal solar wind, the sudden shock compresses its magnetic field violently. Scientists thought this massive shockwave might finally trigger X-ray auroras on Saturn. But after carefully analyzing the data, they found… nothing. The elusive X-ray auroras remained hidden, proving that Saturn’s magnetic response is completely different from Earth’s or Jupiter’s.

Original Paper: ‘Searching for Saturn’s X-rays during a rare Jupiter Magnetotail Crossing using Chandra’

This is the first X-ray campaign of its kind to look at a planet’s magnetospheric response during such extreme conditions.
— D. M. Weigt

This is NOT just a shadow blocking light. A magnetotail is a giant, invisible teardrop of plasma shaped by the Sun’s radiation. Think of it like a windsock in a hurricane. The Salient Idea here is the ‘flapping’ motion. Because the solar wind is constantly changing, Jupiter’s massive tail flaps back and forth every 2 to 3 days. When Saturn is behind Jupiter, it gets repeatedly dunked in and out of this magnetic tail. Inside the tail, the environment is an incredibly empty and calm void. Outside, it is a chaotic blast of solar wind. Going from a calm, empty void back into a high-pressure solar storm is like stepping out of a quiet room straight into a hurricane. Scientists hoped this violent transition would energize particles enough to glow in X-rays, but Saturn’s atmosphere absorbed the punch without flashing.

The structure and movement of the tail are both determined by the variable solar wind dynamic pressure surrounding the jovian magnetosphere.
— Research Team

Auroras on Earth are created when our magnetic field funnels charged particles from the solar wind into our atmosphere, lighting up the sky. Saturn has brilliant ultraviolet (UV) auroras, but X-ray auroras require way more energy. Jupiter makes X-ray auroras by stripping electrons off volcanic sulfur from its moon Io. Saturn is dominated by water and oxygen from its icy moon Enceladus. The mystery is why Saturn’s magnetic field can’t seem to generate enough voltage to charge these heavier ions to X-ray levels. Even with the massive shock of exiting Jupiter’s tail, the energy just wasn’t there. This teaches us that not all auroras are built the same, and a planet’s moons completely change how its magnetic shield reacts to space weather.

The field potentials at Saturn are too low to sufficiently charge strip magnetospheric plasma… to generate observable X-ray ion aurora.
— Hui et al., cited in study

How do you measure something that isn’t there? It comes down to incredible precision and math. The team didn’t just snap a photo; they mapped a grid over the Chandra telescope’s detector to count individual X-ray photons hitting the pixels where Saturn was supposed to be. They had to filter out background noise caused by high-energy cosmic rays streaking through space. They calculated the ‘Signal-to-Noise’ ratio and found it was too low—the photons from Saturn were no higher than the random background radiation of space. Instead of giving up, they calculated an ‘upper limit’—the absolute maximum amount of X-rays Saturn *could* be producing without us seeing it. This sets the baseline for future, more powerful telescopes like Athena and Lynx to finally crack the case.

With the non-detection of Saturn throughout each of the observations, our analysis suggests that even with such a variable external driver, the dramatic compressions are still not enough.
— D. M. Weigt

Q: If scientists didn’t find X-rays, does that mean Saturn has no auroras at all?
A: Not at all! Saturn has massive, beautiful auroras that glow in ultraviolet (UV) and infrared light. X-rays require a much more violent, high-energy process, which seems to be missing on Saturn.