New interpretation of the moon’s magnetic mystery dating back half a century – ScienceDaily

The rocks returned to Earth during NASA’s Apollo program from 1968 to 1972, and provided large amounts of information about the history of the Moon, but they were also a source of enduring mystery. Analysis of the rocks revealed that some of them appeared to have formed in the presence of a strong magnetic field – a field that rivals Earth in strength. But it wasn’t clear how a moon-sized object could generate such a strong magnetic field.

Now, research led by a Brown University geologist proposes a new explanation for the moon’s magnetic mystery. The study published in the nature of astronomy It shows that giant rock formations sinking into the moon’s mantle can produce a type of internal convection that generates strong magnetic fields. These processes may have produced strong magnetic fields intermittently during the first billion years of the moon’s history, the researchers say.

“Everything we’ve thought about how magnetic fields are generated by planetary cores tells us that a moon-sized object shouldn’t be able to generate a field as strong as Earth’s,” said Alexander Evans, assistant professor at Earth. Environmental and Planetary Science at Brown University and co-author of the study with Sonia Tiko of Stanford University. “But instead of thinking about how to run a strong magnetic field continuously over billions of years, perhaps there is a way to get a high-density field intermittently. Our model shows how that might happen, and it is consistent with what we know about the Moon’s interior.”

Planetary bodies produce magnetic fields through what is known as a basic dynamo. The slow dissipation of heat causes molten metals to move into the planet’s core. The continuous undulation of electrically conducting materials is what produces a magnetic field. This is how the Earth’s magnetic field is formed – which protects the surface from the sun’s most dangerous rays.

The Moon lacks a magnetic field today, and models of its core suggest that it may have been too small and lacked the convective strength to produce a persistently strong magnetic field. In order for the core to have a strong carrying momentum, it needs to dissipate a lot of heat. In the case of the early moon, Evans says, the mantle surrounding the core was not much cooler than the core itself. Since the core’s heat had nowhere to go, there wasn’t much convection in the core. But this new study shows how sunken rocks can provide intermittent enhancements to convection.

The story of these sunken stones begins a few million years after the formation of the moon. Very early in its history, the moon was thought to be covered in an ocean of molten rock. As the vast ocean of magma began to cool and solidify, minerals such as olivine and pyroxene that were denser than liquid magma sank to the bottom, while less dense minerals such as anorthosite floated to form the crust. The remaining liquid magma was rich in titanium as well as heat-producing elements such as thorium, uranium, and potassium, so it took longer to solidify. When the layer of titanium finally crystallized under the crust, it was denser than the minerals that had previously solidified beneath it. Over time, titanium formations sank through the less dense mantle rocks beneath, a process known as gravitational inversion.

In this new study, Evans and Tycho model the dynamics of how titanium formations sink, as well as the potential impact when they eventually reach the lunar core. The analysis, which was based on the moon’s current composition and estimated mantle viscosity, showed the formations likely fractured into small points up to 60 kilometers in diameter, and sank intermittently over the course of about a billion years.

The researchers found that when each of these blobs eventually reached the bottom, they would give a major jolt to the Moon’s underlying dynamo. Perched just under the moon’s crust, the titanium formations were relatively cool in temperature – much cooler than the estimated core temperature of somewhere between 2,600 and 3,800 degrees Fahrenheit. When the cold blobs came into contact with the hot core after it sank, the temperature mismatch could have increased the core convection — enough to push a magnetic field on the Moon’s surface as strong or stronger than Earth’s.

“You can think of it a bit like a drop of water hitting a hot frying pan,” Evans said. “You have something really cold touching the core, and suddenly a lot of heat can flow out. That increases ripple in the core, giving you these intermittently strong magnetic fields.”

The researchers say there could have been as many as 100 of these flash events on the lunar surface during the first billion years of the moon’s existence, and each one would have produced a strong magnetic field lasting for a century or so.

Evans says that the discontinuous magnetic model explains not only the strength of the magnetic signature found in the Apollo rock samples, but also the fact that magnetic signatures vary widely in the Apollo group—with some strong magnetic signatures while others not. .

“This model is able to explain both the density and the diversity that we see in the Apollo samples – something no other model has been able to do,” Evans said. “It also gives us some time constraints on this titanium tumble, which gives us a better picture of the early evolution of the Moon.”

Evans says the idea is also testable. It means that there must have been a weak magnetic background on the Moon punctuated by these high-strength events. That should be evident in the Apollo group. While the strong magnetic signatures in the Apollo samples looked like sore thumbs, Evans says, no one has ever looked for the weaker signatures.

Having those weak signals combined with the strong ones will give this new idea a huge boost, which could eventually put the moon’s magnetic puzzle to rest.

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Andrew Naughtie

News reporter and author at @websalespromo