Inge Lehmann overturned the idea that our planet’s metallic core is entirely molten liquid. She used mathematics to analyze the way energy released by earthquakes travels through the earth.
She discovered something eternally concealed from the naked eye – thousands of miles below our feet, at its center, the earth is solid. In fact, it has a solid inner core and a liquid outer core.
Inge Lehmann is also remarkable in that she is one of the longest lived scientists in history, living to 104 years of age.
Beginnings
Inge Lehmann was born in Denmark’s capital city, Copenhagen, on May 13, 1888. Her father, Alfred Georg Ludvik Lehmann, was a psychologist and her mother, Ida Sophie Tørsleff, was a housewife. Both parents came from prominent families.
Inge was a very shy girl, who did not enjoy being in the spotlight. She continued to be shy throughout her long life.
She was schooled at a private coeducational school called Fællesskolen – which translates as shared school. The school was new: it had been founded when Inge was 5 years old by Hanna Adler, a wealthy woman.
Hanna Adler’s new school was unusual in that boys and girls were treated identically, studying the same subjects and taking part in the same sports and activities. The children were not disciplined as rigorously as in other schools of that time.
Inge Lehmann enjoyed her time at the Fællesskolen, but she was sometimes bored, because she did not feel challenged enough by the schoolwork.
In 1906, at age 18, she passed the entrance examination for Copenhagen University with a first rank mark.
University
Lehmann started freshman courses in mathematics, chemistry, and physics at Copenhagen University in 1907. She finally graduated in 1920.
It took her an exceptionally long time to get a degree: in 1911 she returned to Copenhagen completely burned out after a year at Cambridge University. She abandoned her degree to do actuarial work for an insurance company. In 1918, she returned to Copenhagen University, graduating with a mathematics degree in 1920, age 32.
In 1923, she began working as an assistant in Copenhagen University’s actuarial department.
In 1925, she transferred to seismology work with Professor Niels Nørlund, who showed her how the internal structure of our planet can be unveiled with earthquake data. She visited seismic stations in Germany, the Netherlands, and France learning about techniques for analyzing the earth’s movements.
Lehmann was captivated by her new academic field and, in 1928, age 40, she obtained a Master of Science degree in geodesy (the science of making measurements related to planet Earth).
Seismic data measured at different stations of an earthquake in Mexico in 1928 published by Inge Lehmann in 1930. Lehmann, I., The earthquake of 22 III 1928, Gerlands Beitr. Geophys., 28, 151, 1930
Earth Research
In 1928, Lehmann was appointed head of the Department of Seismology at the Royal Danish Geodetic Institute, with responsibility for running the Copenhagen, Ivigtut, and Scoresbysund seismographic observatories.
Her job was administrative, but she made time for scientific research, including improving the coordination and analysis of measurements from Europe’s seismographic observatories. This was important, because it ensured data from the observatories could be better compared and interpreted. Lehmann’s improvement of the trustworthiness of measurements lay at the heart of her later discovery.
A 1926 magazine cover illustrating a scene from Jule Verne’s Journey to the Center of the Earth.
Dreaming of a World Deep Below
The interior of our planet has long held a fascination for philosophers and story tellers.
Some have speculated that another inhabited world lies beneath our own.
In 1864, Jules Verne’s novel Journey to the Center of the Earth, described the fictional adventures of explorers traveling under our planet’s surface.
It was a best-seller.
People wondered if the world Verne described could be real.
Disappointed Dreamers
By the time Lehmann was awarded her Master’s degree in 1928, scientists already knew that seismographic data from earthquakes could be used to deduce what sort of stuff Earth’s interior is made of.
To the dismay of many dreamers, seismologists had ruled out Jules Verne’s ideas of another inhabited world below Earth’s surface. They had figured out that vibrations from earthquakes travel all the way through the earth. Some travel as transverse waves (S-waves,) and others as longitudinal waves (P-waves). The time these waves take to travel from an earthquake’s epicenter to different seismic observatories around the world reveals information about the paths the waves take.
The path of earthquake waves through our planet depends on the materials the waves travel through and the boundaries between these materials.
Paths of different wave-types moving out from the focus of an earthquake. The P-waves are fast moving longitudinal waves. The S-waves are slower moving transverse waves.
In 1906, Richard Dixon Oldham analyzed seismic waves from several earthquakes and concluded that the earth has a large, liquid, metallic core. He calculated the size of the core, finding it forms the inner 40 percent of our planet’s radius. (We now know the core Oldham discovered comprises the innermost 3470 km of Earth’s 6360 km radius.)
A Puzzle
Although Oldham had discovered Earth’s metallic core, seismologists still did not completely understand the meaning of the data recorded at their observatories.
Lehmann and other workers were puzzled about the behavior of the P-waves. Earthquake data from observatories showed these were not traveling through Earth in the way they were expected to. They were appearing in locations they ought not to.
Lehmann had an idea. People believed that Earth, below its solid crust, was molten. She wondered if our planet’s inner core might actually be solid. If it were solid, surrounded by molten liquid, would that account for the odd behavior of the P-waves?
She developed mathematical models of our planet featuring a solid inner core and… Eureka! Such a planet agreed with the observed data. Lehmann was able to conclude that P-waves were appearing in unexpected locations because they were being refracted and reflected to these locations by the boundary between the Earth’s solid inner core and liquid outer core. The inner core, she calculated, had a radius of about 1400 km.
Lehmann published her findings in 1936, in a paper entitled very simply P’.
Within a few years her new model of Earth’s inner structure had been generally although not universally accepted by the scientific community. With the passage of time, as ever more accurate seismic measurements were taken confirming Lehmann’s work, the solid core became accepted in full.
We now know the solid core Inge Lehmann discovered:
- is about the same temperature as the sun’s surface
- is made of iron-nickel alloy
- is solid because of the enormous pressure from the outer layers of the earth pushing down on it
- has a radius of 1220 km, making it somewhat smaller than the moon, whose radius is 1737 km, and similar in size to Pluto, which has a radius of 1188 km.
Pluto and Earth.
The huge, solid chunk of metal at Earth’s center is slightly bigger than Pluto. The metal – most likely an iron-nickel alloy – is very dense. Its mass is about 10 times greater than Pluto’s and close to double the mass of the moon.
Inge had great energy, both mental and physical. She loved mountains, particularly in Switzerland. In the citation for the Bowie medal in 1971 Francis Birch drew attention to the economy of the title of her 1936 paper, P’, and provided what could be a fitting epitaph, describing her as “the master of a black art [seismology calculations] for which no amount of computerizing is likely to be a complete substitute”.
Retirement? Not Really
Lehmann retired from her position at the Geodetic Institute in 1953, age 65. This freed her from administrative work, allowing her to spend more time on her true passion – scientific research – much of which she carried out in lengthy stays in the USA and Canada.
During her ‘retirement’ she discovered Lehmann discontinuities in 1959, which have not been fully explained even today. (A Lehmann discontinuity is a step-change increase in seismic wave speeds in the earth’s mantle at depths of 180 to 250 km below the surface.)
In 1987, age 99, she wrote her last scientific article: Seismology in the Days of Old. In 1988, she attended the party held for her hundredth birthday at her old workplace, the Geodetic Institute.
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. The hanging lamp swayed. It was very strange. My father came into the room. “It was an earthquake,” he said. The center had evidently been at a considerable distance, for the movements felt slow and not shaky. In spite of a great deal of effort, an accurate epicenter was never found. This was my only experience with an earthquake until I became a seismologist 20 years later.
Honors
1938: Tagea Brandt Award
1941, 1944: Chair of Danish Geophysical Society
1950: President of the European Seismological Federation
1960: Gordon Wood Award
1964: Emil Wiechert Medal of the German Geophysical Society
1965: Gold Medal of the Danish Royal Society
1969: Elected Fellow of the British Royal Society
1971: The William Bowie Medal
1977: Medal of the Seismological Society of America
The American Geophysical Union established the Inge Lehmann Medal in 1997 to be awarded for outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.
The End
Inge Lehmann died at age 104 on February 21, 1993. She had not married and had no children. She left all of her possessions to The Danish Academy.
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