Ten minutes. That’s all the warning Mars Express had before the X-rays arrived. On the morning of 15 May 2024, while half of Earth was still sharing aurora photographs on social media, the same solar storm that had lit up skies from London to Mexico was barrelling towards the Red Planet at the speed of light. When it got there, things went a bit sideways: both of ESA’s Mars orbiters started throwing computer errors, and the thin Martian atmosphere began swelling with electrons like it never had before.
“The impact was remarkable: Mars’s upper atmosphere was flooded by electrons,” says Jacob Parrott, a research fellow at the European Space Agency and lead author of a study published today in Nature Communications. “It was the biggest response to a solar storm we’ve ever seen at Mars.”
How big, exactly? The lower ionospheric layer, a region about 110 kilometres up known to planetary scientists as the M1, ballooned to 278% of its typical electron density. A second layer roughly 20 kilometres higher grew by a more modest 45 percent. Both shifted upward by 6.5 kilometres in altitude, shoved bodily by heating that had been building for days. By any measure this was record-breaking; nothing in the existing catalogue of Martian ionospheric data comes close for the M1 layer.
The storm itself was a triple act. First came a coronal mass ejection on 11 May, a broad eruption of magnetised plasma that took three days to cross the gap between the Sun and Mars. That was followed by a punishing burst of solar energetic particles, protons accelerated to a fair fraction of the speed of light, which peppered the upper atmosphere and likely left residual ionisation lingering for hours. Then the main event: an X3-class flare whose soft X-ray photons, travelling at light speed, slammed into Mars’s dayside and began stripping electrons off carbon dioxide molecules wholesale. TGO’s radiation monitor, meanwhile, racked up a dose equivalent to 200 normal days in just 64 hours.
“The storm also caused computer errors for both orbiters, a typical peril of space weather, as the particles involved are so energetic and hard to predict,” says Parrott. “Luckily, the spacecraft were designed with this in mind, and built with radiation-resistant components and specific systems for detecting and fixing these errors. They recovered fast.”
What makes this more than a neat anecdote about glitchy spacecraft is the technique that caught the storm in action. Since November 2020, ESA has been pioneering something called mutual radio occultation at Mars. The principle is straightforward enough: Mars Express fires a radio signal at TGO at exactly the moment one orbiter is vanishing behind the planet’s limb. As the signal dips through successive layers of atmosphere, each layer bends it a little differently, and from those distortions you can reconstruct a vertical profile of electron density. Sort of like shining a torch through fog and working out how thick it is from the way the beam spreads.
“This technique has actually been used for decades to explore the Solar System, but using signals beamed from a spacecraft to Earth,” says Colin Wilson, ESA’s project scientist for both orbiters and a co-author on the study. “It’s only in the past five years or so that we’ve started using it at Mars between two spacecraft, such as Mars Express and TGO, which usually use those radios to beam data between orbiters and rovers. It’s great to see it in action.”
The team have completed 124 mutual occultation measurements so far, yielding 74 usable electron density profiles. They only manage about two observations per week, so capturing a major solar event was always going to be largely down to luck. And lucky they were. “Fortunately, we were able to use this new technique with Mars Express and TGO just 10 minutes after a large solar flare hit Mars,” says Parrott. “Currently we’re only performing two observations per week at Mars, so the timing was extremely lucky.”
What the data revealed, though, goes beyond simply confirming that solar storms hammer Mars harder than Earth (which they do, Mars having no global magnetic field to deflect the incoming particles). The really interesting finding is that the M1 layer’s 278% enhancement doesn’t square with the established physics. Previous work on a larger X11-class flare suggested that the increase in soft X-ray flux should scale as the square of the density enhancement. By that logic, a 2.78-fold jump in electron density would require roughly an eightfold increase in flux. But MAVEN’s instruments, orbiting nearby, measured only about a threefold increase. Something else was going on.
Parrott and colleagues reckon the answer lies in secondary ionisations. When a solar flare hardens the X-ray spectrum, shifting it towards higher energies, the photoelectrons it creates carry more kinetic energy. Those energetic photoelectrons then collide with other molecules on their way down, triggering a cascade of additional ionisations that amplifies the original effect. The team estimate that the number of ion-electron pairs produced per photon increased by at least 2.58 times during the flare. It is, in a way, a chain reaction in the upper atmosphere, and it means we’ve been underestimating how efficiently flares can ionise the M1 layer.
At Earth, the whole drama played out rather differently. Our magnetic field shunted most of the incoming particles towards the poles, which is why Londoners got auroras rather than satellite blackouts. Mars has no such luxury. Without a global magnetic shield, the planet’s upper atmosphere takes the full brunt of every solar outburst, and over billions of years that steady bombardment has probably stripped away most of Mars’s original atmosphere and much of its water.
“The results improve our understanding of Mars by revealing how solar storms deposit energy and particles into Mars’s atmosphere, important as we know the planet has lost both huge amounts of water and most of its atmosphere to space, most likely driven by the continual wind of particles streaming out from the Sun,” says Wilson. But there is a practical wrinkle too. If a storm can pack Mars’s ionosphere with electrons this densely, those same electrons could interfere with the radar signals we use to probe the planet’s surface and subsurface. That is not a hypothetical concern; it’s the kind of thing mission planners will need to account for as we send more hardware, and perhaps eventually people, to Mars.
Study link: https://www.nature.com/articles/s41467-026-69468-z
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