Summary
Scientists using the Hubble Space Telescope created the first complete map of the aurora on Ganymede, Jupiter’s largest moon. They discovered its auroral lights aren’t complete ovals like Earth’s, but are split into two glowing crescents, a pattern unique in our solar system.
Quick Facts
Ganymede is the largest moon in our solar system, bigger than the planet Mercury.
It's the only moon known to have its own magnetic field.
Its aurora is created by glowing oxygen atoms, visible in ultraviolet light.
The auroral lights form two bright 'crescents' instead of a continuous ring.
This map was created using 46 observations from the Hubble Space Telescope over 19 years.
The Discovery: Mapping a Moon's Crescent Lights
For years, scientists knew Ganymede had an aurora, a faint glow powered by its unique magnetic field. But seeing the whole picture was impossible. Using a massive dataset of 46 observations from the Hubble Space Telescope spanning from 1998 to 2017, a team of researchers painstakingly stitched together the first-ever global brightness map of Ganymede’s ultraviolet aurora. The result was a huge surprise. Instead of a continuous oval of light at each pole, like the ones we see on Earth or even Jupiter, Ganymede’s aurora is distinctly broken. The map revealed two intensely bright auroral crescents on opposite sides of the moon, while the regions in between were dramatically dimmer. This structure had never been seen anywhere else and points to the strange and complex physics happening around Jupiter’s giant moon.
Read the original research paper on arXiv
Our map reveals Ganymede’s auroral ovals are structured in upstream and downstream ‘crescents’.
— Joachim Saur, Corresponding Author
The Science Explained Simply
Imagine Ganymede as a large rock in a fast-moving river. The ‘river’ is the plasma—a gas of charged particles—that fills Jupiter’s enormous magnetosphere and flows past Ganymede at incredible speed. The brightest parts of the aurora, the crescents, appear on the upstream side (where the plasma hits the moon head-on) and the downstream side (in its wake). This is where the interaction is most intense, accelerating particles into Ganymede’s thin oxygen atmosphere and making it glow. The sides of the ‘rock’ parallel to the flow—the flanks facing toward and away from Jupiter—experience a much weaker interaction. This causes the aurora to be 3 to 4 times fainter in these regions, creating the ‘broken’ or crescent shape. It’s a visual map of how Ganymede battles the constant stream of plasma from its parent planet.
The Aurora Connection
Auroras are the ultimate sign that a planet or moon has a magnetic field. Ganymede is the only moon in our solar system with one, creating what scientists call a mini-magnetosphere. This map of its broken aurora is a stunning visualization of that mini-magnetosphere in action. Unlike Earth’s global magnetic field which stands strong against the solar wind, Ganymede’s field is tiny and completely embedded within Jupiter’s colossal magnetosphere. The crescent shape shows us exactly where Ganymede’s magnetic field lines connect with Jupiter’s, creating channels for energetic particles to slam into its atmosphere. Studying this unique, ‘sub-Alfvénic’ interaction helps scientists understand the physics of magnetism on a smaller scale and provides clues about how moons can protect a fragile atmosphere even in the harshest environments.
This map will be useful to understand the processes that generate the aurora in Ganymede’s non-rotationally driven, sub-Alfvénic magnetosphere.
— The Research Team
A Peek Inside the Research
Creating this map was a cosmic puzzle. The researchers used the Space Telescope Imaging Spectrograph (STIS) on Hubble, which observes in ultraviolet light invisible to the human eye. Each of the 46 exposures only captured one hemisphere of Ganymede at a time. The science team had to precisely determine Ganymede’s position and orientation for each image, carefully subtract the glare of reflected sunlight from its icy surface, and correct for the viewing angle. They then projected each clean image onto a flat, global map, similar to how a map of Earth is made from satellite photos. By averaging all 46 maps together, weighted by their exposure time, they built up a complete, high-quality picture of the entire auroral system. This meticulous process turned nearly two decades of snapshots into the first definitive atlas of Ganymede’s alien auroras.
Key Takeaways
Ganymede's auroral ovals are not continuous rings like Earth's.
The brightest parts are two crescents on the sides facing into and away from the plasma flow from Jupiter.
The sides facing directly toward and away from Jupiter are 3-4 times fainter, creating a 'broken' appearance.
This unique shape is caused by the interaction between Ganymede's small magnetic field and Jupiter's giant one.
The map serves as a blueprint for understanding 'mini-magnetospheres' and their plasma interactions.
Sources & Further Reading
Frequently Asked Questions
Q: What color are Ganymede’s auroras?
A: Ganymede’s auroras glow primarily in ultraviolet (UV) light, which our eyes cannot see. The color comes from oxygen atoms in its thin atmosphere being excited by charged particles. If we could see in UV, they would likely appear as a purple or faint whitish glow.
Q: Why is Ganymede the only moon with a magnetic field?
A: Scientists believe Ganymede has a molten iron core, similar to Earth’s. The churning motion within this liquid metallic core generates a magnetic field. Other moons are either too small to have retained enough internal heat, or their core composition is different.
Q: Why is it important to map Ganymede’s aurora?
A: The aurora acts like a giant TV screen, showing us what’s happening in Ganymede’s invisible magnetic field and how it interacts with Jupiter. Mapping its brightness and shape helps scientists test their models of plasma physics and understand how this unique ‘mini-magnetosphere’ works.
Q: Will we get a closer look at these auroras?
A: Yes! The European Space Agency’s JUICE (JUpiter ICy moons Explorer) mission is on its way to the Jupiter system and will eventually orbit Ganymede. It carries instruments designed to study Ganymede’s magnetic field and aurora in unprecedented detail, giving us an up-close view of these amazing crescent lights.
