The radiation belt is present about 50% of the time when Mercury is farther away from the sun in its oblong orbit, and about 20% of the time at closer distances. Each belt typically lasts less than 8–12 hours but can persist for several Earth days, the researchers say.

Discovered by analyzing interference with instruments on the MESSENGER probe, the findings could help scientists time operations in future missions to Mercury, including BepiColombo, a joint European and Japanese mission scheduled to enter Mercury's orbit this November.

"When we think about the future exploration of space and what instruments we send to Mercury, we'll want to be aware of this radiation belt and take the right sort of precautions and design around it," said Ryan Dewey, U-M assistant research scientist in climate and space sciences and engineering and co-first author of the study, which was published in Nature Astronomy.

Mercury is no longer an exception

All the other planets in our solar system with magnetic fields—Earth, Jupiter, Saturn, Uranus and Neptune—have radiation belts. Instruments that must pass through these belts are designed with radiation shielding, and crewed missions minimize astronauts' time in the belts.

But scientists didn't think that Mercury's relatively weak magnetic field could trap radiation because it is constantly torn apart by the solar wind, the stream of electrically conductive gas that emanates from the sun. Since Mercury is so close to the sun, the solar wind pressure is six to 30 times greater than at Earth, and the sun's magnetic field—which can break planetary magnetic fields—is four to 10 times stronger.