What are lunar swirls?

The Moon is host to a type of bright features called swirls which are magnetic.

The Moon is host to a type of bright features, quite glaringly called swirls. Seen below is the lunar swirl of Reiner Gamma.

Beautiful and bright lunar swirl, Reiner Gamma. Credit: NASA LRO

Lunar swirls are brighter than their surroundings despite having the same composition. Unlike the rest of the lunar surface, the material within the swirls is not exposed to the constant weathering from solar wind and meteorites. This means the material remains fresh/bright for a long amount of time. But what protects the material from the harsh lunar environment? Local magnetic fields!

The Moon has lost its global magnetic field but it does have local areas that are magnetic. Interestingly, all swirls on the Moon are associated with a local magnetic field. These local magnetic fields provide protection to the surface from the solar wind radiation and help retain the bright, distinct look of swirls. On the other hand, regions besides the magnetic field lines get accelerated radiation weathering and are thus darker. How do such swirls form in the first place though? We don’t know.

To get an understanding of the swirl formation processes, we need to precisely measure the local magnetic field strength of regions within the swirl. We also need to measure the solar wind interaction with the swirl's magnetic field, and see how the local plasma environment is shaped by the magnetic field. A landing mission to Reiner Gamma or other swirls would be key to understanding these intriguing features and their origin.