It is impossible to see, to venture, or even to send probes more than a few miles under the surface of Earth.
Almost all of the 4,000+ miles from the surface to the center is unreachable to humans. Yet scientists could not begin to understand our planet and its formation without having an accurate knowledge of that interior.
Beno Gutenberg provided the first reasonable accounting of Earth’s interior. His discovery proved that Earth wasn’t a solid homogeneous planet, but was divided into layers. Gutenberg was the first to correctly estimate the temperature and physical properties of Earth’s core. His discoveries have been so important that he is often considered the father of geophysics.
Born in 1889 in Darmstadt, Germany, Beno Gutenberg loved science as a boy and always knew he’d be a meteorologist. As he began his second year of university meteorological study in 1907, he saw a notice announcing the formation of a department of the new science of geophysics (Earth physics) at the University of Göttingen.
The idea of a whole new science fascinated Gutenberg. He transferred to Göttingen and, while holding onto a major in meteorology, studied under Emil Wiechert, a pioneer in the emerging science of seismology, the study of seismic waves caused by earthquakes and earth tremors.
By the time of his graduation in 1913, Gutenberg had shifted from meteorology (study of the atmosphere) to geophysics (study of Earth’s interior). It was a case of being in the right place at the right time. Gutenberg had access to all of Wiechert’s data and studies, the most extensive and comprehensive collection of seismic data in the world. Wiechert had focused on collecting the data. Gutenberg focused on studying the patterns of those data.
Gutenberg found that, typically, seismic waves did not reach all parts of the earth’s surface, even when the tremor was strong enough to have been detected everywhere. There always existed a shadow zone more or less straight across the globe from an event where no seismic waves were ever detectable.
He also noticed that seismic waves seemed to travel at different speeds on different trajectories through the earth. This all made Gutenberg suspect that the interior of the earth was not a solid, homogeneous mass. It must have several separate layers or regions.
Gutenberg settled on the image of the earth as an egg. The surface of the earth was thin and brittle like an eggshell. He realized that there must be a core to Earth (like an egg yoke) that was more dense than the surrounding mantle (the egg white).
If this image held true, seismic waves approaching the core would change speeds and be diffracted (bent) because of the density difference between layers. One of the kinds of seismic waves that Gutenberg studied was transverse waves. These waves would not enter the core at all. Knowing that transverse waves dissipate quickly in the liquid ocean, Gutenberg surmised that Earth’s core also had to be liquid.
Gutenberg had enough data from the recorded diffractions of enough seismic waves to calculate how big the core had to be and what its density had to be in order to create the diffraction patterns seismologists recorded. The core of the earth, he said, had a radius of 2,100 miles.
Based on these calculations, on chemical experiments he ran in early 1914, and on the measured chemical composition of meteorites, Gutenberg estimated that the core was a liquid mixture of nickel and iron, while the mantel was made up of rock material.
Gutenberg’s model was quickly accepted and was not improved upon until 1938. In that year, Inge Lehman completed a detailed study of “P” waves (another kind of seismic wave) with vastly improved equipment from that used in 1914. Her research showed that Earth’s core was divided into a solid inner core and a surrounding liquid outer core. She also broke the mantle into an inner and an outer mantle. This discovery completed our basic image of Earth’s interior.
The crust of the earth is solid. So is the inner core. But in between, the outer core and mantle (90 percent of the mass of the earth) are liquid to molten semi-solid. We do not live on a particularly solid planet.