Two Spacecraft Accidentally Caught A Solar Superstorm As It Broke Records On Mars

The Same Storm That Lit Up European Skies Was Breaking Galactic Records

In early May 2024, millions of people stepped outside to catch a glimpse of auroras blazing over London, Spain, and the Mediterranean. A byproduct of the most intense geomagnetic storm to strike Earth since 1989, the light show had people stopping in their tracks. What almost nobody realized was that the same solar eruption was simultaneously hammering Mars, and two European spacecraft were already in place to catch what happened next.

The spacecraft recorded a layer of charged particles high in the Martian atmosphere registered the largest spike in density ever documented for that region, ballooning to nearly three times its normal size within minutes of a powerful solar flare striking the planet. Nobody planned it that way. A routine measurement just happened to be running at the right moment, roughly 10 minutes after the flare arrived.

That coincidence handed scientists their sharpest look yet at what a solar superstorm actually does to another planet’s atmosphere. Published in Nature Communications, the findings also suggest that current models may underestimate how strongly solar flares can boost Mars’ lower ionosphere.

How Two Spacecraft Caught a Mars Solar Storm by Accident

Since 2020, researchers have been running experiments using two Mars orbiters, Mars Express and the ExoMars Trace Gas Orbiter. Rather than bouncing radio signals off Earth-based antennas, the technique has the two spacecraft beam signals directly to each other as they orbit on opposite sides of the planet. As the signal cuts through Mars’ atmosphere, tiny changes in its frequency reveal how densely packed the electrons are at different altitudes. Scientists call this mutual radio occultation, and by May 2024 the campaign had completed 124 measurements.

On the morning of May 15, one of those routine passes was already underway when an X3-class solar flare, among the more powerful categories of solar eruption, arrived at Mars and began rearranging the upper atmosphere. Both spacecraft were already listening.

“This resulted in the largest lower ionospheric layer ever recorded, where it was 278% its typical size,” the authors write.

Mars’ Upper Atmosphere Shattered Records During the Solar Storm

Mars has an ionosphere, a zone where solar radiation knocks electrons loose from gas molecules and creates a band of electrically charged particles. Earth has one too, and it’s what makes long-distance radio communication possible. Mars’ version has two main sub-layers at different altitudes, and both reacted to the storm, though in very different ways.

Called M1, the lower of the two sits roughly 90 to 110 kilometers above the surface. Above it, starting around 130 kilometers up, is the M2 layer. During the May 2024 storm, M1’s density surged by 278%. M2 also grew, but by a comparatively minor 45%. Both layers rose about 6.5 kilometers higher than usual, a sign that the storm had heated the lower atmosphere considerably, most likely due to particle precipitation during the days of CME disturbance leading up to the flare.

Three solar events compounded on each other to produce what the instruments captured. A coronal mass ejection, a massive eruption of solar plasma, struck Mars about 26 hours before the measurement. A wave of high-energy solar particles followed around 16 hours later. Then the X3 flare itself hit almost simultaneously with the observation window.

Scientists’ Mars Solar Storm Models May Be Missing a Key Factor

Researchers have long used models to estimate how a solar flare’s intensity should translate into atmospheric changes on Mars. One established framework predicted that a density surge as large as 278% should require a more than sevenfold increase in solar X-ray flux to produce it.

When the team cross-checked actual flux data from NASA’s MAVEN spacecraft and a NOAA satellite, the number came in far lower. X-ray flux at Mars only tripled during the flare, well short of what the older model would have demanded.

So how did M1 swell so dramatically on roughly three times the usual energy? Researchers believe secondary ionizations are the missing piece. When a high-energy photon slams into an atmospheric molecule, it doesn’t just ionize that one molecule; it can kick off a chain reaction of additional collisions. As the flare pushed the solar spectrum toward higher-energy X-rays, a process called spectrum hardening, each photon appears to have triggered a far bigger cascade than models had accounted for. Authors estimate the number of ion-electron pairs produced per photon climbed by at least 2.58 times.

“We propose that the contribution of secondary ionisations to the ionosphere have been underestimated,” the team writes.

For researchers who model space weather across the solar system, that gap between prediction and observation is worth taking seriously.

Why This Mars Solar Storm Data Matters for Future Astronauts

Mars is no longer just a scientific target. NASA, ESA, and private aerospace companies are actively building toward crewed missions, and any astronauts on the surface will need to contend with exactly this kind of space weather. Storms capable of tripling ionospheric density could potentially interfere with radio signals or radar systems used on the surface, and the broader space-weather risks they represent are ones mission planners are only beginning to quantify.

At 278%, one of the most extreme data points mission planners now have on record comes from a storm that also lit the skies over southern Europe with auroras. Beyond crew safety, the data give engineers a more grounded basis for designing communication systems for landers, rovers, and surface habitats.

The mutual radio occultation campaign was built to run at a near-weekly cadence for exactly this reason: to keep the odds high that a rare solar event would fall within an active measurement window. In May 2024, that bet paid off.

Millions of people on Earth looked up at aurora-lit skies and saw the sun’s power written across the atmosphere. Eleven light-minutes away, two spacecraft were running a routine pass and caught something nobody had seen before at Mars. What they recorded may shift how scientists model the next one.

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