Author: Tetiana Koshliakova, PhD in Geology, Senior Researcher at the Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the National Academy of Sciences of Ukraine.
In science fiction, one can often find various scenarios of catastrophic events associated with large-scale natural processes: earthquakes, tsunamis, volcanic eruptions, floods, asteroid impacts, etc. These phenomena are astonishing and awe-inspiring, and they evoke fear of the unconquerable power of nature. However, we don’t always think about the powerful processes that are constantly taking place deep in the Earth’s interior and how they affect our daily lives.
Planet Earth is a unique cosmic body because it has a magnetic field and an atmosphere that provide the necessary conditions for life. And while the key role of the gas envelope is beyond doubt, the presence of the magnetosphere, at first glance, does not seem to be of such fundamental importance. However, this opinion is misleading.
Our planet is surrounded by a magnetic field that has existed since its formation. It is created by the movement of the hot outer core. Everything on the Earth is under the influence of its invisible power lines. In the first approximation, a magnetic field (geomagnetic field) is a dipole with poles located near the planet’s poles. It is a type of electromagnetic field created by moving electric charges or currents, and forms the magnetosphere, which deflects solar wind particles. They accumulate in the Van Allen radiation belts, two concentric regions around the Earth. Near the magnetic belts, these particles can «fly» into the atmosphere and cause the aurora to appear.
According to modern concepts, the geomagnetic field originated about 4.2 billion years ago (for comparison: the age of the Earth is almost 4.55 billion years) and was caused by sources located inside the globe, in the liquid outer core (by more than 90%) and near-Earth space (magnetosphere and ionosphere). Scientists believe that the formation of the planet’s electromagnetic field directly depends on the temperature of its core. The Earth’s core is heated to about 6000°C (like the surface of the Sun).
How the Earth’s core was formed
According to one of the most common theories, about 4.5 billion years ago the Solar System consisted of a cloud of gases and cold dust particles. At a certain stage, they began to collapse, and gravity brought them together to form a huge rotating disk. Its center turned into the Sun, and the matter in the outer orbits turned into large fireballs of gas and molten liquid, which gradually cooled and condensed.
At the same time, the surfaces of the newly formed planets were constantly bombarded by large cosmic bodies, which generated enormous amounts of heat during collisions and melted cosmic dust. Subsequently, when the Earth was formed, it was a homogeneous ball of hot rock. Radioactive decay kept the temperature high. Eventually, after about 500 million years, the planet’s temperature reached the melting point of iron – 1538°C. Relatively low-melting and volatile materials such as silicates, water, and air remained within the Earth’s outer part, forming the early mantle and crust. Droplets of iron, nickel, and other heavy metals gravitated toward the center, forming the early core. This process is called gravitational differentiation.
At the beginning of the planet’s formation, its core was completely molten. Since then, the Earth has been gradually cooling, releasing heat into space. In the process of solidification, a solid inner core was formed, which continues to grow in size. However, this process is very slow (the inner core grows by about 1 mm per year) because between the hot core and the cold surface, the Earth has a very dense rocky mantle that prevents it from cooling too quickly.
The core is located at a depth of 2900 km from the planet’s surface. It has the shape of a sphere with a radius of about 3500 km and consists of iron with other elements addeed. It is the densest and heaviest part of the planet (it accounts for only 15% of the Earth’s volume and 35% of its mass). The core has two layers: a solid inner layer (with a radius of almost 1300 km) and a liquid outer layer (almost 2200 km). The inner core seems to “float” in the outer liquid layer.
It is worth noting that science knows much less about the Earth’s core than about distant stars. After all, it is currently impossible to penetrate to such depths even using up to date methods, core samples are not available, so all information is obtained by indirect geophysical or geochemical methods.
When the core cools completely and becomes solid, it will have a huge impact on the entire planet. Scientists believe that the Earth will then become a “second Mars” with almost no atmosphere and no magnetic field. The appearance of our planet will change dramatically. There will be no more earthquakes and volcanic eruptions, and the movement of tectonic plates that form the surface relief will stop. Life will be threatened, because the geomagnetic field creates an insurmountable barrier in space that prevents cosmic radiation from reaching our planet (Van Allen radiation belts). The magnetic field’s force lines deflect and capture the solar wind, preventing it from depriving the Earth of its atmosphere. Without the magnetic field, the solar wind would destroy the ozone layer, which protects life from harmful ultraviolet radiation.
How to know when the Earth’s core will cool
There are two main reasons why the Earth’s core is so hot. The first is that heat has been stored in it since the Earth’s formation. The second is that it is heated by the decay of radioactive elements. Scientists are not yet sure what the exact ratio of these heat sources is.
One way to answer this question more accurately is to catch so-called geoneutrinos using special detectors. These are the lightest known subatomic particles released during the decay of radioactive substances in the depths of the planet. Detecting neutrinos and antineutrinos is an extremely difficult task because they react very rarely and weakly with matter. Massive neutrino detectors the size of a small office building are placed at great depths (over 600 meters). This is due to the need to create protection against cosmic rays that can lead to false alarms. In the process, the detector detects antineutrinos during their collisions with hydrogen atoms (two bright flashes are observed inside the device at each collision). By counting their number, scientists can determine the number of uranium and thorium atoms that remain inside our planet.
William McDonough, a professor of geology at the University of Maryland, notes that an accurate understanding of the radiation from the Earth’s core requires at least three years of data from five different antineutrino detectors. In this way, researchers will be able to calculate how much “radioactive fuel” is still left in the Earth’s core. Current hypotheses suggest that it could last for several tens of billions of years.
What will happen first: the cooling of the Earth’s core or the extinction of the Sun?
The thermonuclear reactions that make the Sun shine occur in its core at temperatures above 10 million degrees Celsius. They consume hydrogen, which makes up most of the solar matter. Its reserves will last for another 5-6 billion years, after which our sun will go out.
As mentioned above, the Earth’s core is much colder — about 6000°C. Theoretically, due to heat seeping through the mantle, the core temperature should drop by only one degree per 10 million years or 100 degrees per billion. However, modern model calculations have shown that the Earth’s core is now losing heat much more slowly than it did at any other time in the history of our planet, and this heat will last for at least a billion years of the magnetic field’s existence. This slowdown in the core cooling process is due to its heating due to the decay of radioactive elements. Scientists believe that by the time the Sun goes out, the Earth’s core will have cooled by only a few hundred degrees. However, the extinguished Sun (which will become a white dwarf by then) will be much hotter, and its cooling will last for hundreds of billions of years, meaning that it will extinguish earlier but cool later than the Earth’s core.
Did Mars also have a magnetic field?
While exploring the surface of Mars, scientists have noticed characteristic features that may indicate volcanic activity on this planet in the distant past. This led researchers to believe that ancient Mars could have had a molten core and, accordingly, a magnetic field and atmosphere like on the modern Earth. There is a hypothesis that there was once a Van Allen belt around the Red Planet, which protected it from the solar wind. However, when the core froze, it lost its “shield”. Some scientists suggest that the loss of the magnetic field could have been caused by a cosmic event — a collision with an asteroid.
The fact is that the so-called crustal dichotomy was discovered on the planet — the rocks in the northern hemisphere are much thinner than in the southern hemisphere. According to the American Mars Global Surveyor orbiter, instead of a single magnetic field, there are many local, sometimes quite strong magnetic anomalies on Mars. On the map, they look like a colorful spotted mosaic picture. The magnetic anomalies are especially strong in the Southern Hemisphere, in the area of the giant depression Hellas Planitia with a diameter of 600 km. Canadian astrophysicist Sabine Stanley from the University of Toronto suggests that a powerful meteor impact at the beginning of the planet’s history (about 4 billion years ago) could have damaged the liquid core and affected the magnetic field. It was previously thought that Mars’ core had cooled because it is half the size of Earth, but this theory was disproven by the recent discovery of liquid core inside Mercury, which is even smaller.
The nature of localized magnetic anomalies on Mars remains a mystery, as their magnetization is too high for ordinary rocks. Similar anomalies are also found on Earth (for example, in Eastern Siberia and South Africa). They are associated with a specific type of mineral, stable maghemite, which occurs in impact structures. This mineral is a magnetic iron oxide (Fe2O3). As you know, red-colored weathered crusts are common only on two planets in the Solar System — Earth and Mars. The latter is called the Red Planet because it is covered with a thick layer of red-brown iron oxides and hydroxides in the form of sand and dust. However, these minerals are magnetic. It is likely that the impact of a space body caused the formation of maghemite on Mars.
Such analogies between the planets give grounds to consider Mars as a model of the Earth in the distant future. The rapid development of modern technologies stimulates the emergence of ambitious goals for the colonization of other bodies in the Solar System. Although all plans to create settlements on the Red Planet are still only theoretical, the study of the Earth’s core may be the key to understanding the nature of the magnetic field, which is a necessary element for ensuring the conditions for the existence of living organisms.