Mercury's Secret X-Ray Auroras

Using powerful supercomputer simulations, scientists have confirmed for the first time how the solar wind creates ghostly, invisible auroras made of X-rays on the surface of Mercury.

For years, scientists have puzzled over strange X-ray emissions detected from Mercury by NASA’s MESSENGER spacecraft. They suspected these were a type of aurora, but the exact cause was a mystery. Now, a team of researchers led by Federico Lavorenti has provided the answer using a massive 3D computer simulation. Their model, which is the first to track individual electrons on a planetary scale, shows exactly how the solar wind—a stream of charged particles from the Sun—is responsible. When these electrons are captured by Mercury’s weak magnetic field, they get accelerated to incredible speeds. They then slam into the planet’s rocky surface, causing the atoms in the rock to release energy as X-rays. This process creates an ‘aurora’ not in an atmosphere, but on the solid ground itself, providing a clear explanation for the ghostly glow MESSENGER saw.

Read the original research paper on arXiv: ‘Solar-wind electron precipitation on weakly magnetized bodies: the planet Mercury’

We’ve shown for the first time, using a numerical approach, that solar-wind electrons are the source of Mercury’s X-ray auroras.
— Federico Lavorenti, Lead Researcher

Think of Mercury’s magnetic field as a leaky shield. It’s not strong enough to block all of the incoming solar wind like Earth’s field does. Instead, it acts more like a funnel or a slingshot. It captures some of the electrons from the solar wind and channels them towards the planet. As the electrons spiral down the magnetic field lines, they get a massive energy boost, accelerating to about 100 times their initial energy. This is a crucial difference compared to a body with no magnetic field, like our Moon. The Moon gets hit by solar wind over its entire sun-facing side, but the particles arrive with low energy. On Mercury, the magnetic field focuses these super-charged electrons into specific zones, making their impact much more powerful and capable of generating X-rays. This ‘filtering and acceleration’ effect is what makes Mercury’s space environment so unique and dynamic.

Here on Earth, the Northern and Southern Lights are born when solar wind particles, guided by our powerful magnetic field, collide with oxygen and nitrogen atoms high in our atmosphere. Those atoms get excited and release their energy as visible light. But Mercury has no significant atmosphere to create a light show in the sky. Instead, the super-charged electrons crash directly into the rocky surface. The impact is so energetic that it excites the atoms in the planet’s crust—like silicon, magnesium, and calcium—causing them to emit X-rays. So while the underlying cause is the same (charged particles guided by a magnetic field), the result is totally different. Earth has atmospheric auroras you can see; Mercury has surface auroras that are completely invisible. This discovery highlights the critical role a magnetic field plays in creating auroral phenomena, whether in the sky or on the ground.

Mercury’s magnetosphere turns the planet’s surface into the screen for its own unique auroral light show.
— NorthernLightsIceland.com Science Team

To solve this puzzle, scientists couldn’t just watch Mercury—they had to build a virtual one inside a supercomputer. They used a fully-kinetic plasma model, a type of simulation so detailed it tracks the motion of billions of individual virtual electrons and ions as they interact with magnetic fields. The team ran two main scenarios. In one, the Sun’s magnetic field (called the Interplanetary Magnetic Field or IMF) pointed northward. In this case, the simulation showed electrons raining down on Mercury’s polar cusps. When the IMF was flipped southward, the model showed electrons hitting the planet’s night side near the equator. These predicted ‘hotspots’ of X-ray emission perfectly match the fragmented observations from past missions and give scientists a map of what to look for with future spacecraft, like the joint European-Japanese BepiColombo mission currently on its way to Mercury.

Q: Can we see Mercury’s auroras with a telescope?
A: No, you can’t. These auroras are made of X-rays, which are a high-energy form of light that is invisible to the human eye. We can only detect them using special X-ray telescopes on spacecraft orbiting the planet.

Q: Why are they called auroras if they’re invisible and on the ground?
A: They’re called auroras because the fundamental process is the same as Earth’s: energetic particles from the Sun are guided by a planet’s magnetic field and cause something to glow. The main difference is what’s being hit—our atmosphere versus Mercury’s rocky surface.

Q: Does this mean Mercury is radioactive?
A: No, not in the way we usually think of it. The X-rays are only generated when the solar wind is actively hitting the surface, a process called fluorescence. The rock itself isn’t radioactive; it’s just temporarily glowing in response to being bombarded by energetic electrons.

Q: Why is it important to study this?
A: Understanding Mercury helps us learn about the thousands of rocky exoplanets being discovered around other stars, many of which may have weak magnetic fields and thin atmospheres. Mercury is our closest natural laboratory for studying how these types of worlds survive in their stellar environments.