In May 2024, Earth was struck by the most powerful solar storm in over two decades. The same storm reached Mars — and what happened there was extraordinary. A new study published in Nature Communications now reveals the full picture.
ESA's two Mars orbiters — Mars Express and the ExoMars Trace Gas Orbiter — were perfectly positioned to observe the event. The storm arrived in three waves: a radiation flare, a burst of high-energy particles, and a coronal mass ejection. Together, they bombarded Mars's upper atmosphere with magnetised plasma and X-rays, stripping electrons from neutral atoms and flooding the ionosphere. Electron densities at 110 km altitude jumped by 45%, and at 130 km by a staggering 278% — the highest ever recorded at Mars.
To measure this, researchers used a technique called radio occultation between two spacecraft: Mars Express beamed a radio signal to TGO just as it disappeared over the Martian horizon, and the atmospheric layers bent the signal in ways that revealed their composition. The storm also caused computer errors on both orbiters, though the spacecraft recovered quickly. The radiation dose recorded aboard TGO — equivalent to 200 normal days — arrived in just 64 hours.
On Earth, the magnetic field softened the blow and redirected particles toward the poles, producing auroras visible as far south as Mexico. Mars has no such shield, so it took the full force of the storm. This difference is more than academic: the relentless solar wind is the leading explanation for why Mars lost most of its atmosphere and water billions of years ago — and events like this one show that process is still ongoing.