Planets with a sufficiently strong magnetic field trap high-energy charged particles around them in their “radiation belts”. To better understand the mechanisms of formation of planetary radiation belts, a team led by the University of Berkeley and including scientists from CNRS Terre & Univers turned to Ganymede, the largest moon in the Solar System, in orbit around Jupiter. This satellite has a magnetic field sufficient to create a mini magnetosphere nested within that of Jupiter, but can it have its own radiation belts?
To find out, the researchers dug up the data acquired by NASA’s Galileo probe during its flyby of Ganymede on May 20, 2000. After a new analysis of the energetic electrons captured by the probe, the researchers showed that the particle flows reached a maximum in the direction perpendicular to the local magnetic field. This type of distribution is called “pancake”. The result shows that the electron pancakes come from electrons trapped around Ganymede, confirming the existence of radiation belts around the icy moon! After a few orbits of Ganymede, particles from Jupiter’s magnetosphere can return to the latter or impact the surface of the moon.
This new analysis of old Galileo data tells us what the ESA JUICE mission will be able to explore on its way to the Jupiter system. JUICE will be placed in a low polar orbit around Ganymede in 2033. It will then draw up a complete 3D map of the radiation belts of the icy moon and will study the underlying physical processes in detail to better understand this phenomenon discovered thanks to Galileo.
