This Month in Physics History

September 1936: Seismologist Inge Lehmann Concludes That Earth Has an Inner Core

By carefully studying earthquake shock waves, Lehmann realized that the prevailing view of Earth’s structure was incomplete.

By Liz Boatman | August 10, 2023

Credit: Background drawn from pgs. 88 and 97 of Lehmann, I. (1936): P', Publications du Bureau Central Seismologique International, Série A, Travaux Scientifique, 14, 87–115. Foreground is public domain.

In 1936, Danish seismologist Inge Lehmann proposed that Earth has a solid inner core.

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In the early 1900s, scientists were pondering a tricky question: What lies deep — very deep — beneath our feet? Relying on seismographs, which measure the shock waves of earthquakes, geophysicists had assumed that the Earth’s core was a hot, molten sphere. But the theory had holes, failing to explain stray waves measured in unexpected places.

Then, in September 1936, a Danish geophysicist named Inge Lehmann published a paper, succinctly called “P′,” arguing for what scientists now take for granted: That buried inside the Earth’s molten layer is a solid core.

Born in 1888, Lehmann attended the first coeducational school for children in Denmark, run by Hanna Adler, aunt of physicist Niels Bohr. At Fællesskolen, boys and girls alike learned traditional subjects, like math and science, as well as crafts like woodworking and needlepoint. “No difference between the intellect of boys and girls was recognised,” she wrote later, “a fact that brought some disappointment [to me] later in life when I had to recognise that this was not the general attitude.” (There were few women in science in Lehmann’s era.)

After completing a mathematics degree at the University of Copenhagen in 1920, Lehmann spent a few years working as an assistant for an actuarial science professor. But she had also taken chemistry, physics, and astronomy courses during school, and found the topics interesting.

At the time, many geologists, geodesists (who study Earth’s geometry), and seismologists were captivated by questions of Earth’s geological history and structure. One of these scientists was Niels Erik Nörlund, the director of the Danish geodetic institute, the Den Danske Gradmaaling. In 1925, he invited Lehmann to work as his assistant. Nörlund was on a mission to construct the most advanced and most accurate seismograph stations in the world, building a network throughout Denmark and Greenland to study earthquakes.

At the time, Lehmann knew little about earthquakes. “I may have been 15 or 16 years old when, on a Sunday morning, I was sitting at home together with my mother and sister, and the floor began to move under us,” she wrote in a 1987 article. “This was my only experience with an earthquake until I became a seismologist 20 years later.”

Working with Nörlund, Lehmann learned that seismographs, which record shock waves, enabled scientists to pinpoint an earthquake’s origin, or epicenter. And because waves move differently through different materials, “knowledge of the earth’s interior composition could be obtained from the observations of the seismographs,” she wrote — a roundabout way to peer inside the planet, since no direct samples could be taken from so far beneath the surface.

An exceptional mathematician, Lehmann took to the calculations quickly. By the late 1920s, Lehmann’s expertise in seismographic interpretation had led her straight to an unsolved problem in geophysics.

Seismic data had long recorded two types of waves. P-waves, or pressure waves, cause matter to compress and expand in the direction the waves are traveling. P-waves can move through solids, liquids, or gases. S-waves, or shear waves, are different — they cause matter to slide across itself, perpendicular to the wave’s direction of motion, and they can only travel through solid matter or some extremely viscous liquids.

Because pressure waves travel faster in solids than in liquids, the trajectory of an earthquake’s P-wave “bends” as it moves from Earth’s solid mantle into its liquid core, similar to how light bends through a prism.

Since the late 1880s, seismologists had observed that, unlike P-waves, S-waves could not be detected on the other side of the world from an earthquake epicenter. Something inside Earth was stopping them in their tracks. In 1926, British geophysicist Harold Jeffreys concluded that Earth must have a liquid core, which prevented the S-waves from passing through. Jeffreys’ model also meant that P-waves shouldn’t make it to certain places on the other side of the world: The waves, bent as they traveled through the liquid core, created "shadow zones” — patches far from the epicenter that waves didn’t reach.

But there was a problem: Newer, better seismographs were detecting P-waves in shadow zones. They were few and faint, and geophysicists treated them as errant. German seismologist Beno Gutenberg, for example, called them “diffracted waves, with no explanation given,” Lehmann wrote.

Then, in 1929, a massive New Zealand earthquake provided more concrete proof: P-waves had undeniably been detected in the earthquake’s shadow zones. Lehmann, poring over the new earthquake data, was perplexed. If indeed Earth had only a mantle and a liquid core, why did P-waves reach those zones?

Then she had an idea. Perhaps another layer in Earth’s core was interacting with the waves — a core nested within a core. She revised seismologists’ working model of Earth and, in September 1936, published her theory in a paper titled simply “P′” (The prime denoted waves that had passed from the mantle into the core). In it, she wrote, “We take it that, as before, the earth consists of a core and a mantle, but that inside the core there is an inner core in which the velocity is larger than the outer one.”

The theory solved the problem of the P-waves “emerging at distances where it had not been possible to predict their presence,” she wrote later.

To Gutenberg, Lehmann had found the missing piece of the puzzle. Jeffreys wasn’t convinced until 1939, when other seismologists carried out calculations with more accurate data and confirmed that Lehmann’s model matched their experimental observations.

In the 1940s, other geophysicists proposed that the innermost core was solid, as Lehmann’s paper had implied. And in 1952, geophysicist Francis Birch published a detailed study that concluded Earth’s inner core must be solid — probably crystalline iron.

Lehmann went on to serve as chief of Gradmaaling’s seismological department until she retired in 1953. During those years, she published 35 papers. Even in retirement, she continued her research, directing her interests toward the Earth’s mantle.

Ultimately, it was Jeffreys who, in a letter to Niels Bohr in 1962, suggested that Lehmann’s discovery of Earth’s inner core deserved to be honored in her home country of Denmark. In 1965, Lehmann was awarded the Gold Medal of the Danish Academy of Sciences and Letters.

In 1971, Birch awarded Lehmann with the American Geophysical Union’s highest honor, the Bowie Medal. He remarked that Lehmann’s 1936 revelation “was discovered through exacting scrutiny of seismic records by a master of black art for which no amount of computerisation is likely to be a complete substitute” — a reference to the incredible number of hand calculations that Lehmann had performed.

In her honor, the geology community named the boundary between the Earth’s inner core and outer core “the Lehmann discontinuity.”

Lehmann died in 1993, at the age of 104, making her one of history’s longest-lived scientists. Her last writing, a manuscript called “Seismology in the days of old,” was published six years before her death, in 1987. It closed with words that still ring true today.

“The first results for the properties of the inner core were naturally approximate,” she wrote. “Much has been written about it, but the last word has probably not been said.”

Liz Boatman is a staff writer for APS News.

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