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A new answer for the Moon’s half-century magnetic riddle has been discovered


A new study explains how the Moon may have been a magnetic powerhouse on occasion early in its existence, an issue that has perplexed scientists since NASA’s Apollo mission started in the 1960s.

Rocks returned to Earth during NASA’s Apollo mission, which ran from 1968 to 1972, have yielded a wealth of knowledge about the Moon’s past, but they’ve also remained a subject of intrigue. Some of the rocks seemed to have developed in the presence of a powerful magnetic field that equaled Earth’s in intensity, according to analysis. However, it remained unclear how a mass the size of the Moon could have created such a powerful magnetic field.

A new explanation for the Moon’s magnetic riddle has been proposed by research conducted by a Brown University geoscientist. Giant rock formations plunging into the Moon’s mantle might have caused the type of inner convection that creates powerful magnetic fields, according to a research published in Nature Astronomy. According to the researchers, the processes might have created sporadically powerful magnetic fields over the first billion years of the Moon’s existence.

“Everything we’ve thought about how planetary cores generate magnetic fields tells us that a body the size of the Moon shouldn’t be able to generate a field as strong as Earth’s,” said Alexander Evans, an assistant professor of Earth, environmental, and planetary sciences at Brown University and co-author of the study with Sonia Tikoo of Stanford University. “Instead of pondering how to sustain a powerful magnetic field for billions of years, maybe there is a method to generate a high-intensity field on demand. Our model explains how this may happen, and it’s in line with what we know about the interior of the Moon.”

A core dynamo is a mechanism that produces magnetic fields in planetary bodies. Convection of molten metals in a planet’s core is caused by slowly dispersing heat. A magnetic field is created by the continual churning of electrically conducting material. This is how the Earth’s magnetic field is created, which shields the surface from the sun’s most hazardous radiation.

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The Moon does not have a magnetic field now, and simulations of its core show that it was probably too tiny and lacking the convective force to have ever formed one. A core must dissipate a lot of heat in order to have a strong convective churn. Evans claims that the mantle covering the early Moon’s core was not substantially colder than the core itself. There wasn’t much convection in the core since the heat didn’t have someplace to go. However, this new research suggests that sinking rocks may have supplied occasional convective boosts.

The origins of these sinking stones may be traced back a few million years after the Moon was formed. The Moon is assumed to have been covered by an ocean of molten rock at some point in its history. Minerals that were denser than the liquid magma sank to the bottom as the enormous magma ocean cooled and solidified, while less dense minerals like anorthosite floated to create the crust. Because the leftover liquid magma had high levels of titanium as well as heat-producing components such as thorium, uranium, and potassium, it took a little longer to solidify. This titanium layer was denser than the earlier-solidifying minerals beneath it until it ultimately crystallized just under the crust. The titanium structures sunk through the mantle rock underneath them over time, a process known as gravitational overturn.

Evans and Tikoo studied the mechanics of how those titanium deposits would have sank, as well as the impact they may have had once they reached the Moon’s core, for this new research. The formations would likely break up into blobs as tiny as 60 kilometers in diameter and descend periodically over a billion years, according to the calculations, which was based on the Moon’s present composition and predicted mantle viscosity.

The researchers discovered that when each of these blobs ultimately touched bottom, they would have given the Moon’s core dynamo a significant shock. The titanium structures would have been relatively chilly in temperature, having been sitting just under the Moon’s crust — significantly lower than the core’s estimated temperature of between between 2,600 and 3,800 degrees Fahrenheit. When the cold blobs collided with the hot core after sinking, the temperature difference caused greater core convection, which might have resulted in a magnetic field at the Moon’s surface that was as strong as or stronger than Earth’s.

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“Imagine a drop of water striking a hot griddle,” Evans said. “When something very cold comes into contact with the core, a large amount of heat might be released. This promotes increased churning in the core, resulting in these occasionally intense magnetic fields.”

According to the researchers, there might have been as many as 100 of these downwelling episodes during the Moon’s first billion years of life, each producing a powerful magnetic field that lasted a century or more.

The intermittent magnetic model, according to Evans, not only explains for the intensity of the magnetic signature detected in the Apollo rock samples, but also for the fact that magnetic signatures in the Apollo collection vary significantly, with some having strong magnetic signatures and others not.

“No other model has been able to explain both the intensity and the variability we detect in the Apollo samples,” Evans added. “It also puts a temporal limit on the foundering of this titanium material, giving us a fuller understanding of the Moon’s early genesis.”

Evans claims that the concept may also be put to the test. It means that on the Moon, there should have been a faint magnetic background punctured by these high-intensity occurrences. The Apollo collection should demonstrate this. While the Apollo samples’ powerful magnetic signals stood out like a sore thumb, Evans claims that no one has ever sought for weaker signs.

The existence of both weak and strong signs would give this new theory a major boost, perhaps putting an end to the Moon’s magnetic riddle.

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