Powerful superstorm gives rare glimpse into Mars’s atmosphere

A solar superstorm that struck Mars in 2024 has given scientists their most detailed view yet of how the Red Planet responds to extreme space weather.

The findings, published in Nature Communications, were shaped by analysis led at Imperial and enabled in part by data-processing improvements developed within the Department of Physics.

The event occurred in May 2024 as the Sun reached the peak of its 11-year activity cycle. Earth witnessed auroras visible as far south as London, Mexico and mainland Europe. That same intense pulse of radiation and solar material travelled across the Solar System and struck Mars shortly afterwards.

In a stroke of luck, ESA’s two orbiters, Mars Express and ExoMars Trace Gas Orbiter (TGO), were positioned perfectly to observe the planetary response. As both spacecraft crossed the Martian horizon, their instruments detected a surge of charged particles and severe disturbances in the upper atmosphere.

The storm produced dramatic increases in electron density in two atmospheric layers, resulting in the largest lower ionospheric layer ever recorded, at 278% its typical size. TGO’s radiation monitor also measured a dose equivalent to 200 normal days of exposure in just 64 hours.

Lead author Jacob Parrott, who carried out much of this work as a PhD student at Imperial, said,

“The impact was remarkable: Mars’s upper atmosphere was flooded by electrons. It was the biggest response to a solar storm we’ve ever seen at Mars.”

The intense radiation also caused temporary computer errors on both orbiters, although both spacecraft quickly recovered thanks to their radiation-hardened designs.

Capturing a rare planetary event

As the solar flare struck the Martian atmosphere, Mars Express transmitted a radio signal directly to TGO as it passed behind the planet’s horizon. The signal passed through layers of the upper atmosphere, bending in response to the charged environment before being received by TGO. The data allowed researchers to reconstruct the structure of Mars’s atmosphere immediately after the storm.

The measurement relied on a technique known as mutual radio occultation, developed at ESA. However, Imperial also played a key role in enabling this particular observation.

In earlier work, Jacob and his supervisor Professor Ingo Müller‑Wodarg optimised the sampling strategy used during measurements. By reducing the data volume required for each observation, they made it possible for ESA to conduct many more measurements per week, increasing the chance of capturing rare atmospheric events.

That optimisation proved decisive. Just minutes after a major solar flare hit Mars, the orbiters collected a measurement revealing a huge rise in electrons.

“At Imperial, the sampling frequency requirements were optimised so that each measurement didn’t take up as much hard‑drive space on the spacecraft. This led to an increase in the number of measurements we could take per week.” said Jacob,  

“Because of this increased cadence, we were able to get this ‘super‑lucky’ measurement just 10 minutes after a massive solar flare hit Mars.”

Professor Müller‑Wodarg added “When Jacob showed me the first measurements, I said ‘Now that’s a result! This is worth a paper just in itself.”

What the storm reveals about the red planet

Understanding how solar activity affects the Solar System is crucial, as intense events like solar storms can pose risks to astronauts and interfere with satellites and communications systems. However, the Sun releases radiation and energetic particles unpredictably, making direct measurements difficult to obtain. This is what makes the results from Jacob and his team particularly significant.

In the study, the researchers captured the aftermath of three solar events, a flare, a burst of high-energy particles and a coronal mass ejection, all part of the same storm. Analysing how each interacted with Mars’s atmosphere revealed clear differences in the way energy and particles were deposited.

The results show that Mars responds quite differently to solar activity than Earth. While Earth’s magnetic field deflects many incoming particles and channels others towards the poles, producing auroras, Mars is directly exposed to the solar wind. When the storm hit, fast-moving solar plasma and X-rays stripped electrons from neutral atoms in the upper atmosphere, rapidly filling the region with charged particles.

By comparing measurements from Mars Express and TGO with simultaneous observations from NASA’s MAVEN mission, the team was able to build a detailed picture of how the storm unfolded across the Martian atmosphere.

Colin Wilson, ESA project scientist for Mars Express and TGO, and co-author of the study, said “The results improve our understanding of Mars by revealing how solar storms deposit energy and particles into Mars’s atmosphere[...] But there’s another side to it: the structure and contents of a planet’s atmosphere influence how radio signals travel through space. 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 – and impacting our ability to investigate other worlds.”

Jacob and colleagues are now planning further analysis using the same approach to investigate the nightside of Mars, where the lower ionosphere has historically been difficult to observe.

They are also assessing the potential of reflectometry, which involves bouncing radio signals off the surface rather than transmitting them between orbiters, to study surface roughness or probe for subsurface ice. This remains at an experimental stage, but could offer another way of extracting additional science from instruments already in orbit.