New results from China’s Chang’e 5 mission suggest that the Moon possessed a magnetic field well into its midlife, much longer than previously documented. In work published Jan. 1 in Science Advances, researchers report rocks recovered from the sample-return mission that are weakly magnetized — and just 2 billion years old.
While not conclusive, the results add to a growing body of evidence documenting the early Moon’s dynamism. Combined with previous sample-return data and observations by spacecraft in lunar orbit, the new results make a strong case that not only did the young Moon have a magnetic field, but that it — and the molten lunar interior that must have generated it — persisted for much of the Moon’s lifetime.
The research — led by Shuhui Cai with the Chinese Academy of Sciences in Bejing — also continues the prolific science output from the 2020 Chang’e 5 mission. The mission’s bounty of 3.82 pounds (1,731 grams) of Moon rocks and soil comprised the first lunar samples returned to Earth in 44 years. This has provided a treasure trove of insights into the Moon’s history and physical nature. For instance, in results published last year, researchers were amazed to find evidence of lunar volcanism a mere 125 million years ago, upending assumptions that the Moon has been volcanically inactive for a billion years.
The magnetic Moon
One of the Moon’s ongoing mysteries is whether it once possessed a magnetic field, and if so, how long it was present. Analysis of samples from both the Apollo missions and the USSR’s uncrewed Luna missions have revealed that over 4 billion years ago, the Moon had a weak global magnetic field about 1/20th the strength of Earth’s present magnetic field. But robotic and crewed explorations have determined that the field is non-existent today.
Planetary magnetic fields are created by what is known as a dynamo effect, where a magnetic field is created through an electrically conductive fluid — usually molten iron — by the rotation of the planet. A planetary dynamo also requires a source of internal heat to drive convection motions through the molten fluid. These conditions still exist on Earth and sustain our planet’s magnetic field.
These conditions could also have existed on the Moon early in its life. The Moon formed close to Earth from the debris created by the collision of a Mars-sized body with the proto-Earth. Prior to being tidally locked into synchronous rotation — which keeps the same face of the Moon pointed toward Earth — the Moon rotated much faster than today. Its closer proximity to Earth also created tidal forces that maintained internal heat, promoting the dynamo effect and creating a lunar magnetic field.
Currently, there are regions on the Moon with localized magnetic fields reaching hundreds of nanoteslas, such as the Reiner Gamma region on Oceanus Procellarum (Ocean of Storms). These small local fields could be the remains of a strong early global magnetic field. But scientists are unsure when that global field disappeared.
The samples returned by Chang’e 5 offered an opportunity for a fresh data point, adding to those provided by samples from previous missions. Such analysis also presented a unique set of challenges. Simply sealing and packaging a sample is sufficient to protect their chemistry. But the collection, transport, and storage of samples can induce magnetism, which must be accounted for in any analysis.
The magnetic analysis of Chang’e 5 basalt samples by Cai and his colleagues revealed that the Moon had a magnetic field of 2,000 to 4,000 nanoteslas when the Moon was 2 billion years old, or at about the middle of its existence. The underlying implication of this magnetic finding is that the planetary dynamo effect was still present at this lunar age, meaning the Moon’s interior was still molten and experiencing convective forces that could sustain volcanism.
Impacts on exploration
The absence of a magnetic field also has implications for water on the Moon’s surface — and for future explorers who may seek to make use of it.
With no magnetic field, the particles of the solar wind strike the Moon’s surface unimpeded. (The Moon is briefly shielded from the solar wind every lunar month when the Moon passes through the extended tail of Earth’s magnetic field, which sweeps away from the Sun. But this respite is too short to have significant long-term impact.)
When hydrogen and helium atoms from the solar wind strike the Moon’s surface, they interact with lunar minerals and oxides to create a variety of 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 include the potentially valuable resource of Helium-3 (3H), which could one day be used to make rocket fuel. The promise of water on the Moon is one of the prime drivers for the ongoing Artemis crewed lunar program.
But what if the Moon once possessed a magnetic field that shielded it from the solar wind? A lunar magnetic field would limit the amount of solar hydrogen and lunar oxygen interaction and subsequently reduce the amount of native water frozen in permanently shadowed polar regions. Additionally, Helium-3 reserves may not be as abundant as hoped.
The upcoming lunar explorations by NASA and international space flight partners will soon find out just how much water exists, continuing to strip away the mystery from Earth’s lonely moon.
