Mars orbiters witness solar superstorm striking the Red Planet: 'The timing was extremely lucky'

Two European Space Agency Mars-orbiting missions, the Mars Express and ExoMars Trace Gas Orbiter spacecraft, watched as a powerful solar "superstorm" that ravaged Earth also struck the Red Planet. The storm caused glitching spacecraft and a super-charged upper Martian atmosphere.

The solar storm slammed the Earth on May 11, 2024, proving to be the biggest recorded on our planet for over 20 years. It generated bright auroras, usually restricted to our planet's poles, as far toward the equator as Mexico. And indeed, the storm also impacted Mars, with the Mars Express and the ExoMars Trace Gas Orbiter (TGO) belted with 200 days' worth of radiation in the space of just 64 hours.

"The impact was remarkable: Mars' upper atmosphere was flooded by electrons," ESA Research Fellow and team leader Jacob Parrott said in a statement. "It was the biggest response to a solar storm we've ever seen at Mars."

The solar storm increased the number of electrons in two layers of the Martian atmosphere: A 45% increase at an altitude of 68 miles (110 kilometers) and a whopping 278% increase at 81 miles (130 kilometers) above the Martian surface. That represents the most electrons planetary scientists have ever seen in the atmosphere of the Red Planet.

The solar superstorm also demonstrated the negative impact that such space weather can have on space technology, a risk that drives the desire of researchers to develop better space weather prediction.

"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," Parrott continued. "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."

Parrott and colleagues used a pioneering technique called radio occultation to investigate the effect of this solar storm. This involved Mars Express beaming a radio signal to the TGO as it dipped below the horizon of Mars. This signal was then refracted back at the Mars Express by the atmospheric layers of the Red Planet. This revealed details of these layers.

"This technique has actually been used for decades to explore the solar system, but using signals beamed from a spacecraft to Earth," team member Colin Wilson, ESA project scientist for Mars Express and TGO, said. "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."

What this revealed was the fact that Earth and Mars respond very differently when under bombardment from charged particles from the sun.

Solar storms impact Mars and Earth differently

The key difference between Mars and Earth in their reaction to solar storms is the fact that our planet has a protective magnetic field, the magnetosphere. This mutes the impact of solar storms on our atmosphere and also diverts charged particles both away from Earth and toward the poles, where they can cause auroras.

The lack of a magnetosphere around Mars makes it hard to compare the Red Planet to Earth. The study of space weather around our neighboring planet is also complicated by the fact that the sun is erratic in its dispersal of charged particles and radiation.

"Fortunately, we were able to use this new technique with Mars Express and TGO just 10 minutes after a large solar flare hit Mars," Jacobs explained. "Currently, we're only performing two observations per week at Mars, so the timing was extremely lucky."

The scientists looked at the aftermath of three solar events that were part of the same overall storm: a flare of radiation, a blast of high-energy charged particles, and material launched by a coronal mass ejection (CME).When the radiation and material from these events hit the Martian atmosphere, electrons were ripped away from neutral atoms, causing these negatively charged particles to fill the atmosphere in numbers never before recorded.

The team's study of the event could also help us understand how the Red Planet became an arid and barren landscape.

"The results improve our understanding of Mars by revealing how solar storms deposit energy and particles into Mars' 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," Wilson added. "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' 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."

The team's results were published on Thursday (March 5) in the journal Nature Communications.