One of the most difficult objects for research is the state of matter in which it is located in the bowels of the planets. This state is called warm dense matter. This name emphasizes that the substance, firstly, is strongly compressed by pressure forces, which per atom can exceed the forces of interatomic interaction, and secondly, it is heated to high, but not too high temperatures: the substance is on the verge of ionization, but still is not fully plasma as, for example, the matter of hot stars.
The complexity of the research is due to the fact that it is extremely difficult to obtain such conditions in the laboratory, and you cannot really penetrate into the bowels of the planets. Nevertheless, we still manage to do something.
The easiest way to achieve the desired state is by hitting something hard on the sample: powerful focused laser pulses are suitable, for example. Then the substance will shrink for a split second and slightly (by several thousand degrees) heat up. The problem with this approach is that, firstly, it is still a dynamic process, and in the bowels of the planets, matter is in a stationary state, and can behave differently, and secondly, due to the extremely short duration of the compression state, sophisticated diagnostic techniques to measure something relevant.
There is another approach: a small piece of substance is placed between ultra-sharp diamond needles, which are placed under high pressure. Due to the small area and high hardness of the diamond, a pressure of millions of atmospheres is achieved. This method, called the diamond anvil method, has gained immense popularity in recent years due to the successful implementation of the idea of a multi-stage anvil: when a smaller anvil is placed in the anvil, inside which the substance under study is already located.
But there is another problem. At such high pressures, many substances begin to flow: they turn into a liquid state. This is exactly what happens, including with iron, of which, according to modern concepts, the earth’s core mainly consists.
The most reliable way to determine the density of a substance in an anvil is X-ray diffraction. X-rays are diffracted by atoms, and due to interference, the scattered signal contains information about the distances between the atoms. And knowing this distance and the mass of nuclei, it is easy to determine the density. But this is easy to do when the substance forms a crystal: all distances are the same, and the signal is pronounced. In a liquid, the distances between atoms are very different, and the signal is blurred.
In a recent article published in PRL, the scientists managed to solve this problem by applying a new, more sophisticated way of processing X-ray diffraction data. This made it possible for the first time to measure the density of iron at pressures up to 1.16 million atmospheres and a temperature of about 4000 ° C. It turned out that its density at these parameters reaches a value of 10.1 g / cm³, which is 29% higher than under normal conditions. Extrapolating these data to higher pressures showed that the density of pure iron is 7.5% higher than the density of the Earth’s outer core, which was measured by seismological methods, which most likely means that there is a significant amount of a lighter element in the core.
It is natural to assume that oxygen is such an element. Only now it is well known that oxygen is extremely poorly soluble in iron, so it settled deeper, in the solid inner core, about which it is known that it really has a lower density than a pure iron crystal. But what provides the low density of the liquid core is still unknown.