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A mysterious underground world
Humanity has successfully sent an exploration probe to Pluto, a dwarf planet of an average distance of 5.6 billion kilometers away from Earth, placed seismographs on Mars, and even brought back samples of rocks from asteroids. Yet, when it comes to the world spread out right under its feet, humanity has only managed to drill to a depth of 12 kilometers: the Kola Superdeep Borehole in Russia. Exploring the underground world is that difficult. Sending a measuring device to the center of Earth, 6,370 kilometers from the surface, is impossible with current technology. That is why Professor Kei Hirose says that Earth's interior is “a mysterious world indeed.”
“In geoscience, often, we cannot directly look at evidence, or take samples from deep within Earth. Therefore, many well-known geological phenomena have not yet been explained scientifically. Geoscience investigates phenomena from various directions to solve their mysteries. I contribute to these efforts by artificially creating the materials that would constitute Earth's deep interior by high-pressure and high-temperature experiments and investigating their properties.”
So, which mysteries is he trying to solve? When people think of earthquakes and volcanoes, tectonic plate motions might come to mind, which are, of course, essential characters in a much larger play. The play that Hirose and other geoscientists are trying to “write” using the language of science is the epic story of Earth, from the birth of the planet to the future of human civilization. Geoscience is necessary because everything that happens on Earth, in a sense, is brought about by Earth. Hirose is particularly fascinated by the emergence of life.
“As an extension of my research into how Earth came to be and what is going on inside it, I have a strong interest in how life on Earth came to be and how universal life is. Life emerged as a result of various chemical reactions that took place on Earth's surface. What formation processes make a planet suitable for life? I am continuing my research to find the answer to this question.”
First to reach a depth of 2,600 km and a pressure of 120 GPa
A device called a diamond-anvil cell is used in experiments to create high-pressure and high-temperature conditions by inserting a sample between diamonds of about 3.5 mm in diameter. Using this diamond-anvil cell, Hirose was the first person in the world to artificially create a substance thought to be at the bottom of the mantle. This material was named post-perovskite, and the paper announcing its discovery was featured on the cover of the journal Science in 2004, sending a wave of impact over the world.
“Earth's interior is structured like the inside of an egg, with the crust, mantle, and core. The core is divided into the liquid outer core and the solid inner core closer to the center. The mantle has several layers. We explored the structure of Earth's interior by analyzing the propagation of seismic waves in detail, but we had not yet determined the materials present in the interior. Therefore, we conducted experiments that created pressure conditions corresponding to the depth of Earth's interior. This resulted in a “pressure race,” in which researchers worldwide competed to create pressure conditions corresponding to greater and greater depths. The lowest part of the mantle is more than 2,600 km deep and under pressures of more than 120 GPa (gigapascal). We were the first ones to get there and reveal the materials hiding in the deep.”
It was thought at the time that the lowest part of the mantle would consist of perovskite-type material, being composed mainly of magnesium, silicon, and oxygen. However, the propagation of seismic waves differed from what was expected. The lowest part of the mantle sat directly above the liquid metallic iron of the outer core. Therefore, researchers believed that chemical reactions with the core changed the chemical composition of the lowermost part of the mantle. That is, before the discovery of post-perovskite.
“There were various hypotheses, but I thought the discrepancy indicated a change of crystal structure from perovskite. When we successfully pressurized perovskite to over 120 GPa, we could see that the pressure changed the crystal structure drastically.”
The result was post-perovskite. Still, why did the discovery of the material in the lowest part of the mantle have such an impact?
“There is the atmosphere, there is the rocky crust and mantle, then there is the core in the center. Earth has such a three-layered structure, but the interactions at the boundaries between the layers are important. For example, many things happen on Earth's surface, at the boundary between the atmosphere and the rocky crust. The atmosphere, ocean, and rocks are in contact, extracting various chemical elements from the rocks, which is how life emerged. The next important boundary is between the mantle and the core. What is happening there determines the evolution of the mantle and core. So, to understand the evolution of the entire Earth, it is critical to clarify the materials at the boundary between the mantle and the core.”
Loss of magnetic field stripped Mars of its oceans
Earth is a giant electromagnet: currently, its magnetic south pole is in the geographic north, and its magnetic north pole is in the geographic south, but the magnetic poles have reversed many times. The cooling of the core at the boundary between the mantle and the core is vital for generating of Earth's magnetic field.
“The convection of liquid in the outer core generates electricity, which creates the magnetic field. For thermal convection to occur, the liquid in the outer core must be cooled. After we learned that post-perovskite removes heat from the outer core liquid, we could study its thermal conductivity to understand how it interacts with the core and generates convection. For example, the types of rocks that make up a particular place determine the environment on the surface. Similarly, the constitution of the lowest part of the mantle determines the interaction between the mantle and the core, so it is important to grasp its constitution.”
The magnetic field plays a much greater role than we imagine. Take Mars, for instance. Once it had a magnetic field but lost it around 4 billion years ago. The loss of its magnetic field might have played a significant role in the subsequent loss of its oceans.
“Earth's magnetic field creates a barrier that prevents solar wind and cosmic rays, which are harmful to living organisms on land, from reaching the planet’s surface. However, if the magnetic field disappears, this barrier disappears, and light elements such as hydrogen in the atmosphere will be blown away by the solar wind, dissipating into space. Eventually, the oceans may dry up. Thus, the disappearance of the oceans from Mars may be due to the disappearance of its magnetic field.”
Will Earth lose its magnetic field in the future?
“If the inner core continues to grow larger and larger, there will be less and less room for convection in the liquid of the outer core. Then, the magnetic field will gradually weaken and ultimately disappear. But I am talking about a scale of a billion years from now.”
Hirose, however, says that even when the magnetic field has disappeared and oceans are lost, life on Earth will not vanish.
“Once life emerges, its adaptability is great. It can evolve rapidly in response to environmental changes. Even if Earth were to become like Mars, I do not think life would go extinct. Mars must have also had a rich environment in the past. So, life may have emerged on Mars as well. Even today, there may be life underground adapted to the harsh Martian environment.”
A second life form underground on Earth?
Terrestrial life synthesizes proteins based on information coded in DNA, which are responsible for the metabolism of living organisms. All life on Earth is of this type, and only this one type of life is known to exist. However, Hirose believes a different life form might exist and be discovered on Mars.
“If life is discovered on Mars and it is of a different type than terrestrial life, the discovery will have a tremendous impact on our knowledge of the variations, origin, and universality of life.”
On the other hand, Hirose also says there may be a different type of life underground on Earth.
“It would not be so surprising if life different from ours arose even 100 times on Earth in prehistoric times. We could think of this as 100 different forms of life emerging, but only our ancestors could adapt to their environment. Or perhaps there was another kind of life that took refuge in the depths of our planet. There is a chance we find a form of life with a different origin than ours on Mars or here on Earth, underground. It would be an amazing discovery,” says Hirose happily smiling.
“The changes and fluctuations of Earth's environment, on a long timescale, are often driven by the interior of the planet. The one exception is the appearance of oxygen in the atmosphere, the result of photosynthesis. However, most other environmental changes are driven by the interior. Therefore, we cannot understand the environmental changes on Earth's surface without understanding the changes in Earth's interior. Moreover, it is important to understand Earth's interior to understand what kind of environments are possible on other planets and whether there is a possibility of life there.”
A series of major events divided Earth into the atmosphere, mantle, and core. The Theia Impact was a hypothesized event in which Earth collided with another huge astronomical object. Another event was the so-called "Magma Ocean" when hot magma covered the entire planet. The third event was the formation of the core at the center of Earth. The details of these major events remain unclear. However, by learning more about Earth's interior, especially about the chemical composition of the core, we can get a closer look at what happened in the past. One of Hirose's current research projects is determining the chemical composition of the core.
Identifying the chemical composition of the core is the key to breakthroughs
The core is composed mainly of iron, but not pure iron: it contains impurities such as carbon, silicon, or even a large amount of hydrogen. However, the specific impurity elements and their concentrations are unclear. Hirose says that many things about the core still cannot be understood.
“The presence of impurities is important because differences in impurities influence the convection currents, which create the planet's magnetic field, in the liquid, outer core. Impurities in the core are light. So, the liquid rich in impurities becomes light and floats up, creating convection. There is also a significant difference in hardness and other physical properties between pure iron and iron alloys, such as steel that contains carbon. In other words, we cannot understand much unless we grasp the types and amounts of impurities in the core. Currently, we do not even know the temperature of the core. Therefore, determining the chemical composition is the key to the most significant breakthroughs, and that is what we are putting our efforts into now.”
Of course, researchers cannot drill down to the core to test for impurities. So, how do they find out?
“We observe under an electron microscope the samples created in high-pressure, high-temperature experiments to determine what kind of chemical reactions have taken place and create a phase diagram. Then, based on the phase diagram, we try to infer what conditions and liquid compositions could produce a solid inner core. This is one possible approach. However, researchers must leverage various approaches to answer any major question in geoscience. There are approaches from chemistry or condensed matter physics. There is also paleomagnetism, an approach based on the traces of magnetic fields left in rocks on Earth's surface. Geoscience requires many different approaches to address a single major question.”
Another target Hirose is focusing on is planets other than Earth. He says he is not sure if this is due to his fascination with the remarkable recent achievements using space exploration probes or due to the enthusiasm of his students, many of whom are already studying the internal structures of other planets.
“NASA says their targets for the next decade are Uranus and Neptune. Uranus and Neptune are mainly composed of ice (H2O), but we still know very little about the physical properties of water at the high pressure and temperature conditions inside these planets. It is difficult to know their internal structures just by looking at them from the outside. But I believe we can clarify the answers through ultrahigh-pressure, high-temperature experiments.”
Earth's interior is no longer a black box
When asked about his long-term goals, Hirose replies as follows.
“It is important to view Earth as one of the planets in our solar system. With the Earth in focus, I would like to think in a unified way about how the planets of the solar system were formed. Clarifying what is happening deep inside Earth clarifies what is happening overall on Earth. Studying the core is equivalent to studying the origin of Earth and its building blocks. The knowledge gained will be key to understanding planets other than Earth.”
Until recently, the Earth's deep interior was a black box. However, Hirose says it has become less so thanks to the rapid progress in the last 20 years since the discovery of post-perovskite. The progress is due to the increasing number of researchers who use diamond cells, accelerating the flow of research dramatically.
“The theory of plate tectonics was a major revolution in geoscience, but that was 50-odd years ago. I am sure we will see another critical revolution in the future.”
Finally, he has the following message for the next generation.
“Since I was a high school student, I have always thought that the natural sciences were the most interesting fields of study. Although they may not be directly related to our daily lives, I think the pursuit of truths like the origins of the natural world is the most exciting part of science. Students interested in pursuing the truth should not hesitate to join us at the School of Science.”
Hirose’s hobby is mountain climbing. In the summer of last year, he climbed Mt. Hotaka, a steep mountain with a summit of more than 3,000 meters above sea level. While climbing, Hirose sometimes wondered about the kinds of rocks that made up Mt. Hotaka.
“But I did not climb Mt. Hotaka because I wanted to learn about its geology,” Hirose laughs. “I just wanted to spend time at the mountain lodge.”
※Year of interview:2024
Interview/Text: OTA Minoru
Photography: KAIZUKA Junichi
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