Moon’s Magnetic Field, Now Gone, Likely Shielded Early Earth

Like a protective sibling, the moon shielded a nascent Earth from the ravages of space with its magnetic field — a power that has long since disappeared.

The moon is seen during a complete lunar eclipse over Jakarta, Indonesia, on Aug. 28, 2018. (AP Photo/Tatan Syuflana)

(CN) — Earth’s moon was once quite different from the barren landscape visited by the Apollo astronauts in the 1960s and 1970s — it had volcanoes, an atmosphere and a strong magnetic field which may have helped shield Earth from destructive solar storms. 

In a study released Wednesday in the journal Science Advances, NASA scientists modeled the joint Earth-moon magnetospheres to show how their combined effects may have ensured Earth’s habitability.

“It looks like the magnetic fields of certain rocks were created at a time when the moon generated its own magnetic field,” NASA chief scientist James Green said in a phone interview. “This was a significant result that combined with our current knowledge really forced us to move in the direction of modeling what the magnetic field of the moon would have looked like.”

Earth’s magnetosphere acts like a protective barrier, deflecting harmful charged solar particles away from the planet which would otherwise erode its lower atmosphere. This magnetosphere is created through the interaction of powerful solar winds and Earth’s own magnetic field powered by its iron core.

Solar winds compress Earth’s magnetosphere on the daylit side, while the dark side extends over six times further into space. The boundary between the solar wind and the magnetosphere is called the magnetopause, which is governed by the pressure of the solar winds pushing against the Earth’s magnetic field.

“The prevailing model for the generation of the moon is the giant impact hypothesis in which an approximately Mars-sized object, named Theia, collided with the proto-Earth during the early formation of the solar system,” the study authors wrote. “Under this hypothesis, the resulting lunar iron core would be much smaller than the core Theia is thought to have had. For decades, it was assumed that the moon would not have the ability to generate and maintain a substantial magnetic dynamo from this small core.” 

The first magnetic measurements of the moon came from the Soviet Luna 1 spacecraft that flew by on Jan. 4, 1959. Those measurements confirmed the moon retained only a minuscule magnetic field, but it wasn’t until Apollo 12, with its more powerful magnetometer, that scientists discovered the field had declined to around a thousand times weaker than that of Earth.

After reexamining lunar rock samples returned by the Apollo astronauts, paleomagnetologists recently determined that a more powerful magnetic field almost certainly existed during the moon’s first several hundred million years — but how it began and why it faltered are topics that are still hotly disputed. The early moon’s magnetic field may have even been stronger than Earth’s at one time, but geological forces have eliminated much of Earth’s early rock record, making it difficult to determine exactly to what extent.

Researchers modeled the overlapping magnetic dipole fields of Earth and its sole moon in their distant past, when the two bodies were about three times closer together than they are today. They found that the moon’s magnetic field functioned as a sort of protective barrier for the early Earth, absorbing much of the solar wind that would have otherwise stripped away Earth’s nascent atmosphere.

“The magnetosphere of the moon creates a previously unrecognized barrier to the solar wind that must be taken into account and which may mitigate many of the effects of solar radiation extremes,” the study authors wrote. “Previous research called into question the ability of the early Earth’s magnetic field to shelter its atmosphere from erosion by intense solar wind, ultraviolet and X-ray radiation from the young sun, which would have promoted the loss of volatiles, including water.” 

Green and his team believe NASA’s Artemis program, which will return humans to the moon for the first time since Apollo 17 departed in 1972, may finally allow them to test their hypothesis.

One of the primary scientific goals of Artemis is to retrieve the first core samples from the permanently shadowed areas of the moon around its south pole, which many scientists believe might contain frozen water and other volatile-rich materials. By gauging the chemical compositions of these volatiles, they may be able to confirm the early existence of a conjoined magnetosphere.

“The whole idea of the lunar and Earth magnetospheres joining and coupling in this intense way produces in the polar regions what was actually obtained from the early Earth. The atmosphere from the early Earth is actually trapped in the regions of the north and south pole in ice cores, like we find in Antarctica and Greenland,” Green said. “We would do that in the permanently shadowed regions on the moon, bringing back those cores and being able to see where the early atmosphere of the Earth gets trapped in these polar regions.”

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