“Galactic Charger: The Enigmatic Magnetic Field of the Middle-Aged Moon”

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New discoveries indicate that the Moon retained its magnetic field for the majority of its existence — suggesting it has less water accessible to future astronauts than previously anticipated.

Recent findings from China’s Chang’e 5 mission indicate that the Moon had a magnetic field well into its middle age, significantly longer than previously noted. Research published on Jan. 1 in Science Advances reports that the rocks obtained from the sample-return mission are only weakly magnetized — and are just 2 billion years old.

While not definitive, these results contribute to an expanding collection of evidence documenting the dynamism of the early Moon. When combined with prior sample-return data and observations made by spacecraft orbiting the Moon, the new results strongly suggest that the young Moon not only possessed a magnetic field, but that it — along with the molten lunar interior that must have produced it — endured for a significant portion of the Moon’s existence.

The study, directed by Shuhui Cai from the Chinese Academy of Sciences in Beijing, continues the prolific scientific output from the 2020 Chang’e 5 mission. The mission collected 3.82 pounds (1,731 grams) of Moon rocks and soil, marking the first lunar samples returned to Earth in 44 years. This has yielded a wealth of insights into the Moon’s history and physical composition. For example, in findings published last year, researchers discovered evidence of lunar volcanism just 125 million years ago, challenging long-held beliefs that the Moon had been volcanically dormant for a billion years.

The magnetic Moon

One of the Moon’s ongoing enigmas is whether it ever had a magnetic field, and if it did, for how long it was present. Examination of samples from both the Apollo missions and the USSR’s unmanned Luna missions has shown that over 4 billion years ago, the Moon had a weak global magnetic field, approximately 1/20th of the strength of Earth’s present magnetic field. Yet, robotic and human exploration has concluded that this field is absent today.

Planetary magnetic fields emerge from a process known as the dynamo effect, where a magnetic field is generated by an electrically conductive fluid — typically molten iron — through the rotation of the planet. A planetary dynamo also necessitates a source of internal heat to drive convective movements within the molten fluid. These conditions still prevail on Earth and sustain our planet’s magnetic field.

Such conditions may have also been present on the Moon during its early formation. The Moon formed in proximity to Earth from the debris resulting from a collision with a Mars-sized body. Before being tidally locked into synchronous rotation — which keeps one side of the Moon directed toward Earth — the Moon spun much more rapidly than it does now. Its closer distance to Earth also created tidal forces that helped maintain internal heat, fostering the dynamo effect and producing a lunar magnetic field.

Today, localized areas on the Moon exhibit magnetic fields reaching hundreds of nanoteslas, such as the Reiner Gamma region on Oceanus Procellarum (Ocean of Storms). These minor local fields could be remnants of an intense early global magnetic field, but scientists remain uncertain as to when this global field ceased to exist.

The samples returned by Chang’e 5 provided a chance for new data, supplementing those obtained from earlier missions. Such analysis also posed a distinctive set of challenges. Simply sealing and packaging a sample suffices to protect their chemistry. However, the collection, transport, and storage of samples can induce magnetism, which must be considered in any analysis.

The magnetic analysis of Chang’e 5 basalt samples conducted by Cai and his team determined that the Moon had a magnetic field of 2,000 to 4,000 nanoteslas when it was 2 billion years old, or roughly in the middle of its life span. The underlying consequence of this magnetic discovery is that the planetary dynamo effect was still in play at this point in lunar history, suggesting that the Moon’s interior remained molten and was experiencing convective forces capable of sustaining volcanism.

Impacts on exploration

The absence of a magnetic field also has ramifications for the presence of water on the Moon’s surface — and for future explorers who may wish to utilize it.

Without a magnetic field, solar wind particles hit the Moon’s surface unobstructed. (The Moon briefly receives shielding from the solar wind each lunar month when it navigates through the extended tail of Earth’s magnetic field, which extends away from the Sun. However, this shielding period is too brief to have any significant long-term effect.)

When hydrogen and helium atoms from the solar wind collide with the Moon’s surface, they interact with lunar minerals and oxides to create various products. These include hydroxyls (one hydrogen atom and one oxygen atom) as well as water molecules (two hydrogen atoms and one oxygen atom). Other combinations yield the potentially valuable resource of Helium-3 (³H), which could eventually be employed to produce rocket fuel. The potential of water on the Moon is one of the key motivators behind the ongoing Artemis crewed lunar program.

However, what if the Moon once had a magnetic field that protected it from the solar wind? A lunar magnetic field would limit the interaction between solar hydrogen and lunar oxygen, consequently diminishing the quantity of native water frozen in permanently shadowed polar regions. Furthermore, Helium-3 deposits may not be as plentiful as anticipated.

Future lunar explorations by NASA and international space flight collaborators will soon determine how much water actually exists, continuing to unveil the mysteries surrounding Earth’s solitary moon.