Summary
By the end of this article, you will understand how an icy rock hurtling through space generates its own aurora, and how solar wind causes cometary atmospheres to glow in the dark.
Quick Facts
Surprise: Comets can have auroras even though they have zero magnetic field.
Salient Idea: The aurora on Comet 67P shines in far-ultraviolet light, which is completely invisible to the human eye.
Surprise: In the comet's southern hemisphere, the aurora is fueled by carbon dioxide, not water.
Surprise: The glowing light flares up when high-speed streams of solar wind hit the comet.
The Discovery: Finding Light in the Dark
During the amazing Rosetta mission, scientists pointed the Alice ultraviolet spectrograph at the shadowed, night side of Comet 67P. They expected to see nothing but pitch blackness. Instead, they found a Surprise: the comet was glowing brightly in far-ultraviolet light. Because the comet’s surface was completely in the shadows, this light couldn’t just be a simple reflection of the sun. The team realized they were looking at something spectacular: a true comet aurora. On Earth, we see auroras near the poles, but this glow was scattered all across the comet’s gassy envelope, known as the coma. They had discovered a brand new type of cosmic weather happening right in our solar neighborhood, proving that auroras aren’t just for planets.
Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P
The FUV emissions are auroral in nature.
— Research Team
The Science Explained Simply
This is NOT like the Northern Lights on Earth. On Earth, a powerful magnetic field funnels charged particles gracefully toward the north and south poles. Comets are completely unmagnetized. When high-speed electrons from the solar wind hit the comet’s gas cloud—which is mostly carbon dioxide in its southern hemisphere—they crash straight in. The Salient Idea here is a process called dissociative excitation. The electron acts like a wrecking ball, hitting the carbon dioxide molecule so hard that it breaks apart. The broken oxygen and carbon fragments are left energized, and to calm down, they release a flash of ultraviolet light. It is a permanent, chaotic crash-zone creating a diffuse bubble of light around the comet.
The Aurora Connection
Earth’s beautiful, ribbon-like Northern Lights are a product of our planet’s magnetic shield. Because Comet 67P lacks this shield, its aurora looks much more like the diffuse auroras found on Mars, where the solar wind slams directly into an unprotected atmosphere. By studying the aurora on Comet 67P, we get a front-row seat to how the solar wind behaves when there are no defenses in place. During events called Corotating Interaction Regions—powerful gusts of solar wind—the comet’s aurora flares up dramatically. It shows us that auroras are the universe’s way of making invisible space weather visible, teaching us about atmospheric survival and planetary protection.
These emissions are driven by electrons which have been accelerated on large scales rather than locally heated.
— Dr. Marina Galand
A Peek Inside the Research
How do we know the solar wind is pulling the trigger? It comes down to incredible Knowledge and Tools. The researchers couldn’t just rely on one camera. They used a multi-instrument analysis to build a complete picture of the comet. While the Alice spectrograph watched the flashes of ultraviolet light, another sensor called RPC/IES was physically counting the exact number and energy of the electrons hitting the comet. Meanwhile, the ROSINA mass spectrometer ‘sniffed’ the local gas to prove carbon dioxide was the main target. By perfectly lining up the spikes in electron counts with the spikes in ultraviolet brightness, the team mathematically proved that the electrons were causing the glow.
The close correlation observed between the FUV auroral brightness and the electron flux allows spectroscopy to be used as a measure of solar wind.
— Lead Researchers
Key Takeaways
Solar wind electrons act like hammers, breaking apart comet gas to create light.
Without a magnetic field to guide them, cometary auroras form a diffuse, shapeless glowing bubble.
Multi-instrument spacecraft analysis is required to match electron spikes to flashes of light.
Auroras are a universal sign of space weather interacting with planetary atmospheres.
Sources & Further Reading
Frequently Asked Questions
Q: What color is the comet’s aurora?
A: If you were standing next to the comet, you wouldn’t see it! It glows in far-ultraviolet light, a wavelength that is completely invisible to human eyes but easily seen by specialized space cameras.
Q: Why does the aurora behave differently in the southern hemisphere?
A: The comet has distinct seasons and different ice compositions. In the southern hemisphere, the comet outgasses mostly carbon dioxide, whereas the northern hemisphere outgasses mostly water. Different gases create different light patterns when smashed by electrons.
