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Strong, sporadic magnetic fields could explain one of the Moon’s enduring mysteries

It is half a century since the Apollo missions returned from the Moon, and yet the lunar samples they brought home continue to confuse us.

Some of these rocks are more than 3 billion years old and appear to have been formed in the presence of a strong geomagnetic field, such as that on Earth. But the Moon today does not have a magnetosphere; it is too small and dense, frozen all the way to the core.

Unlike Earth, the Moon's interior does not constantly collapse with electrically conductive material, which produces a geomagnetic field in the first place. So why does the moon stone tell us otherwise?

It is possible that the Moon did not freeze as fast as we thought; a few billion years ago, its core could still have been easily melted.

But even though the field was maintained for a surprisingly long time, the strength of this field - given the size of the Moon - is unlikely to match what the surface rocks tell us.

Some scientists suggest that the Moon used to sway more, which kept the fluid in the stomach to splash away for a little longer. Constant meteorites could also have given the Moon a boost in energy.

Scientists have previously entertained a new angle on the question, suggesting that spots on the moon's surface were exposed to brief eruptions of intense magnetic activity.

In this latest study, a duo from Stanford and Brown University in the United States have proposed a model that describes how these short-lived but powerful fields can be formed.

"[I]Instead of thinking about how to operate a strong magnetic field continuously over billions of years, there might be a way to get a high-intensity field at intervals, "explains planetary scientist Alexander Evans.

"Our model shows how that can happen, and it's consistent with what we know about the Moon's interior."

For the first billion years or so of the Moon's existence, its core was not much hotter than the mantle above. This meant that the heat from the Moon's interior had nowhere to spread, which is what usually causes molten material to move. The lighter, warmer pieces tend to rise until they cool, while the denser, colder pieces sink until they heat up, and so on, and so forth.

Something else must have touched the pot and generated a magnetic field.

In its youth, a sea of ​​molten rock probably covered the Moon, and as the object cooled, this rock would have solidified at slightly different speeds.

The densest minerals, such as olivine and pyroxene, would have sunk to the bottom and cooled first, while lighter elements like titanium would have floated to the top and cooled last.

Titanium-rich rock, however, would have weighed more than the solids below, causing bits near the Moon's crust to fall through the mantle, right into the core.

Scientists believe that this sinking effect continued until at least 3.5 billion years ago, when at least a hundred blobs of titanium-rich material hit the 'bottom' in a billion years.

Each time one of these massive plates, about 60 kilometers (37 miles) in radius,connected to the core, the mismatch in temperature would have temporarily restarted a surprising convection current, strong enough to generate a strong magnetic pulse.

"You can think of it a bit like a drop of water hitting a hot frying pan," Evans says.

"You have something really cold that touches the core, and suddenly a lot of heat can flow out. It causes the core to rise, giving you these sometimes strong magnetic fields."

The new models can help explain why different moonstones show different magnetic signatures. The moon's magnetosphere may not have been a constant or consistent phenomenon.

The authors are now testing their explanation by looking back at the moon's rocks to see if they can detect a faint magnetic background that is only occasionally pierced by a stronger force. The presence of a weaker magnetic hum suggests that a stronger magnetosphere was the exception and not the rule.

The study was published in Nature astronomy.


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