NASA has announced what causes the moon to have its distinctive pattern of darker and lighter swirls. According to the agency, the Moon’s crustal magnetic fields and solar wind work together to give the moon its pattern, as seen in the image below. NASA used data from its ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun) mission to make the discovery.
The Sun has a continuous outflow of particles and radiation called the solar wind. Solar wind hits all the planets, moons, and other bodies in the solar system creating the heliosphere that extends far past the orbit of Pluto. Since the solar wind is magnetized, the natural magnetic field of the Earth deflects the solar wind around the planet allowing only a small fraction of it to reach the atmosphere of Earth.
The moon, on the other hand, has no global magnetic field. The Moon does have localized spots of magnetic fields due to magnetized rocks on its surface that produce local magnetic fields. Those fields are anywhere from hundreds of yards to hundreds of miles in size.
The small magnetic fields act as a type of sunscreen, says NASA, that deflects solar wind particles. The deflection is on a much smaller scale than on Earth. While these magnetic fields can’t protect astronauts from solar radiation, they can protect the material on the surface of the moon from the Sun’s particles by deflecting the solar wind to the outside of the magnetic field.
A chemical reaction with the surface of the moon, called regolith, causes the areas right outside the magnetic bubbles to darken. That darkening gives the Moon its pattern of light and dark areas.