Jupiter's Auroras: Cosmic Chemical Thieves!

Decades after a comet crashed into Jupiter, scientists discovered its powerful auroras are actively scrubbing a specific chemical from the atmosphere, revealing that these beautiful light shows are also massive chemical factories.

Back in 1994, the world watched as fragments of Comet Shoemaker-Levy 9 spectacularly crashed into Jupiter. This cosmic collision was more than just a fireworks display; it delivered a cocktail of new chemicals, including carbon monoxide (CO) and hydrogen cyanide (HCN), into the gas giant’s stratosphere. Scientists have been tracking these chemicals ever since, using them as tracers to understand Jupiter’s winds and chemistry. Fast forward to 2017. Using the powerful ALMA telescope, researchers mapped these two molecules with stunning detail. The results were puzzling. The CO had spread out evenly across the entire planet, just as expected. But the HCN was a different story. In the regions around Jupiter’s north and south poles, where the brilliant auroras dance, the HCN had almost completely vanished. It was a cosmic mystery: two chemicals delivered together were now behaving in completely different ways.

Read the original research paper: ‘Evidence for auroral influence on Jupiter’s nitrogen and oxygen chemistry revealed by ALMA’

Seeing CO spread so uniformly confirmed our models, but the massive depletion of HCN in the auroral regions was a total surprise.
— T. Cavalié, Lead Researcher

Imagine dropping two different colored dyes into a swimming pool. You’d expect them both to spread out and mix evenly over time. That’s what scientists thought would happen with CO and HCN in Jupiter’s stratosphere. Both were deposited at similar altitudes by the same comet impact. The fact that CO is now found everywhere from the equator to the poles tells us that Jupiter’s high-altitude winds are very effective at mixing things up. This makes the disappearance of HCN even weirder. If the winds are mixing everything, why is there a giant hole in the HCN distribution right over the poles? A simple ‘dynamical barrier’ or wind pattern can’t be the answer, because it would block CO as well. The solution had to be chemical, and it had to be something happening only at the poles.

The prime suspect? Jupiter’s incredibly powerful auroras. Just like on Earth, auroras are created when energetic particles from a planet’s magnetosphere slam into atmospheric gases. But on Jupiter, this process is supercharged. The paper proposes that this intense energy drives the formation of complex organic molecules, which then clump together to form aerosols — essentially a fine, high-altitude haze or smog. This is where the story takes a turn. The researchers believe that HCN molecules are ‘sticky’ and readily bond to the surface of these auroral aerosol particles. In contrast, the more stable CO molecules do not. Once HCN is locked onto these heavier aerosol particles, they slowly sink deeper into the atmosphere, effectively removing, or ‘scrubbing’, the HCN from the upper layers where ALMA can observe it. The aurora isn’t just a light show; it’s an active chemical trap!

We propose that heterogeneous chemistry bonds HCN on large aurora-produced aerosols… causing the observed depletion.
— The Research Team

This discovery was made possible by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. ALMA isn’t one telescope, but an array of 66 high-precision radio antennas working together. This allows it to act like a single, giant telescope, achieving incredible resolution. By tuning to the specific frequencies (or colors) of light emitted by CO and HCN molecules, ALMA can create detailed maps of their location and abundance. The key was its ability to resolve Jupiter’s disk and isolate the polar regions from the rest of the planet. The team analyzed the lineshape of the signal, which reveals the vertical distribution of the gas—telling them not just *if* the chemical was present, but at what altitude. By combining this with temperature data from the Gemini telescope, they could confidently confirm that the HCN wasn’t just hiding; it was truly gone from the upper stratosphere in the auroral zones.

Q: What is hydrogen cyanide?
A: Hydrogen cyanide (HCN) is a simple molecule made of hydrogen, carbon, and nitrogen. While it’s toxic on Earth, it’s a common building block for more complex organic molecules found throughout space, especially in comets.

Q: Why doesn’t carbon monoxide (CO) get trapped too?
A: Carbon monoxide is a very stable and less reactive molecule. The leading theory is that its chemical properties don’t allow it to easily bond to the surface of the organic aerosol particles in the way that HCN can. It simply bounces off while the HCN gets stuck.

Q: Are Jupiter’s auroras like the ones on Earth?
A: They are created by a similar process—charged particles hitting the atmosphere—but Jupiter’s are on a completely different scale. They are thousands of times more energetic and are mainly driven by Jupiter’s immense magnetic field and particles from its volcanic moon, Io. Earth’s auroras are primarily driven by the solar wind.

Q: So the auroras are both destroying and creating HCN?
A: It’s a fascinating paradox! The research suggests that in the upper layers, auroral aerosols are removing HCN. However, deep inside the main auroral oval, there’s evidence that the same energetic particles are creating *new* HCN from nitrogen gas welling up from below. It’s a complex cycle of creation and destruction happening in the same region.