We know very little of what we have under our feet. An ignorance that increases as we get closer to the center of the Earth: it is a fact long lamented by geologists that we know the center of the Sun better than that of our planet , despite the fact that the former is more than 150 million miles away. kilometers and the second a few thousand. If that were not enough, geophysics and geology do not enjoy the congratulations of journalists and scientific popularizers: they are more attracted to stories of black holes located thousands of light-years away or invisible subatomic particles than what happens 6 000 kilometers under their houses.
Knowing the interior of the Earth is a very difficult undertaking, and what makes it more complicated is that its properties cannot be unequivocally determined with measurements from the surface. The way geologists work is very similar to when we receive a Christmas present and shake the box to see if we can guess from the sound what is inside: a music CD, a computer video game or a small book can sound the same. This is what is called the problem of the uniqueness of the solutions: different answers can account for the data obtained. In our case, those Christmas shocks are the dreaded earthquakes .
What is the core of the Earth like?
The core of our planet is about 3,500 km in radius and contains 32% of the entire mass of the Earth. It is made up of a fluid outer zone, which is where the magnetic field is supposed to be generated, and a solid inner zone, described by the Danish Inge Lehmann in 1936 in the article with the shortest title in the history of science: “P’” . We may be tempted to think that the center of our planet is an iron block like the ones we are used to seeing, but it is not. The answer may be even more surprising: In 1995 geophysicists at the Carnegie Institution of Washington hinted that there may be a huge crystal at the center of the Earth.
It all started when they discovered something amazing: there is a gap in the arrival time of seismic waves when they travel from pole to pole and when they travel through the equator: those that cross the poles arrive four seconds earlier . This means that the solid core of the earth is anisotropic, that is, it is not the same in all directions. Or put another way, in the center of our planet there is some kind of structure but we don’t know what it is. Can there really be a single crystal in the center of the Earth? The proposal is so radical that different laboratories around the world are trying to see experimentally if it is possible.
The center of the Earth never ceases to amaze us… and to grow. As the Earth cools over time, the size of the inner core grows as the molten iron solidifies. Calculations suggest that the central core began to form between 1 and 3 billion years ago and has not stopped growing since then. In addition, the difference in temperature that exists between both parts is enough to cause a convection movement in the outer core: the hot material rises and the cold material falls, analogous to what happens when we heat a pot of water. And it is this convection that animates and maintains one of the great unknowns of geophysics: the Earth’s magnetic field.
Where does the Earth’s magnetic field come from?
There are only two ways to produce a magnetic field : through permanent magnetization, as happens in magnets, or through electric currents. Whatever the explanation, it must answer four well-established basic facts: that the magnetic field is produced inside the Earth; that has been with our planet practically since its origin; that varies in intensity and direction and, finally, the most mysterious of all: that from time to time there is a reversal of the field, so that magnetic north becomes south and vice versa.
A permanent magnetization is not able to explain all this, so we are only left with the proposal made in 1919 by the Irishman Joseph Larmor , when he suggested that certain electric currents should produce the magnetic field.
The so-called terrestrial dynamo , the mechanism that creates the magnetic field of our planet, has marked one of the longest and most mathematically complex scientific adventures that exist. We begin to understand it thanks to a German who at the age of 15 discovered that he was of Jewish origin and had to emigrate to the United States, far from Nazi Germany: Walter Elsasser . He began to lay the groundwork for the new theory in 1941, in the spare time he had left after his hours of service in the US Army Signal Corps. Little by little other researchers were adding pieces that were going to give consistency to the theory until in 1996 Gary Glatzmaier and Paul Roberts proposed the first realistic model of how the Earth’s magnetic field is generated : almost 400 years had passed since we discovered that our planet behaved like a Great Magnet. That yes, in spite of all the advances and discoveries that we have lived, the nucleus of the Earth continues being, still, that unknown country.
References:
Merrill, R. T. (2010) Our magnetic Earth, The University of Chicago Press
