The Moon, our natural satellite, has always intrigued scientists with its unusual magnetic properties. Although it no longer has a significant magnetic field of its own like Earth, some areas of its surface, especially on the far side, have recorded strong magnetic anomalies in the rocks. The puzzle of why this is so has persisted for decades, and recently, scientists from MIT have offered a convincing explanation that could resolve this mysterious phenomenon.

The Moon's Magnetism - where did it go?

Modern research of the Moon, from samples brought back during NASA's Apollo missions in the 60s and 70s to precise measurements from lunar orbits, has shown that there are traces of an ancient magnetic field on the surface. However, the Moon today does not possess a global magnetic field like Earth. The traditional model of planetary magnetism explains the origin of a magnetic field through a so-called dynamo – a process in the liquid metal core that produces a magnetic field. Since the Moon's iron core is much smaller and cooler, scientists suspected that a dynamo might never have been able to produce a magnetic field strong enough to explain the intensity of the magnetic traces found in lunar rocks, especially on the far side of the Moon.

A new hypothesis: Synergy of dynamo and impact

In light of new findings, scientists at MIT propose a more complex picture that includes a combination of an existing, albeit weak, dynamo magnetic field and the influence of a large collision with the Moon. According to their research, which was just published in the journal Science Advances, a powerful impact from a huge asteroid could have momentarily significantly amplified the weak lunar magnetic field, creating a transient but sufficiently intense magnetic pulse that was then recorded by certain rocks on the far side of the Moon.

How does an impact amplify magnetism?

When a massive asteroid strikes the Moon's surface, the impact energy vaporizes huge amounts of material, creating a cloud of ionized particles – a plasma. According to the MIT team's simulations, this plasma cloud does not disappear immediately but spreads around the Moon and concentrates on the side opposite the impact site. On that opposite side, the interaction of the plasma with the existing weak magnetic field leads to its temporary amplification.

This process, although very fast – lasting only about 40 minutes – is long enough for certain rocks in that area to record the change in the magnetic field. In addition to the amplified magnetic pulse, the large impact also sends powerful seismic waves through the Moon's interior, which "shake" the rocks on the opposite side, causing the electrons in the minerals to reorient themselves in accordance with the new magnetic field. This phenomenon is similar to throwing cards in the air and watching them fall, where the orientation of the "compass" on the cards changes just as the cards return to the ground.

The key role of the great Imbrium basin impact

It is particularly interesting that the area of strong magnetism on the far side of the Moon is located exactly opposite the large Imbrium impact basin on the near side of the Moon, one of the largest impact craters in the Moon's history. It was the impact that formed the Imbrium basin that likely triggered the said plasma cloud, which, traveling around the Moon, amplified the magnetic field on the far side and left a magnetic trace in the rocks.

Previous theories and new simulations

Previous theories suggested that the solar magnetic field, which is extremely weak at the Moon's distance from the Sun, was not strong enough to explain the strong magnetic anomalies found on its surface. Simulations conducted in 2020 ruled out the possibility that an impact by itself, without the presence of a dynamo, could significantly amplify the magnetic field.

In the new study, the researchers started from the assumption that the Moon once had a dynamo that produced a weaker magnetic field – about 50 times weaker than Earth's magnetic field today. From there, they used computer models to simulate a large impact, which generated plasma and the interaction of the plasma with the magnetic field. The results showed how the plasma collects and amplifies the magnetic field on the side opposite the impact, which is consistent with observations of magnetic anomalies on the far side of the Moon.

Research Methodology

The simulations were performed using the advanced computing resources of the MIT SuperCloud system. In addition, the impact simulations were developed with the help of expert Katarina Miljković, while the plasma flow and magnetism interaction models were created in collaboration with researchers from the University of Michigan and Cambridge. This interdisciplinary approach enabled an in-depth analysis of the complex processes that occurred during and after the impact.

Significance of the research for future missions

This new theory is testable and provides concrete guidelines for future missions to the Moon, especially those planning research on the far side and around the south pole. NASA's Artemis program, as well as other planned missions, could take rock samples from these areas to further confirm or refute the hypothesis of a synergy between the dynamo and impact events in creating magnetic anomalies on the Moon.

Broader context - what does the Moon's magnetism tell us?

Understanding the origin and characteristics of the Moon's magnetic field is not only important for geology and planetary sciences but also for the broader context of space exploration. Magnetic fields play a key role in protecting planets from solar and cosmic radiation and in shaping the conditions for the emergence of life. Also, knowledge about the Moon's magnetism can help in interpreting similar phenomena on other celestial bodies in the Solar System.

Given that the Moon does not have an active dynamo today, its magnetic legacy recorded in the rocks can be seen as a kind of "fossil record" of its geophysical and cosmic events, including the large impacts that shaped its surface and interior.

The role of international scientific collaboration

The research is the result of collaboration between many scientists and institutions around the world – from MIT and the University of Michigan to Cambridge and Curtin University in Australia. This multidisciplinary cooperation shows how modern planetary science uses a combination of computer models, laboratory analyses, and space missions to illuminate the complex processes in our Solar System.

What awaits us next?

While many unknowns about the Moon's magnetism are now clearer, scientists emphasize that this is just the beginning of a new era of research. In the coming years, new missions and more precise measurements will contribute to an even more detailed understanding of not only the Moon but also other celestial bodies. We might soon have the opportunity to directly analyze rocks from the areas on the far side of the Moon covered by these new findings, which will clarify many questions and open the door to new scientific discoveries.

Source: Massachusetts Institute of Technology