Moon's magnetic field was twice as strong as Earth's until 1 billion years ago: Study

A strong field may have shielded the Moon's surface from the solar wind, the supersonic plasma emitted from the Sun.

                            Moon's magnetic field was twice as strong as Earth's until 1 billion years ago: Study
The Apollo 11 experiment (NASA)

The Moon lost its protective shield, its magnetic field, over a billion years ago, a new study suggests. This left the Moon deprived of its ability to protect itself from the harsh solar winds, whose radiation is dangerous to any life.

"A strong field on the Moon may have shielded the [Moon's] surface from the solar wind, the supersonic plasma emitted from the sun," study author Benjamin Weiss told Newsweek. "This may have prevented the surface from being space weathered and the soil becoming rich in solar gases like it is today," says Weiss.

Scientists had no idea that the Moon ever had a magnetic field. But they found evidence of the Moon's magnetic field preserved in the four-billion-year-old rocks, picked up by Apollo astronauts. When they measured its magnetic strength, they found signs of a strong magnetic field of around 100 microteslas in the rocks. Earth's magnetic field, on the other hand, is of around 50 microteslas, says the study. 

Since the return of the lunar rocks, scientists have traced the journey of the Moon's magnetic field. About 4 billion years ago, its magnetic field was twice as strong as that of the Earth. During this time, the Earth helped the Moon produce its own magnetic field. But that changed when the Moon began moving away from the Earth, says a statement. Because of this, the lunar dynamo, which produces the magnetic field, started weakening.

In 2017, Weiss and his colleagues pinned down the timing of the lunar dynamo's fall by studying lunar samples collected from NASA’s Apollo project. In one relatively new rock, estimated to be about 2.5-billion-year-old, they recorded a weak magnetic field: below 10 microteslas, indicating that the Moon's magnetic field started getting to wither by then. It eventually shut down over one billion years ago, and so its magnetic field ceased to exist, believes Weiss.

What is lunar dynamo? How did the Moon lose its magnetic field?

The insides of Earth or the core has a metallic liquid: Iron. Its movement generates electric currents. And when the Earth rotates on its own axis, these electric currents form a magnetic field. This process is called the dynamo effect. Like Earth, the Moon also produced its magnetic field through the dynamo. 

The Moon's dynamo was active as long as it stayed close to the Earth. Scientists believe that the Moon remained in close proximity to Earth until about four billion years ago. This close proximity meant that the Earth's gravity exerted some influence on the Moon. In response to Earth’s gravity, the Moon may have wobbled. This wobbling, in turn, caused the fluid in the Moon's core to stir up, explains the study.

A new analysis of Moon rocks pins down the end of the lunar dynamo, the process by which the Moon once generated a magnetic field. (Hernán Cañellas and Benjamin Weiss/MIT)

This, however, was not permanent. As the Moon began moving away, the Earth's influence withered: its gravity could not shake up the Moon's core, resulting in a weak dynamo.

About 2.5 billion years ago, the dynamo was replaced by another process, which continued to maintain the Moon's magnetic field for another billion years, while turning the liquid core into solid. Called core crystallization, it produced a weaker magnetic field that continued to dissipate as the Moon’s core eventually fully crystallized. 

Weiss and his colleagues hope to study younger lunar rocks to understand whether the dynamo stopped permanently or if it entered a "stop-start regime" before eventually ceasing altogether.

The group is looking to measure the direction of the Moon’s ancient magnetic field to paint a clear picture of the Moon’s evolution.

The study was published in Science Advances.

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