Summary
By the end of this article, you will understand how astronomers use giant eclipses to spot glowing auroras on Jupiter’s moons, and what these light shows reveal about alien atmospheres.
Quick Facts
Surprise: Jupiter's moons have auroras, but you can only see them when the moons are hiding in Jupiter's shadow.
Surprise: Astronomers detected the first 'optical' (visible to the human eye) auroras on Ganymede and Callisto.
Salient Idea: The auroras act like chemical fingerprints—specific red and green glows prove the atmospheres are mostly oxygen gas.
Surprise: Past UV studies thought these moons had wet, water-filled atmospheres, but this new visible-light data shows almost no water.
The Discovery: Glowing in the Dark
To see a faint glow next to a glaring star, you need to turn off the lights. Astronomers used the Keck telescope in Hawaii to stare at Jupiter’s moons—Europa, Ganymede, and Callisto—exactly when they passed into Jupiter’s giant shadow. Stripped of harsh, reflected sunlight, the moons revealed a Surprise: they were glowing. These were the first visible-light auroras ever detected on Ganymede and Callisto. By measuring the exact colors of this light, scientists discovered these atmospheres are dominated by oxygen gas. The Salient Idea here is that eclipses aren’t just cool visual events; they are nature’s way of dimming the background so we can see the faintest secrets of the solar system.
Original Paper: ‘The Optical Aurorae of Europa, Ganymede and Callisto’
We present the first detections of Ganymede’s and Callisto’s optical aurorae… and place upper limits on hydrogen.
— Dr. Katherine de Kleer and team
The Science Explained Simply
This is NOT like Earth’s thick atmosphere where rain and weather happen. The atmospheres on these icy moons are incredibly thin—almost a vacuum. But Jupiter’s powerful magnetic field acts like a particle accelerator, slamming electrons into the moons’ surfaces. When these electrons hit gas molecules, the molecules get ‘excited’ and release light. We call this an aurora. The Salient Idea is that different gases glow in different colors. Oxygen glows red and green. If there were a lot of water vapor, we would see a strong hydrogen glow (a specific red line called H-alpha). Because the scientists saw intense oxygen lines but almost no hydrogen lines, they proved these atmospheres are mostly oxygen, debunking recent theories of water-dominated skies.
The simultaneous measurement of multiple emission lines provides robust constraints on atmospheric composition.
— The Research Team
The Aurora Connection
On Earth, our magnetic field pulls solar wind to the poles, creating the Northern and Southern Lights. But Jupiter’s moons sit inside Jupiter’s massive, spinning magnetic field. This means their auroras aren’t driven by the Sun, but by Jupiter itself! As the moons orbit, they plow through a sheet of plasma (charged particles) trapped by the giant planet. In fact, Europa’s aurora gets brighter and dimmer depending on how deep it is inside this plasma sheet. Understanding these alien auroras helps us understand how magnetic fields interact with atmospheres across the universe, either protecting them or slowly stripping them away.
Europa’s auroral brightness correlates with magnetic latitude… and variations in the electron density.
— The Research Team
A Peek Inside the Research
How do you measure the exact gases on a moon hundreds of millions of miles away? The team used a tool called a spectrograph (the HIRES instrument on Keck). A spectrograph splits light into a rainbow barcode. Each chemical element has a specific set of lines on this barcode. The researchers looked for the exact wavelengths of oxygen (like 6300, 5577, and 7774 Angstroms) and hydrogen. It takes immense patience. The team observed just ten eclipses over 23 years (1998 to 2021) to gather enough light. This meticulous Knowledge and Tool combination allowed them to separate true auroral light from cosmic rays and background noise.
These constitute the first detections of emissions at 7774 and 8446 Angstroms at a planetary body other than Earth.
— The Research Team
Key Takeaways
Observing moons during a Jupiter eclipse blocks out harsh sunlight, revealing faint atmospheric glows.
Different molecules (like oxygen or water) emit specific colors of light when hit by high-energy electrons.
Europa and Ganymede have extremely thin atmospheres made almost entirely of O2 (Oxygen).
Callisto's first visible-light aurora detection proves it also has an oxygen-rich atmosphere.
Sources & Further Reading
Frequently Asked Questions
Q: Could we breathe the oxygen on these moons?
A: No. Even though the atmosphere is made of oxygen, it is incredibly thin—billions of times thinner than Earth’s atmosphere. It is practically a vacuum!
Q: Why did previous studies think there was water?
A: Previous studies looked at ultraviolet (UV) light on the sunlit sides of the moons. This optical (visible light) study looked at the dark sides during an eclipse, providing a different set of ‘chemical fingerprints’ that strongly point to pure oxygen.
