Did the Moon Ever Have a Magnetosphere?

In a few years, NASA will be sending astronauts to the Moon for the first time since the Apollo Era (1969-1972). As part of the Artemis Program, the long-term goal is to create the necessary infrastructure for a “sustained program of lunar exploration.” The opportunities this will present for lunar research are profound and will likely result in new discoveries about the formation and evolution of the Moon.

In particular, scientists are hoping to investigate the long-standing mystery of whether or not the Moon had a magnetosphere. In anticipation of what scientists might find, an international team of geophysicists led by the University of Rochester examined samples of lunar material brought back by the Apollo astronauts. Based on the composition of these samples, the team determined that the Moon’s dynamo was short-lived.

The research was led by John A. Tarduno, the William R. Kenan Jr., Professor, of Geophysics and the dean of research for Arts, Sciences & Engineering at the University of Rochester. He was joined by researchers from Rochester’s Department of Earth and Environmental Sciences, the Planetary Science Institute (PSI), Michigan Technological University (MTU), the Geological Survey of Japan, and universities across the US, UK, Canada, and France.

For the sake of their study, the team examined samples of lunar glass from a young impact crater (about 2 million years old). This impact caused material on the surface to comingle with the material in the mantle that dates back to shortly after the Moon formed (ca. 4.5 billion years ago). In the past, examination of lunar rocks has revealed indications of strong Earth-like magnetization, indicating exposure to a magnetic field.

In the case of Earth, our planetary magnetic field (aka. magnetosphere) is the result of a geodynamo deep in our planet’s core. This is created by the movement of the molten outer core around the solid inner core, which generates the powerful electric currents that make up the Earth’s magnetic field. For some time, scientists have understood the important role our magnetic field plays in the maintenance of habitability here on Earth.

Were it not for this field, the surface of our planet would be bombarded by intense amounts of solar radiation and cosmic rays. In addition, interaction with charged particles from the Sun (solar wind) would have slowly stripped our atmosphere away over the course of eons (which is what happened on Mars). While the Moon has no magnetic field to speak of today, it did at one time, which raises the question of how long it existed.

Moreover, scientists have many unresolved questions about how the Moon could have sustained a magnetic field given its size and mass. As Tarduno explained in a recent Rochester Newscenter release:

“This is a new paradigm for the lunar magnetic field. Since the Apollo missions, there has been this idea that the moon had a magnetic field that was as strong or even stronger than Earth’s magnetic field at around 3.7 billion years ago.

The core of the moon is really small and it would be hard to actually drive that kind of magnetic field. Plus, the previous measurements that record a high magnetic field were not conducted using heating experiments. They used other techniques that may not accurately record the magnetic field.”

For years, Tarduno has been a leader in the field of paleomagnetism, where geophysicists study the development of Earth’s magnetic field to learn more about planetary evolution, environmental change, and how these are interrelated. Using carbon dioxide (CO₂) lasers, Tarduno and his team heated the lunar glass samples for short periods of time, then measured their magnetic signals using highly-sensitive superconducting magnetometers.

This allowed them to obtain more accurate readings of their magnetization without altering them, which may have been a factor in the past and led to misleading results. Unfortunately, the researchers determined that the readings they obtained could be the result of impacts from meteorites or comets, not a magnetic field. Similarly, their examination of other samples showed that they had the potential to record strong core dynamo–like magnetic fields.

However, these samples didn’t show any magnetization, another indication that the Moon has never possessed a long-lived magnetic field. Said Tarduno:

“One of the issues with lunar samples has been that the magnetic carriers in them are quite susceptible to alteration. By heating with a laser, there is no evidence of alteration in our measurements, so we can avoid the problems people may have had in the past.

If there had been a magnetic field on the moon, the samples we studied should all have acquired magnetization, but they haven’t. That’s pretty conclusive that the moon didn’t have a long-lasting dynamo field.”

These results contradict previous research led by MIT’s Department of Earth, Atmospheric, and Planetary Sciences, where analysis of lunar rocks collected by the Apollo 15 mission suggested the Moon had a magnetic field as recent as 1 and 2.5 billion years ago. Prior to this, scientists theorized that the Moon’s magnetic field disappeared about 1 billion years after it formed (ca. 3 to 3.5 billion years ago).

The implications of these findings are quite significant in terms of our understanding of the Moon’s composition and evolution. Without the protection of a magnetic field, the Moon would be susceptible to solar wind which could have caused volatile compounds to become implanted in the lunar soil. These include carbon, hydrogen, water, but also compounds like helium 3, which are not in abundance here on Earth. Said Tarduno:

“Our data indicates we should be looking at the high end of estimates of helium 3 because a lack of magnetic shield means more solar wind reaches the lunar surface, resulting in much deeper reservoirs of helium 3 than people thought previously.

“With the background provided by our research, scientists can more properly think about the next set of lunar experiments to perform. These experiments may focus on current lunar resources and how we could use them and also on the historical record of what is trapped in the lunar soil.”

When astronauts begin conducting long-term stays on the lunar surface, they will need to rely on local sources of ice and other resources to support their operations – a process known as in-situ resource utilization (ISRU). This research could help inform field research, the creation of essential infrastructure, and meeting power needs. Helium-3, for example, is currently used for medical imaging and cryogenics, and could someday be used as fuel for fusion reactors.

A short-lived magnetosphere also means that the lunar surface has a more comprehensive record of solar wind emissions. This could allow scientists to reconstruct a record of Solar activity and the Sun’s evolution by examining soils of various depths. The study that describes the team’s findings, titled “Absence of a long-lived lunar paleomagnetosphere,” recently appeared in the journal Science Advances.

Further Reading: Rochester University