How did the Moon magnetic field formed?

With missions to the Moon in the 1970s and with the collection and return to Earth of the first lunar samples, several scientists began to believe that the Moon might actually have had a magnetic field. The samples, in fact, were characterized by a certain level of magnetization. However, over the years doubts have arisen about the method used to verify magnetism, and whether a nucleus as small as the lunar nucleus could actually produce a powerful magnetic field. To date, there have been two main hypotheses on why the lunar crust has this slight magnetization: the presence of a primordial dynamo and meteoric impacts.

Missions

Apollo astronauts collected 382 kg of lunar samples during six missions, between 1969 and 1972. Numerically, the samples were over two thousand and they are not the only testimonies that came to us from our satellite. Soviet Moon probes also collected some lunar samples, about 300 grams, during the early 1970s. Additionally, spacecraft observations of the lunar magnetic anomalies were performed by the Lunar Prospector (1998–1999) and Kaguya (2007–2009). One of the scientific surprises that arose from these missions was the discovery of intense magnetic fields concentrated in certain points of the lunar crust. A surprising discovery due to the fact that lunar rocks are poor in metallic iron, a characteristic that makes them intrinsically not very magnetic.

Theory 1: Paleodynamo

The first theory suggests that the crustal magnetization was generated in the early years of our satellite, when the molten metal core was still functioning as a dynamo, which generated a magnetic field by rotation. Studies of the rocks from the Apollo missions have shown that the Moon must have had a strong magnetic field in the past (around 110 μT, against the current 50 μT of the Earth’s one) which has increasingly weakened, reaching around 20 μT. 3.6-3.1 billion years ago (Wieczorek et al., 2006). However, the origin, intensity and duration of this ancient field have long remained uncertain. Thanks to the development of a new generation of analysis and accurate simulation models of the satellite’s thermal evolution that have allowed the creation of realistic simulations of the possible lunar geodynamo, it was possible to establish that, according to current model data, between 4.5 and 3.56 billion years ago the Moon had a magnetic field of similar intensity to that of Earth today, which would have decreased by at least an order of magnitude by 3.3 billion years ago. The initial high intensity of the field required a decidedly powerful power supply, such as that which could be provided by the differential movement between the mantle and a ferrous core, and in particular the fluid outer part of the core (Weiss et al., 2014).

The researchers hypothesized that when the Moon was young, about 4 billion years ago, it was much closer to Earth, whose gravitational force would have agitated the liquid core of the satellite, which would have created a powerful dynamo that formed a magnetic field. As the Moon receded, the force that stirred the dynamo weakened, so the magnetic field also lost strength (Dwyer et al., 2011). 2.5 billion years ago, Earth’s gravity ceased to have an effect on the lunar core, which began to crystallize. That crystallization caused the liquids to move and this explains why the Moon’s core continued to produce a magnetic field, although at that point it was much weaker. When the core completely crystallized, the dynamo stopped existing. However, it is still unclear whether the dynamo has stopped permanently or if it has entered an “active pause” cycle before shutting down forever (Weiss et al., 2014).

Theory 2: Meteor impacts

According to the second theory, most of the Moon’s magnetic anomalies come from the highly magnetized remnants of a large asteroid that crashed in the early stages of the Moon’s formation. These debris would then ‘record’ the magnetic fields that the Moon possessed in ancient times, keeping track of them up to the present day (Wieczorek et al., 2012). The hypotheses advanced start from the fact that two conditions are needed to have magnetic anomalies: the existence of a magnetic field and the presence of minerals capable of recording it. On the Moon, the main ‘messengers’ of magnetization phenomena are the metal alloys of iron and nickel, materials that are however extremely rare in the composition of the crust and upper mantle of our satellite, and therefore cannot be held solely responsible for the magnetic anomalies recorded. Much more plausible is the scenario that sees the presence of deposits of material of meteoritic origin, which has much higher concentrations of iron-nickel alloys. Wieczorek and his team focused on the hidden face of the Moon, in particular on the Aitken impact basin, on whose northern edge most of the lunar magnetic anomalies are concentrated. According to their computer reconstructions, the meteorite that created Aitken basin must have been around 200 kilometers in diameter and must have impacted from south. The huge cloud of debris produced by its disintegration would then have fallen mainly on the northern edge of the basin, thus explaining the concentration of magnetic anomalies.

Which theory?

The problem with the paleodynamo theory is the small size of the lunar core and the lack of certainty about how it generated. Almost all the samples collected were in fact composed of regolith, whose original orientation of the magnetic field is problematic to derive. Furthermore, the main magnetic anomalies mapped, concentrated in relatively recent impact basins, were compatible with both the dynamo and the creation by impact theories. For these reasons, Tarduno et al. (2021) suggested that the Moon never had a magnetic field for prolonged periods of time, as they found that the level of magnetization of some samples is to be explained by impacts, also considering that other samples did not show a level of magnetization.

Gastine et al. (2012) noted that no single dynamo generation mechanism proposed at that point could readily reproduce the paleointensity record inferred from Apollo samples. One possible solution is that the dynamo may have been powered by at least two distinct mechanisms operating during early and late lunar history. A second possible solution is that dynamo was powered by a single bistable mechanism that transitioned from a strong dipole dominated state to a weaker multipolar state after 3.56 billion years ago.

However, a more recent study by Oran et al. (2020) provided results of simulations carried out with a multitude of different scenarios, supported the paleodynamo theory as opposed to the impact theory. Although the impact theory does not actually exclude the dynamo theory, as impactors transiently enhance the Moon magnetic field, Oran and her team’s simulations provided data far from those hypothesized by the impact theory. The inconsistency between the collected and theoretical data thus led the researchers to exclude the second theory.

In fact, the results suggested that although the plasmas produced by the impacts may have temporarily increased the surface magnetic field of the Moon, the strength of the resulting fields was at least three orders of magnitude less than what would have been necessary to explain the detected magnetic anomalies. They concluded that a dynamo generated by the ancient rotating core of the Moon is the only plausible option for the current magnetization of the crust.

Future applications

In addition to the Moon, Mercury, some meteorites and other small planetary bodies all have a magnetic crust and might have had an equivalent dynamo mechanism. The definitive confirmation of this scenario and the determination of when the dynamo started and then stopped will be the task of future explorations that aim at obtaining more accurate measurements of paleointensity of the Moon magnetic field. Particularly useful would be non-regulitic samples of the crust, coupled with on-site measurements of their orientation, so that it will be possible to establish the geometry and frequency of the ancient inversions of the lunar field.

Researchers also agree that the missions part of the Artemis program, the first of which will be launched in February 2022, can help shed light on the origin and development of the Moon magnetic field.

Adapted from part of an assignment for my university course for the current academic year (21-22; already handed in at the time of posting).