Mercury does indeed possess a radiation belt formed by energetic electrons. Its existence had remained debated since the first measurements by Mariner 10 in the 1970s. A new analysis now reveals a ring-shaped structure around the planet, whose stability depends directly on the intensity of the solar wind received by the planet.
Radiation belts are regions where charged particles remain trapped by a magnetic field. They notably surround Earth. Mercury, however, has a much smaller magnetosphere, that is, the region dominated by its magnetic field. This reduced size made the long-term presence of such a belt uncertain.
Distributions and configuration of the magnetic field under different solar wind pressures.
The team combined indirect measurements from the MESSENGER probe, statistical methods, particle simulations, and physical models. MESSENGER studied Mercury between 2011 and 2015. Its instruments did not directly measure all the energetic electrons, but their effects made it possible to reconstruct their presence and distribution.
The results reveal populations of electrons capable of orbiting stably around Mercury. This belt appears about half the time when the planet is near aphelion, the point farthest from the Sun. Its lifetime often remains shorter than 8 or 12 hours, but it can sometimes persist for several Earth days.
The solar wind strongly controls this stability: this flow of particles from the Sun compresses and agitates the magnetosphere. When its action remains weak, electrons remain trapped longer. When it becomes strong, their trajectories separate near the magnetic equator. The particles then quickly escape the belt.
Mercury's highly elliptical orbit accentuates this phenomenon. The planet experiences a different solar wind pressure depending on its distance from the Sun. Near perihelion, its closest point to the Sun, electron losses increase. Near aphelion, the presence of a belt becomes more probable and more stable.
This discovery makes Mercury a natural laboratory for studying radiation belts under extreme conditions. The BepiColombo mission, which is to begin its scientific observations around Mercury in 2027, will be able to examine this structure with two probes and instruments better suited to energetic particles.