International fleet of spacecraft captured coronal mass outburst

Rare ground-level enhancement

The European Space Agency (ESA) stated: “The event that took place on 28 October 2021 is an example of a rare ‘ground level enhancement.’”

The agency explained that during such circumstances, particles from the Sun are energetic enough to pass through the magnetic bubble that surrounds Earth and protects us from less energetic solar outbursts. 

“This was only the 73rd ground-level enhancement since records began in the 1940s, and none have been recorded since,” ESA added. 

Since the Moon and Mars lack their own magnetic fields, they are more susceptible to direct contact with solar particles. Further, these particles interact with the soil on these celestial bodies leading to the generation of secondary radiation.

However, Mars's thin atmosphere provides some protection by stopping lower-energy solar particles and slowing down the more energetic ones.

The research said that extreme SEP events can induce much higher (∼100 times) radiation doses on the Moon than on Mars.

Studying solar particles

ESA stresses the importance of understanding these solar events and their potential impacts on humans as the Moon and Mars are noted as the focus of future human exploration.

If astronauts in future expeditions encounter these solar particles, they could potentially face the risk of radiation sickness. 

“A radiation dose above 700 milligray – the unit for the absorption of radiation – may induce radiation sickness via destruction of the bone marrow, resulting in symptoms such as infection and internal bleeding,” stated ESA, “If an astronaut receives more than 10 gray, they are extremely unlikely to survive more than two weeks.”

Upon measuring the dose in lunar orbit, Nasa’s Lunar Reconnaissance Orbiter revealed 31 milligrays. 

The scientist Jingnan Guo said, “Our calculations of the past ground-level enhancement events show that on average one event every 5.5 years may have exceeded the safe dose level on the Moon if no radiation protection had been provided.”

She accentuated that understanding these events is crucial for future crewed missions to the Moon’s surface.

Marco Pinto, an ESA research fellow working on radiation detectors, also emphasized the need to study the impacts of solar energetic particles. 

“Currently, we live in a golden age of Solar System physics. Radiation detectors aboard planetary missions such as BepiColombo, on its way to Mercury, and Juice, cruising to Jupiter, add a much-needed coverage to study the acceleration and propagation of solar energetic particles,” Pinto said.

The study was published on 2 August 2023 in the journal Geographical Research Letters.

Abstract:

On 28 October 2021, solar eruptions caused intense and long-lasting solar energetic particle (SEP) flux enhancements observed by spacecraft located over a wide longitudinal range in the heliosphere. SEPs arriving at Earth caused the 73rd ground level enhancement (GLE) event recorded by ground-based neutron monitors. In particular, this is also the first GLE event seen on the surface of three planetary bodies, Earth, Moon, and Mars, by particle and radiation detectors as shown in this study. We derive the event-integrated proton spectrum from measurements by near-Earth spacecraft and predict the lunar and martian surface radiation levels using particle transport models. Event doses at the lunar and martian surfaces of previous GLE events are also modeled and compared with the current event. This statistical and comparative study advances our understanding of potential radiation risks induced by extreme SEP events for future human explorations of the Moon and Mars.