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During NASA's Apollo program from 1968 to 1972 rocks returned to Earth and provided a lot of information about the Moon's history, but they have also been the source of an enduring mystery. The analysis of the rocks showed that some appeared to have formed in the presence of a strong magnetic field. It wasn't clear how a Moon-sized body could generate a strong magnetic field.
A new explanation for the Moon's magnetic mystery has been proposed. A study published in Nature Astronomy shows that giant rock formations sinking through the Moon's mantle could have produced the kind of interior convection that produces strong magnetic fields. The first billion years of the Moon's history could have produced intermittently strong magnetic fields.
Alexander Evans, an assistant professor of Earth, environmental and planetary sciences, said that a body of the Moon's size should not be able to generate a field that is as strong as Earth's. There's a way to get a high-intensity field intermittently, instead of thinking about how to power a strong magnetic field continuously over billions of years. Our model shows how that can happen, and it's in line with what we know about the Moon's interior.
The magnetic fields produced by planetary bodies are known as a core dynamo. The molten metals in a planet's core are caused by slowly dissipating heat. The magnetic field is created by the constant churned of the material. Earth's magnetic field is formed from that.
The models of the Moon's core suggest that it was too small and lacked the convective force to produce a strong magnetic field. A strong convective Churn can be achieved by a core that has dissipated a lot of heat. Evans says that the mantle surrounding the core wasn't much cooler than the core itself. The core's heat didn't have a place to go. The new study shows how the sinking rocks could have provided intermittent convective boost.
The story of these sinking stones begins a million years after the Moon's formation. The Moon is thought to have been covered in molten rock very early in its history. As the ocean cooled and solidified, minerals like olivine and pyroxene that were denser than the liquid magma sank to the bottom, while less dense minerals like anorthosite floated to form the crust. It took a bit longer to solidify because the liquid magma was rich in titanium and other elements. The denser titanium layer was 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- The titanium formations sank through the less dense mantle rock.
Evans and Tikoo modeled the dynamics of how those titanium formations would have sunk and the effect they would have on the Moon's core. The analysis showed that the formations would break into small blobs as small as 60 kilometers and sink intermittently over the course of a billion years.
The researchers found that when these blobs hit bottom, they would give a big boost to the Moon's core. The titanium formations would have been cooler than the core's estimated temperature of between 2,600 and 3,800 degrees, because they were just below the Moon's crust. The Moon's surface would have a magnetic field stronger than Earth's if the cool blobs came in contact with the hot core after sinking.
Evans said that it was like a drop of water hitting a hot skillet. A lot of heat can evaporate from the core when it is cold. The strong magnetic fields are caused by that.
The researchers say that there could have been as many as 100 downwelling events over the first billion years of the Moon's existence.
Evans says that the intermittent magnetic model accounts for the strength of the magnetic signature found in the Apollo rock samples, as well as for the fact that magnetic signatures vary widely in the Apollo collection.
"This model is able to explain both the intensity and the variability we see in the Apollo samples, something that no other model has been able to do," Evans said. It also gives us some time constraints on the foundering of this titanium material, which gives us a better picture of the Moon's early evolution.
Evans says that the idea is testable. It is implied that there should have been a weak magnetic background on the Moon. The Apollo collection should show that. The strong magnetic signatures in the Apollo samples stuck out like a sore thumb, but no one has ever looked for weaker signatures.
The strong signatures along with the weak ones would give this new idea a big boost, which could finally put the Moon's magnetic mystery to rest.
Alexander Evans, An episodic high-intensity lunar core dynamo, is available in Nature Astronomy. www.nature.com/articles/s41550-021-01574-y
Nature Astronomy journal has information.
There is a new explanation for the Moon's half-century magnetic mystery.
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