Why the Moon Lost Its Magnetism and Earth Did Not

Why the Moon Lost Its Magnetism and Earth Did Not

As human bodies age, thermodynamics, gravity, and electromagnetism take their toll. And what is true for human beings is also true for planets and moons.

As planets and moons get older, thermodynamics, gravity, and electromagnetism take their inevitable toll. Astronomical bodies begin with a magnetic attractiveness, but over time that attractiveness wanes and eventually vanishes. Some retain magnetism a lot longer than others.

Earth is the only known rocky body that has retained a strong magnetic field—key for advanced life—for several billion years. Recent measurements on magnetic minerals in ancient Greenland rocks show that Earth’s magnetic field has remained stronger than 0.1 gauss (10 microtesla) for at least the past 3.7 billion years.1

History of the Moon’s Magnetic Field
Other rocky bodies have not been so fortunate. The surface magnetic field of Mercury is only 1 percent that of Earth’s and part of that field comes from solar wind interaction.2 Analysis of Apollo lunar rocks reveal that when the Moon was younger than 0.9 billion years (4.47–3.57 billion years ago) it had a magnetic field strength ranging from 1 to 40 microtesla (0.01–0.40 gauss).3 This is comparable in strength to Earth’s present-day magnetic field of 25–65 microTesla (0.25–0.65 gauss).4

At that time, a core dynamo—powered by thermal convection in a liquid outer core—sustained the Moon’s relatively strong magnetic field. (This was similar to what has operated on Earth for the past 3.7+ billion years.) At 3.57 billion years ago the Moon’s core dynamo shut down. From 3.57 to 2.0 billion years ago mantle precession powered a lunar magnetic field 10–100 times weaker. Mantle precession occurred when the Moon was much closer to Earth. Earth’s gravity at that time generated powerful tidal forces on the Moon and caused it to wobble. This wobbling stirred up the fluid in the Moon’s core, which generated a weak magnetic field.

As the Moon continued to move away from Earth, the wobbling became less pronounced. By 2.0 billion years ago, the wobbling no longer could sustain a lunar magnetic field.

There is some evidence for an even weaker lunar magnetic field existing about 1.5 billion years ago.5 Such a field could have been generated by the process of the lunar core gradually crystallizing.

End of the Moon’s Magnetic Field
When the Moon’s core becomes fully crystallized, the Moon’s magnetic field will completely disappear. A team of six planetary scientists endeavored to determine if indeed the Moon has lost its magnetic field, and if so, when.6 The team used a superconducting rock magnetometer to look for any indication that the matrix glass in the two youngest lunar breccia brought back from the Apollo 15 mission showed evidence for an ambient lunar magnetic field.

Lunar breccia 15465, dated to be 0.44±0.01 billion years old, revealed an upper limit for a lunar magnetic field of 0.06 microtesla (0.0006 gauss). Lunar breccia 15015, dated to be 0.91±0.11 billion years old, revealed an upper limit of 0.08 microtesla (0.0008 gauss). Consequently, the six planetary scientists concluded that the Moon lost its magnetic field sometime between 1.92 and 0.80 billion years ago.

How the Lunar Magnetic Field Persisted
That the Moon retained a magnetic field until 1.92–0.80 billion years ago is remarkable. Mars, which is nine times more massive than the Moon, lost its magnetic field about 4.2 billion years ago.7 However, unlike Mars and Earth’s other planetary partners, the Earth-Moon system experienced a different formation history. About 95 million years after its formation through the accretion of planetesimals, the primordial Earth collided/merged with Theia, a planet 20–90 percent the primordial Earth’s mass.

The collision/merger of the primordial Earth and Theia substantially modified Earth and gave rise to the formation of the Moon.8 One consequence of the collision/merger is that both the Moon and Earth ended up with an interior composition and structure different from that of the other solar system planets and moons. This consequence explains, in large part, why the Moon and Earth have had such long-lasting magnetic fields.

Persistence and Strength of Earth’s Magnetic Field
Meanwhile, Earth’s magnetic field has persisted until the present time. The Moon transitioned through three different magnetic field generation mechanisms: (1) core convection, (2) mantle precession, and (3) core crystallization. Earth has experienced only core convection.

Earth’s core, unlike Mars’s core or the Moon’s, contains a low abundance of elements lighter than iron,9 but it has an extremely high abundance of thorium and uranium. In large part due to the heat released from the radiometric decay of thorium and uranium, Earth’s outer core of predominantly ferrous (easily magnetized) elements has remained liquid throughout Earth’s history. Therefore, core convection—the most powerful of the magnetic field generation mechanism—has operated during the entire 3.8-billion-year history of life on Earth.

Earth’s powerful, enduring magnetic field rebuffs deadly high-energy particles flowing in from the Sun and equally deadly high-energy cosmic rays. Earth’s magnetic shield not only protects surface life, it also protects oceans and lakes and Earth’s atmosphere. Without the shield, solar radiation would quickly sputter away both Earth’s atmosphere and surface water into interplanetary space. Such was the fate of Mars some 4 billion years ago.

The Moon has had a remarkable magnetic field history. Yet Earth’s magnetic field history is far more remarkable. It is thanks in part to that history that life has been able to thrive on Earth in great abundance and diversity for 3.8 billion years. Without that great abundance and diversity persisting for 3.8 billion years, human beings and global civilization would be impossible.10

Featured image: Full Moon
Credit: Gregory H. Revera, Creative Commons Attribution


Interested readers might enjoy this short 3-minute video produced by NASA showing the 4.47-billion-year history of the Moon: Evolution_of_the_Moon.gov

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