Solar activity had been building toward a peak in its 11-year cycle when, in May 2024, Earth experienced its most powerful geomagnetic storm in more than two decades. Two European Space Agency spacecraft positioned around Mars were watching — and what they recorded was unprecedented.
The same solar superstorm that struck Earth on May 11, 2024, generating auroras as far south as Mexico, simultaneously hit Mars. According to the report, the Mars Express and the ExoMars Trace Gas Orbiter (TGO) absorbed the equivalent of 200 days’ worth of radiation in just 64 hours.
“The impact was remarkable: Mars’ upper atmosphere was flooded by electrons,” said Jacob Parrott, ESA Research Fellow and team leader. “It was the biggest response to a solar storm we’ve ever seen at Mars.”
What the Numbers Show
The storm drove electron counts in two distinct atmospheric layers to levels planetary scientists have never previously recorded at Mars. At 68 miles (110 kilometers) above the surface, electrons increased by 45%. At 81 miles (130 kilometers), the surge reached 278% — a figure with no prior equivalent in the observational record for the Red Planet.
Both orbiters also experienced computer errors during the event. The charged particles involved are, as Parrott noted, “so energetic and hard to predict.” The spacecraft recovered quickly, having been built with radiation-resistant components and dedicated error-detection systems.
A Technique Decades Old, Applied in a New Way
To measure these atmospheric changes, the team used radio occultation — a method that has existed for decades but has only been applied between two Mars-orbiting spacecraft for roughly the past five years. The process involved Mars Express transmitting a radio signal toward the TGO as it passed below the Martian horizon. The signal refracted off atmospheric layers and returned data revealing the structure and electron density of those layers.
“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 missions. “It’s great to see it in action.”
The findings also highlight a fundamental difference between Mars and Earth under solar bombardment. Earth’s magnetosphere deflects and partially absorbs incoming charged particles, channeling them toward the poles where they produce auroras. Mars has no such shield. That absence makes direct comparisons between the two planets difficult — and it makes the Martian atmosphere considerably more exposed to the full force of space weather events.
The timing of having both orbiters operational and correctly positioned during the largest solar storm in over 20 years was, in the team’s own words, “extremely lucky.” The data collected gives researchers their clearest picture yet of how an unshielded planet responds to extreme solar activity — information that bears directly on efforts to build more reliable space weather prediction systems for future missions.
The findings have been published, with the research team’s next focus directed at refining space weather prediction models using the electron density data captured during the event.
Photo by Pixabay
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