The Moon’s Mysterious Magnetic Field

Today, the Moon has a very weak magnetic field, but researchers from the Massachusetts Institute of Technology (MIT) recently released a paper in Nature showing that the Moon used to have a much stronger magnetic field.

The core of the Earth is surrounded by a mixture of molten iron and nickel. The Earth’s magnetic field is caused by currents of electricity that flow in the core. Image credit: NASA.

In fact, they believe the Moon’s magnetic field was even stronger than the Earth’s. Planetary geologists have been trying to figure out what lies underneath the surface of the Moon for decades, but solid conclusions have eluded them. The Earth’s magnetic field is generated by the rotation of molten metal in the core creating a dynamo, but scientists had not established the mechanism of the Moon’s magnetic field. It was unknown whether or not the Moon had Earthlike composition: with a crust, quasi-mantle and metallic core or if the Moon retained a partially un-melted and undifferentiated interior.

This recent study supports the idea that the Moon’s magnetic field was also generated by a rotating metal core as opposed to other forces. In the past, researchers speculated the Moon’s magnetic field could have been caused by the wobble of the Moon’s spin axis which could have mobilized the core to produce more magnetism. Others have speculated that the magnetic fields could have formed from large basin-forming impacts.

Magnetic Mystery

This is a Moon rock from the Apollo 14 mission. Rocks collected from the Moon by astronauts showed the moon had once had a strong magnetic field. Image Credit: NASA/Sean Smith.

It has been established that a strong magnetic field existed on the Moon from 4.5 billion to 3.56 billion years ago, but suddenly declined by an order of magnitude about 3.3 billion years ago. The long-lived magnetism was surprising to researchers given how small the lunar core is and how quickly it should have cooled off. The MIT researchers deduced that the intense and long-lived magnetic field required an exceptional energy source. Currently, scientists are still stumped as to why the Moon’s magnetic field was so strong. Another puzzling result from this research is how quickly the Moon’s magnetic field diminished, but the solidification of a previously molten core could be a valid explanation. But other researchers do not believe that the Moon’s core has fully cooled. They believe the Moon’s interior is still molten due to the Earth’s gravitational pull. Although this recent study sheds light on our Moon, much remains unknown about the interior and magnetic field of our closest planetary body.