Madrid, 13 (European Press)
A new study has revealed how the moon might have had an episodic magnetic force early in its history, a question that has puzzled researchers since the time of the Apollo program.
Returned to Earth during NASA’s manned lunar program from 1968 to 1972, the rocks have provided large amounts of information about the history of the Moon, but they have also been a source of enduring mystery.
Analysis of the rocks revealed that some of them appeared to have formed in the presence of a magnetic field as strong as Earth’s. But it was not clear how a moon-sized object could generate such a strong magnetic field.
Now, an investigation led by a geologist at Brown University in the US is proposing a new explanation for the moon’s magnetic mystery. The study, published in Nature Astronomy, shows that giant rock formations plunging into the lunar mantle can produce a type of internal convection that generates strong magnetic fields.
The researchers say the processes may have produced strong magnetic fields intermittently during the first billion years of the moon’s history.
“Everything we thought about how to generate magnetic fields in the cores of planets tells us that an object the size of the Moon should not be able to generate a field as powerful as Earth,” explains Associate Professor Alexander Evans. and Planetary Science at Brown and co-author of the study with Sonia Tiko of Stanford University.
“But instead of thinking about how to feed a strong magnetic field continuously for billions of years,” he continues, “there might be a way to achieve an intermittently high-intensity field. Our model shows how this might happen, and is consistent with what we know about the interior of the Moon.”
Planetary bodies produce magnetic fields through what is known as a fundamental dynamo. The slow dissipation of heat causes molten metals to move into the planet’s core. The constant stirring of electrically conductive materials is what produces a magnetic field. This is how the Earth’s magnetic field is formed, which protects the surface from the most dangerous radiation from the sun.
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 strong, persistent magnetic field. In order for the core to have a strong heat load, 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. Because the heat from the core had nowhere to go, there wasn’t much convection in the core, but this new study shows how sunken rock can provide intermittent convection pulsations.
The history of these sunken stones begins a few million years after the formation of the moon. At the beginning of its history, it is believed that the moon was covered with an ocean of molten rock.
As the vast ocean of magma began to cool and solidify, minerals such as olivine and pyroxene, which are 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 this layer of titanium finally crystallized beneath the crust, it was denser than the minerals that solidified earlier. Over time, titanium formations sank through the less dense mantle rocks below, a process known as gravitational inversion.
In this new study, Evans and Techo model the dynamics of how titanium formations sank, as well as the impact they might have when they finally reach the lunar core.
The analysis, which was based on the current composition of the Moon and the estimated viscosity of the mantle, showed that the formations likely fractured into patches up to 60 kilometers in diameter and sank intermittently over the course of about one billion million years.
According to the researchers, when each of these spots hits the bottom, it will vibrate with the force of the dynamo located at the core of the moon. Because it lies just below the lunar crust, the titanium formations had a relatively low temperature, much more than the estimated temperature of the core, between 2,600 and 3,800 degrees Fahrenheit.
When the cold spots came into contact with the hot core after it sank, the temperature mismatch would have resulted in an increase in the core convection, just enough to drive 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,” says Evans. You have something very cold touching the heart, and suddenly a lot of heat can be pouring out. This increases the excitation of the nucleus, giving these intermittently strong magnetic fields.”
Researchers claim that there could have been as many as 100 of these events during the moon’s first billion years of existence, and each of them could have produced a strong magnetic field lasting for a century or so.
Evans notes that the discontinuous magnetic model explains not only the strength of the magnetic signature found in the Apollo rock samples, but the fact that magnetic signatures vary widely in the Apollo group, with some having strong magnetic signals while others do 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 asserts. It also gives us some time constraints in casting this titanium material, giving us a better picture of the early evolution of the Moon.
He adds that the idea is also testable. It means that there must have been a weak magnetic background on the Moon that was dotted with these high resistance events. That should be evident on the Apollo group — while the strong magnetic signatures on the Apollo samples stand out like sore thumbs, no one really looked for the weaker signatures, Evans says.
He concludes that the presence of those weak signals combined with the strong ones will give a huge boost to this new idea, which could put an end to the moon’s magnetic mystery.
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