We live on the most comfortable planet. It may not be visible, but the Earth's magnetic field plays a crucial role in maintaining that comfort. The remaining rocky planets in our solar system have much weaker magnetic fields and are therefore exposed to constant bombardment with high-energy particles from the sun. Yes, our biosphere owes much to an iron melt in the core of our planet.
However, the core is a mystery to us. The extreme conditions make it very difficult to understand: we can not perform experiments that fully mimic the core conditions, and our measurements are indirect because nobody wants to visit the core. That leaves us with computer models. Until recently, these models were rather limited. More and more computing power, however, shows that the core has an interesting story to tell.
Onions not parfait
Our planet, like all planets, was created by force. The accumulation of mass during their growth was due to large impacts and oceans of molten rock. Gravity provided a sort of filter: the heavy elements, such as iron, were pulled to the core, while light elements such as silicon and oxygen floated overhead.
This simple image provides the basic layering of the earth, but does not explain the magnetic field. This requires convection, which drives liquid iron streams that create a magnetic field. However, convection requires a temperature difference between the center of the core and its outer boundary. The thermal conductivity of an iron core, however, makes it difficult to imagine that the temperature differential is sufficient to initiate convection.
To further cloud the liquid iron, convection currents require some mixing. So we have a few different processes in play: gravity drives the stratification, while convection mixes the layers. When mixing the elements then solubility and chemical reactions play a role. Has oxygen in the early nucleus been retained thanks to solubility and chemical reactions? Does the presence of oxygen change the heat flux, which would then change the strength of the magnetic field?
To understand these processes, you have to do very difficult calculations. First, the quantum chemical properties of the elements have to be calculated to determine the configuration with the lowest energy of a mixture – how much oxygen should be contained in the iron. Then this calculation has to be combined with the physical movement of the elements and molecules. All these calculations must be carried out at temperatures of about 5,000 K and pressures of 160 GPa.
Install the GPUs
Twenty years ago, these calculations could not be meaningfully combined because there was simply not enough computing power available. Ten years ago, these calculations were feasible under limited circumstances. And now they can be applied at temperatures and pressures that are relevant to the Earth's core and have a sufficient scale to be meaningful (albeit on a very small scale).
To demonstrate this, a panel of researchers studied oxygen transport and storage through the nucleus. The researchers showed that the early nucleus probably had much more oxygen than the current nucleus. However, this oxygen may have formed a layered oxide layer (though at these temperatures it is probably better to think of this as a mixture of iron and oxygen) that is just below the boundary between the core and the cladding. The stratification and the associated reactions reduced the heat flow from the core. This, in turn, weakens the convection currents that drive the circulation required to form the earth's magnetic field.
In summary, these calculations make it a little more difficult, which was already difficult to explain.
The researchers note that although the core of their findings is reliable, some of the conclusions are poor. For example, the distribution of iron containing iron and oxygen is fixed. However, the model does not have sufficient scale to determine if stratification is stable when exposed to convection currents in the core and thermal gradients in the overlying jacket. This type of calculation requires a different model type.
The researchers also did not include the influence of silicon and magnesium in their calculations. These are the other two main players and could significantly change the results. The researchers treated their calculations as proof of the principle of the technique. The next step is to do a new set of calculations that have a more realistic mix of elements. Then we could get a clearer picture of the chemistry of the early Earth's core.
Physical Review X, 2019, DOI: 10.1103 / PhysRevX.9.041018 (Information on DOIs)