- Moon had a very strong magnetic field, which gradually faded away with time.
- A new model suggests that the Moon’s magma ocean could be responsible for this lost magnetism.
- The magma ocean dynamos might have been a usual event in rocky planets like Mars and Earth.
Earth’s Moon is nearly 4.47 billion years old. In its early stage, it had a very strong magnetic field – like present magnetic field of Earth. But over time, the strength of the Moon’s magnetic field has weakened.
Moon has a much smaller core than Earth’s; then how could it have had such a powerful magnetic field. The question remains a mystery in the saga of Moon’s evolution.
Now, Scientists at Arizona State University and University of Texas Institute have come up with a new model that explains this strong lunar magnetic field. Let’s find out in details what they’ve discovered.
The magnetic field of Earth is responsible for deflecting most of the solar wind and ionizing radiation. It prevents these harmful rays from hitting the atmosphere directly that would otherwise destroy the ozone layer which keeps ultraviolet rays outside the Earth’s atmosphere.
Internal Structure of the Moon | Credit: Wikimedia
While the magnetic field of Earth is produced by what’s called dynamo (caused by fluid motion of conducting metal in outer core), the Moon’s core isn’t large enough to generate magnetic field of that level.
The New Model
The proposed model explains how Moon’s magnetic field could have attained Earth-like magnitudes. In this model, the dynamo is not caused by the small metal core of the Moon, but by a top-heavier layer of molten materials.
Image Credit: Arizona State University
The Moon’s mantle’s bottom-most layer melts to create a metal-rich magma ocean, which resides at the top of metal core. The dynamo is further driven by convection — heat transfer due to bulk motion of molecules within molten rocks — that generates a strong magnetic field. This would have been captured by the cooling crust at the lunar surface, as well as by the samples brought back in Apollo mission.
The concept of this metal-rich magma ocean dynamo has already been presented for magnetic field of early Earth. However, researchers realized that the same methodology could also be applied to the Moon.
They further explained that the base of Moon’s mantle still has a partial molten layer. Since the strength of magnetic field reduces as you go away from the dynamo area, the powerful magnetic field could be achieved at the surface of the moon if the dynamo operated in the mantle, instead in the core.
Reference: ScienceDirect | doi:10.1016/j.epsl.2018.04.015 | ASU
Researchers ran some simulations of Lunar’s core dynamo that showed that the mantle’s lower layer was overheating and melting. In fact, melting process was a crucial factor to this new model.
Gradually magnetic field got weaker
Nearly after a billion year of moon’s formation, its strong magnetic field became weaker. The new model also explains this phenomenon – As the temperature of the moon decreased, the magma ocean would have turned into solid, and in the meantime, core dynamo would have continued creating the later weaker field.
According to the researchers, the magma ocean dynamos, like explained in the model, might have been a usual event in rocky planets like Mars and Earth.
The model could be further enhanced by using tidal forces, which would have imparted additional energy to the magma ocean dynamo, and through interactions with the lunar core.
Previous researchers have shown that a traditional thermochemical core dynamo wouldn’t have been able to create the inferred paleo-fields. Therefore exotic approaches should be taken into account. The magna ocean dynamo seems perfect to explain the all observable evidences, although the mechanism would require a surprisingly high conductivity. Further detailed study is required to analyze whether such scenarios could have existed in the history of Lunar evolution.
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