The mystery of the magnetic field of the moon rock, which has lasted for half a century, is about to be solved.

The rocks brought back to Earth during NASA's Apollo program from 1968 to 1972 provided a great deal of information about the moon's history, but also created an unsolved mystery. The analysis shows that some rocks seem to be formed in the presence of a strong magnetic field, which is as strong as the earth. But it is not clear how the moon-sized object produces such a strong magnetic field.

A few days ago, research led by earth scientists at Brown University has proposed a new explanation for the mystery of the moon's magnetic field. The study, published in the journal Nature Astronomy, suggests that huge layers of rock sinking into the moon's mantle may have triggered internal convection, creating a magnetic field. The researchers say that in the first billion years of the moon's history, this process may have created an intermittent strong magnetic field.

Alexander Evans, assistant professor of earth, environment and planetary science at Brown University, said: "everything we know about the way magnetic fields are generated in the core of the planet suggests that objects the size of the moon should not be able to produce a magnetic field as strong as Earth," said Alexander Evans, an assistant professor of earth, environment and planetary science at Brown University, who completed the study with Sonia Tikoo of Stanford University. "but instead of thinking about how to provide a strong magnetic field continuously for billions of years, there may be a way to get a high magnetic field intermittently. Our model shows how this happens, which is consistent with what we know about the interior of the moon. "

The planet generates a magnetic field through a so-called "core generator". The gentle heat flow will cause the molten metal in the core of the planet to convect. The constantly stirring conductive material is the cause of the magnetic field. This is how the earth's magnetic field is formed, which protects the earth's surface from dangerous radiation from the sun.

Today, the moon lacks a magnetic field, and models suggest that the lunar core may be too small to produce a continuous strong magnetic field. In order for the lunar core to produce strong convective agitation, it needs to emit a lot of heat. In the early days of the moon, the mantle around the core was not much colder than the core itself, Evans said. The heat in the lunar core has nowhere to go, so there is not much convection in the lunar core. But the new study shows how sinking rocks provide intermittent enhanced convection.

These rocks began to sink millions of years after the moon formed. It is thought that the early moon was covered by lava oceans. As the vast ocean of magma begins to cool and solidify, minerals such as olivine and pyroxene, which are denser than liquid magma, sink to the bottom, while lower-density minerals such as plagioclase float to form lunar shells. The remaining liquid magma is rich in titanium and heating elements such as thorium, uranium and potassium, so it takes longer to solidify. When the titanium layer finally crystallizes under the lunar crust, it is denser than the early solidified minerals below it. Over time, titanium strata sink from the lower-density mantle rocks below, a process known as gravity reversal.

In the new study, Evans and Tikoo simulated the sinking of these titanium formations and their possible effects when they eventually reached the core of the moon. The analysis, based on the moon's current composition and estimated mantle viscosity, suggests that these strata may split into "chunks" with a diameter of 60 kilometers and sink intermittently over a period of about 1 billion years.

The researchers found that when these small pieces finally reached the bottom, they caused a lot of vibration to the moon's core generator. Because it is located below the moon's crust, the temperature of the titanium layer is relatively low, well below the core temperature of about 2000 to 2100 degrees Celsius. When the cooled piece sinks and comes into contact with the hot core, the temperature difference is huge, driving the nuclear convection to increase, enough to make the magnetic field on the moon's surface as strong as or even stronger than the Earth's magnetic field.

"you can think of the process as a drop of water hitting a hot frying pan," Evans said. "something very cold touches the nucleus of the moon and suddenly releases a lot of heat. This causes the core to become unstable, resulting in these intermittent strong magnetic fields. "

In the first billion years of the moon's existence, the researchers say, there could be as many as 100 such cooling patches sinking, each of which could produce a strong magnetic field that lasts for a century or so.

Evans said that the intermittent magnetic model not only explains the magnetic field strength found in Apollo rock samples, but also explains the fact that the magnetic field signals of Apollo samples vary widely-some have strong magnetic characteristics, while others do not.

"this model can explain the intensity and variability of the magnetic field we see in the Apollo samples-something that other models cannot do," Evans said. "it also gives us some time frame for the formation of this titanium material, which gives us a better understanding of the early evolution of the moon."

Evans says the idea is also worth testing. This means that there should be a weak magnetic field environment on the moon interrupted by these high-intensity magnetic field events. This is obvious in the Apollo rocks. Evans said that although the strong magnetic signal in the Apollo sample is very prominent, no one has ever really looked for a weaker magnetic signal.

These weak and strong signals strongly support this new conjecture and could eventually end the mystery of the moon's magnetic field.