For many years, Mercury was thought to be the only planet with a magnetic field that did not have radiation belts like Earth’s famous Van Allen belts.
However, a new study has overturned that belief, showing that Mercury can briefly develop its own radiation belts under the right conditions.
The research, carried out by scientists from the University of Michigan Engineering and the University of California, Berkeley, was published in Nature Astronomy.
The discovery could help scientists better protect spacecraft and scientific instruments during future missions to Mercury, including the European and Japanese BepiColombo mission, which is expected to enter Mercury’s orbit this November.
Radiation belts are regions where energetic charged particles become trapped by a planet’s magnetic field. Earth has two permanent radiation belts that can interfere with satellites and pose risks to astronauts.
Other magnetic planets in our solar system, including Jupiter, Saturn, Uranus, and Neptune, also have radiation belts. Until now, Mercury was believed to be the one exception.
Scientists originally thought Mercury’s magnetic field was simply too weak to hold on to these particles. Mercury orbits much closer to the sun than Earth, exposing it to a much stronger solar wind.
The solar wind is a constant stream of electrically charged particles flowing outward from the sun. Near Mercury, this powerful flow places much greater pressure on the planet’s magnetic field and was expected to sweep away any trapped radiation before it could build up.
The new study shows that this is not always the case. Researchers found that Mercury develops temporary radiation belts about half of the time when it is farther from the sun during its stretched, oval-shaped orbit.
When the planet is closer to the sun, radiation belts appear only about 20% of the time. Most of these belts last less than eight to twelve hours, although some can remain for several days.
The reason is linked to Mercury’s changing distance from the sun. When the planet moves farther away, the solar wind becomes less intense. This allows Mercury’s magnetic field to expand, creating more room to trap energetic particles. As Mercury moves closer to the sun again, the stronger solar wind compresses the magnetic field, causing the trapped radiation to escape into space or fall onto the planet.
The discovery came from a fresh look at data collected by NASA’s MESSENGER spacecraft, which orbited Mercury between 2011 and 2015. One of MESSENGER’s instruments was designed to study Mercury’s surface by detecting gamma rays and neutrons. Occasionally, however, the instrument recorded unexpected X-rays produced when energetic radiation struck its metal casing.
At first, these signals appeared to be little more than background noise. By carefully examining when and where they occurred, the researchers realized they were actually evidence of temporary radiation belts surrounding the planet. They then compared the observations with computer models that simulated how charged particles move inside Mercury’s magnetic field. The models closely matched what the spacecraft had detected.
The findings are important for future space exploration. Although scientists still do not know exactly how powerful Mercury’s radiation belts are, they now know these regions exist and appear under predictable conditions. This information can help mission teams decide when to switch off delicate instruments to avoid possible damage and when to activate instruments designed to study the radiation itself.
The upcoming BepiColombo mission will provide the first direct measurements of Mercury’s radiation belts. Those observations should reveal how strong the radiation really is and help scientists better understand not only Mercury but also planets orbiting very close to other stars.
The discovery also shows that Mercury remains an important natural laboratory for studying how planets interact with the harsh environment created by their parent stars.
