Lunar samples solve mystery of the moon’s supposed magnetic shield

In 2024, a new era of space exploration will begin when NASA sends astronauts to the moon as part of its Artemis mission, a sequel to the Apollo missions of the 1960s and 1970s.

Some of the biggest questions scientists hope to explore include determining what resources are present in lunar soil and how those resources might be used to sustain life.

In an article published in the journal Scientists progress, researchers at the University of Rochester, leading a team of colleagues from seven other institutions, report their findings on a major factor that influences the types of resources that can be found on the moon: whether the moon may or may not have had a long-lived magnetic shield at any point in its 4.53 billion-year history.

The presence or absence of a shield is important because magnetic shields protect astronomical bodies from harmful solar radiation. And the team’s findings contradict some long-held assumptions.

“This is a new paradigm for the lunar magnetic field,” says first author John Tarduno, William R. Kenan, Jr., professor of geophysics in the Department of Earth and Environmental Sciences and dean of research for the arts, sciences and engineering in Rochester.

Has the moon ever had a magnetic shield?

For years, Tarduno has been a leader in the field of paleomagnetism, studying the development of the Earth’s magnetic shield as a means of understanding planetary evolution and environmental changes.

The Earth’s magnetic shield originates deep in the core of the planet. There, the swirling liquid iron generates electric currents, causing a phenomenon called geodynamo, which produces the shield. The magnetic shield is invisible, but researchers have long recognized that it is vital for life on the surface of the Earth, because it protects our planet from the solar wind, from the fluxes of radiation from the sun.

But has Earth’s moon ever had a magnetic shield?

While the moon does not have a magnetic shield now, there has been some debate as to whether the moon could have had an extended magnetic shield at some point in its history.

“Since the Apollo missions, there has been this idea that the moon had a magnetic field that was as strong or even stronger than the Earth’s magnetic field about 3.7 billion years ago,” says Tarduno.

The belief that the moon had a magnetic shield was based on an initial 1970s data set that included analyzes of samples collected during the Apollo missions. The analyzes showed that the samples exhibited magnetization, which the researchers believed to be caused by the presence of geodynamo.

But a few factors have since made researchers think.

“The moon’s core is really small and it would be difficult to conduct that kind of magnetic field,” Tarduno explains. “In addition, previous measurements that record a high magnetic field were not made using heating experiments. They used other techniques that may not accurately record the magnetic field.”

When lunar samples meet lasers

Tarduno and his colleagues tested glass samples collected from previous Apollo missions, but used CO₂ lasers to heat lunar samples for a short period of time, a method that allowed them to avoid tampering with the samples. They then used highly sensitive superconducting magnetometers to more accurately measure the magnetic signals of the samples.

“One of the problems with lunar samples is that the magnetic carriers they contain are quite susceptible to modification,” Tarduno explains. “By heating with a laser, there is no evidence of tampering with our measurements, so we can avoid problems that people may have had in the past.”

The researchers determined that the magnetization in the samples could be the result of impacts from objects such as meteorites or comets, and not the result of magnetization due to the presence of a magnetic shield. Other samples they analyzed had the potential to show strong magnetization in the presence of a magnetic field, but showed no magnetization, further indicating that the moon never had an extended magnetic shield.

“If there had been a magnetic field on the moon, the samples we studied would all have acquired magnetization, but they didn’t,” says Tarduno. “It’s pretty conclusive that the moon did not have a long-lasting dynamo field.”

Lack of magnetic shield means abundance of elements

Without the protection of a magnetic shield, the moon was sensitive to the solar wind, which may have caused a variety of volatiles – chemicals and compounds that can be easily evaporated – to implant in the lunar soil. These volatiles can include carbon, hydrogen, water, and helium-3, an isotope of helium that is not abundantly found on Earth.

“Our data indicates that we should be looking at the higher end of the helium-3 estimates because a lack of a magnetic shield means more solar wind is reaching the lunar surface, resulting in helium-3 reservoirs much deeper than what one thought before, ”Tarduno said.

The research could help illuminate a new wave of lunar experiments based on the data that will be collected by the Artemis mission. Data from samples collected during the mission will allow scientists and engineers to study the presence of volatile compounds and better determine whether these materials can be extracted for human use. Helium 3, for example, is currently used in medical imaging and cryogenics and is a potential future source of fuel.

A lack of magnetic shielding also means that ancient lunar soils may contain records of past emissions from the solar wind. The analysis of cores of soil samples could therefore allow scientists to better understand the evolution of the sun.

“With the background provided by our research, scientists can better think about the next round of lunar experiments to be performed,” Tarduno said. “These experiments can focus on current lunar resources and how we might use them, as well as the history of what is trapped in the lunar soil.”