The Moon’s Magnetosphere Used to be Twice as Strong as the Earth’s

For decades, scientists have held that the Earth-Moon system formed as a result of a collision between Earth and a Mars-sized object roughly 4.5 billion years ago. Known as the Giant Impact Hypothesis, this theory explains why Earth and the Moon are similar in structure and composition. Interestingly enough, scientists have also determined that during its early history, the Moon had a magnetosphere – much like Earth does today.

However, a new study led by researchers at MIT (with support provided by NASA) indicates that at one time, the Moon’s magnetic field may have actually been stronger than Earth’s. They were also able to place tighter constraints on when this field petered out, claiming it would have happened about 1 billion years ago. These findings have helped resolve the mystery of what mechanism powered the Moon’s magnetic field over time.

The study, which recently appeared in the journal Science Advances, was led by Saied Mighani, an experimental rock physicist with MIT’s Department of Earth, Atmospheric, and Planetary Sciences. He was joined by members of the Berkeley Geochronology Center at UC Berkeley and the China University of Geosciences, with additional support provided by famed EAPS professor, Dr. Benjamin Weiss.

To recap, Earth’s magnetic field is essential to life as we know it. When incoming solar wind particles reach Earth, they are deflected by this field and form a bow shock in front of Earth and magnetotail behind it. The remaining particles are deposited on the magnetic poles where they interact with our atmosphere, which causes the Aurorae seen in the far northern and southern hemispheres.

Were it not for this magnetic field, Earth’s atmosphere would have been slowly stripped away by solar wind over the course of billions of years and rendered a cold, dry place. This is believed to be what happened on Mars, where a once-thicker atmosphere was depleted between 4.2 and 3.7 billion years ago and all of the liquid water on its surface was either lost or froze as a result.

Over the years, Weiss’ group has helped demonstrate through the study of lunar rocks that roughly 4 billion years ago, the Moon also had a strong magnetic field of about 100 microteslas in strength (whereas Earth’sis around 50 microteslas today). In 2017, they studied samples collected by the Apollo astronauts that were dated to about 2.5 billion years ago and found a much weaker field (less than 10 microteslas).

In other words, the Moon’s magnetic field weakened by a factor of five between 4 and 2.5 billion years ago, then disappeared entirely about 1 billion years ago. At the time, Weiss and his colleagues theorized that perhaps there were two dynamo mechanisms in the Moon’s interior that were responsible for this change.

In short, they argued that a first dynamo effect could have generated a much stronger magnetic field around 4 billion years ago. Then, 2.5 billion years ago, it was replaced by a second dynamo that was more long-lived but sustained a much weaker magnetic field. As Dr. Weiss explained in an MIT News release:

“There are several ideas for what mechanisms powered the lunar dynamo, and the question is, how do you figure out which one did it? It turns out all these power sources have different lifetimes. So if you could figure out when the dynamo turned off, then you could distinguish between the mechanisms that have been proposed for the lunar dynamo. That was the purpose of this new paper.”

Until now, obtaining lunar rocks that are less than 3 billion years old has been a major challenge. The reason for this has to do with the fact that volcanic activity, which was common on the Moon 4 billion years ago, ceased roughly 3 billion years ago. Luckily, the MIT team was able to identify two samples of lunar rock obtained by the Apollo astronauts that were created by an impact 1 billion years ago.

While these rocks were melted by the impact and then resolidified, thus erasing their magnetic record in the process, the team was able to perform tests on them to reconstruct their magnetic signature. First, they analyzed the orientation of the rock’s electrons, which Weiss describes as “little compasses” since they would either align in the direction of an existing magnetic field or appear in random orientations in the absence of one.

In both samples, the team observed the latter, which suggested that the rocks formed in an extremely weak magnetic field of no more than 0.1 microteslas (possibly none at all). This was followed by a radiometric dating technique that was adapted for this study by Weiss and David L. Shuster (a Berkeley Geochronology Center researcher and co-author of the study). These results confirmed that the rocks were indeed 1 billion years old.

Finally, the team conducted heat tests on the samples to determine if they could provide a good magnetic record at the time of the impact. This consisted of placing both samples in an oven and exposing them to the kinds of high temperatures that would have been created by an impact. As they cooled, they exposed them to an artificially-generated magnetic field in the laboratory and confirmed that they were able to record it.

These results confirm that the magnetic strength initially measured by the team (0.1 microteslas) is accurate and that by 1 billion years ago, the dynamo powering the Moon’s magnetic field had likely ended. As Weiss expressed:

“The magnetic field is this nebulous thing that pervades space, like an invisible force field. We’ve shown that the dynamo that produced the moon’s magnetic field died somewhere between 1.5 and 1 billion years ago, and seems to have been powered in an Earth-like way.”

As noted, this study also helps to resolve the debate surrounding what drove the lunar dynamo in its later stages. While multiple theories have been suggested, these new findings are consistent with the theory that core crystallization is responsible. Basically, this theory states that the Moon’s inner core crystallized over time, slowing the flow of electrically-charged fluid and arresting the dynamo.

Weiss suggests that prior to this, precession may have been responsible for powering a much stronger (but short-lived) dynamo which would have produced the strong magnetic field. This is consistent with the fact that 4 billion years ago, the Moon is thought to have orbited much closer to Earth. This would have resulted in Earth’s gravity having a far greater effect on the Moon, causing its mantle to wobble and stirring up activity in the core.

As the Moon slowly migrated away from Earth, the effect of precession decreased and the magnetic field-producing dynamo would weaken. By about 2.5 billion years ago, crystallization became the dominant mechanism by which the lunar dynamo continued, producing a weaker magnetic field that persisted until the outer core finally crystallized a billion years ago.

Studies like this could also help resolve the mystery of why planets like Venus and Mars lost their magnetic fields (contributing to cataclysmic climate change) and how Earth could lose its own someday. Considering its importance to habitability, a greater understanding of dynamos and magnetic fields could also help in the search for habitable exoplanets.

Further Reading: MIT News, Science Advances