When the biggest solar storm in 20 years hit Mars, its atmosphere flooded with electrons

In May 2024, the Sun threw its biggest tantrum in over two decades. A barrage of X-ray flares, high-energy particles, and a massive coronal mass ejection swept through the inner solar system. On Earth, the storm lit up the sky with auroras visible as far south as Mexico. It also hit Mars.

Fortunately, two European Space Agency orbiters were watching. A new study in Nature Communications reveals what the superstorm did to the Red Planet -- and it was dramatic.

A 278% electron spike

When the storm reached Mars, it caused electron densities in two distinct layers of the upper atmosphere to surge. At around 110 km altitude, electron numbers rose by 45%. At around 130 km, the increase was 278% -- the highest electron density ever recorded in that layer of the Martian atmosphere.

"The impact was remarkable: Mars's upper atmosphere was flooded by electrons," said ESA Research Fellow Jacob Parrott, lead author. "It was the biggest response to a solar storm we've ever seen at Mars."

The mechanism is straightforward in principle. When fast-moving, energetic solar particles and X-rays slam into Mars's upper atmosphere, they collide with neutral atoms and strip away their electrons. Without Earth's strong magnetic field to deflect the incoming barrage, Mars's thin atmosphere absorbs the full force. The result: a sudden, massive ionization of the upper atmosphere.

Catching the storm with a new technique

How do you measure electron densities in another planet's atmosphere? Parrott and colleagues used a technique called radio occultation, which ESA has been pioneering for Mars in recent years. Mars Express beamed a radio signal to the Trace Gas Orbiter (TGO) at the precise moment TGO was disappearing over the Martian horizon. As the signal passed through successive layers of the atmosphere, it was bent -- refracted -- in ways that reveal the composition and density of each layer.

The technique has been used for decades to study planetary atmospheres, but traditionally using signals sent from spacecraft back to Earth. Using it between two spacecraft orbiting the same planet is relatively new. "It's only in the past five years or so that we've started using it at Mars between two spacecraft," said Colin Wilson, ESA project scientist for both Mars Express and TGO.

The timing was remarkably lucky. The team performs only about two observations per week, and one happened to fall just 10 minutes after a large solar flare reached Mars. NASA's MAVEN mission provided independent electron density measurements that confirmed the findings.

Glitching spacecraft, resilient design

The storm also caused computer errors aboard both ESA orbiters -- a routine hazard of space weather. Energetic particles can flip bits in spacecraft memory, causing software errors. Both Mars Express and TGO experienced these glitches but recovered quickly, thanks to radiation-resistant components and built-in error-detection systems.

A radiation monitor aboard TGO registered a dose equivalent to 200 normal days of exposure in just 64 hours. For human explorers, radiation events of this magnitude would pose serious health risks -- a consideration that looms over every proposal for crewed Mars missions.

Why Mars and Earth experienced the storm so differently

Earth's response to the same storm was more muted in the upper atmosphere, despite producing spectacular auroras. The difference comes down to magnetic fields. Earth's magnetic field deflects most incoming solar particles, channeling some toward the poles where they produce auroras. Mars, which lost its global magnetic field billions of years ago, has no such shield. Its thin atmosphere takes the full brunt.

This vulnerability is connected to one of the biggest questions about Mars: how the planet lost most of its atmosphere and water over geological time. The solar wind -- the continuous stream of charged particles from the Sun, punctuated by storms -- is a prime suspect. Understanding how individual storms deposit energy and particles into the Martian atmosphere helps quantify that ongoing process.

Implications for radar and communications

There is a practical dimension as well. When Mars's upper atmosphere fills with electrons, it affects how radio signals travel. Radar instruments that probe the planet's surface from orbit could be blocked by a heavily ionized atmosphere, and communications between orbiters, rovers, and future human missions could be disrupted.

"If Mars's upper atmosphere is packed full of electrons, this could block the signals we use to explore the planet's surface via radar, making it a key consideration in our mission planning," Wilson said.

Parrott and colleagues captured the aftermath of three distinct solar events, all part of the same storm but different in character: a radiation flare, a burst of high-energy particles, and a coronal mass ejection. Each deposited energy into the Martian atmosphere in different ways, providing a detailed picture of how different types of solar activity affect the planet.

Parrott began this work as an ESA Young Graduate Trainee, continued it at Imperial College London, and is now a Research Fellow at ESA's European Space Research and Technology Centre in the Netherlands.