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The Earth's core is too hot, not heavy for oxygen, but may have a rusty coating



  Image of magnetic field lines originating from a planetary nucleus.

We live on the most comfortable of the planets. It may not be too visible, but the Earth's magnetic field plays a crucial role in maintaining this comfort. The other rock planets in our solar system have much weaker magnetic fields and as a result are subjected to constant bombardment of high energy particles by the sun. Yes, our biosphere owes a great deal to a molten iron pool at the heart of our planet.

Yet the nucleus presents us with a kind of puzzle. Extreme conditions make it very difficult to understand: we cannot conduct experiments that completely reproduce nuclear conditions, and our measurements are indirect, since no one wants to visit the nucleus. That leaves us with computer models. Until recently, these models were quite limited. However, the ever-increasing computing power is beginning to reveal that the kernel has an interesting story to tell.

Onions are not perfume

Our planet, like all planets, is born of violence. The accumulation of mass during its growth comes through great impacts and oceans of molten rock. Gravity provides a kind of filter: heavy elements such as iron are drawn to the core, while light elements such as silicon and oxygen are left to float on top.

This simple picture provides the basic stratification of the Earth, but do not explain the magnetic field. To do this, you need to turn on convection, which drives currents from liquid iron, which generates a magnetic field. However, convection requires a temperature difference between the center of the core and its outer boundary. But the thermal conductivity of the iron core makes it difficult to imagine that the temperature difference was sufficient to reach convection.

To obscure the liquid iron further, the convection currents require a certain amount of mixing. So, we have several different game processes: gravity leads to stratification, while convection mixes layers. Then, as the elements are mixed, solubility and chemical reactions come into play. Is oxygen retained in the early nucleus due to solubility and chemical reactions? Does the presence of oxygen alter the heat flux that would subsequently change the strength of the magnetic field?

To understand these processes, you have to perform a very difficult set of calculations. First, the chemical properties of the elements have to be quantified to determine what the lowest energy configuration of the mixture is ̵

1; how much oxygen there is to be in the iron. Then this calculation must be combined with how the elements and molecules move physically. All these calculations must be carried out at temperatures of about 5,000 K and a pressure of 160GPa.

Turn on GPUs

20 years ago you couldn't combine these calculations in any useful way because there simply wasn't enough computing power available. Ten years ago, these calculations were viable in limited circumstances. And now they can be applied at temperatures and pressures that are relevant to the Earth's core and of a scale sufficient to make sense (albeit a very small scale).

To demonstrate this, a group of researchers examined the transport and retention of oxygen from the core. Researchers have shown that the early nucleus probably had significantly more oxygen than today's nucleus. However, this oxygen may have formed a stratified oxide layer (although at these temperatures it is probably better to think of it as a mixture of iron and oxygen) just below the boundary between the core and the mantinella. Stratification and concomitant reactions reduce the amount of heat flow from the nucleus. This in turn weakens the convection currents that drive the circulation required to form the Earth's magnetic field.

In summary, these calculations make something that was already difficult to explain, a little more difficult to explain.

The researchers note that, although the core of their results is reliable, some of the conclusions are numerous. For example, the separation of iron and oxygen-containing iron is solid. However, the model is not of sufficient scale to determine whether the stratification is stable when exposed to convection currents in the core and thermal gradients in the mantle above. These types of calculations require a different type of model.

The researchers also do not include the influence of silicon and magnesium in their calculations. These are the other two main elements and can change the results significantly. The researchers treated their calculations as proof of the principle of technology. The next step is to perform a new set of calculations that have a more realistic combination of elements. Then we can get a clearer picture of the chemistry of the early Earth's core.

Physical Review X, 2019, DOI: 10.1103 / PhysRevX.9.041018 (About DOIs)


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