A new theory has been proposed to explain the intensity of the magnetic field of the Moon more than 4 billion years ago. It would have been generated by dynamo effect, not only in its core, but also in an ocean of magma above this core, in the lunar mantle.
The moon in the past generated its own magnetic field. How do we know? Simply because traces of this fossilized magnetic field have been found in the lunar rocks brought back by the Apollo missions and because remnant of magnetic fields have too been measured from space, as the Clementine probe has shown.
Some of these fossil magnetic fields, however, could have been formed by the impact of small celestial bodies and for that reason it was not certain that the measured magnetic fields had their origin in the bowels of our satellite.
Still, an analysis of a lunar rock sample indicated that there must have been a large and stable magnetic field on the surface of the Moon, 4.2 billion years ago. This field has weakened over time to leave only a low value and scientists now consider that it is no longer produced by the Moon. It’s known that on Earth it is quite different and that the self-excited dynamo, whose processes are being explored using the VKS experiment, is still active.
This lunar magnetic field poses multiple problems to selenologists because it would have been generated for at least 800 million years, even a few billion years. However, because of the small size of the Moon, it should not have enough energy reserves, either in the form of radioactive elements or heat of residual accretion, to have been able to keep a convective core producing the magnetic fields measured.
More surprising, the intensity of the lunar magnetic field was to be comparable to that of the Earth just after the formation of the Moon. How would that have been possible?
Researchers from the Institute of Geophysics at the University of Texas, Princeton University and Arizona have just come up with a new solution to this riddle with an article published in Earth and Planetary Science Letters.
Their solution? The existence of an ocean of magma rich in iron and convection in the lower lunar mantle, forming an envelope just above the metal core of the Moon.
A magnetic field comparable to that of the Earth, even temporary, would be too intense to be able to be generated by the small core, most likely of iron and nickel of the Moon. It appears indeed reasonable that a different structure is involved.
We have reason to think that today this core is about 330 kilometers in radius and that it is still partially melted. Modern analyzes of the seismic data delivered by Apollo missions also suggest that this core is surrounded by a partially melted layer (10 to 30%) of the lower mantle, with a thickness of about 150 kilometers.
This layer would have been completely melted, more than 4 billion years ago, and had to contain enough iron for a magnetic field to be generated by a dynamo effect in this ocean of convective magma. In fact, the idea had already been proposed in the case of the magnetic field of the primitive Earth and it is the latter that inspired the American researchers.
In the case of the Moon, by creating magnetic fields closer to its surface in the model, the scientists made sure that they will be stronger, since the intensity of these fields decreases when the distance from the place of generation increases. As a bonus, the simulations conducted by the planetologists would explain why these fields became less strong about 3.56 billion years ago. The ocean of magma would have eventually disappeared, leaving functional just the dynamo of the core.
Scientists believe that this scenario also occurred with other rocky planets, like Mars for example.