Inge Lehmann

Inge Lehmann (13 May 1888 – 21 February 1993) was a Danish seismologist and geophysicist. In 1936, she discovered that the Earth has a solid inner core inside a molten outer core. Before that, seismologists believed Earth's core to be a single molten sphere, being unable, however, to explain careful measurements of seismic waves from earthquakes, which were inconsistent with this idea. Lehmann analysed the seismic wave measurements and concluded that Earth must have a solid inner core and a molten outer core to produce seismic waves that matched the measurements. Other seismologists tested and then accepted Lehmann's explanation. Lehmann was also one of the longest-lived scientists, having lived for over 104 years.
Early life and education
Inge Lehmann was born and grew up in Østerbro, a part of Copenhagen. She was very shy as a child, a behaviour that continued throughout her life. Her mother, Ida Sophie Tørsleff, was a housewife; her father was experimental psychologist Alfred Georg Ludvik Lehmann (1858–1921).
She received her school education at Fællesskolen, a pedagogically progressive high school that treated girls and boys equally, enrolling them in the same curriculum and extracurricular activities. This school was led by Hanna Adler, Niels Bohr's aunt. According to Lehmann, her father and Adler were the most significant influences on her intellectual development.
At age 18, she achieved a first rank mark in the entrance exam for Copenhagen University. In 1907, she started her studies in mathematics, chemistry and physics at the University of Copenhagen and University of Cambridge. These studies were interrupted by poor health. She continued her studies of mathematics in Cambridge from 1910 to 1911 at Newnham College. In 1911, she returned from Cambridge feeling exhausted from the work and put her studies aside for a while. She developed good computational skills in an actuary office she worked in for a few years until she resumed studies at Copenhagen University in 1918. She completed the candidata magisterii degree in physical science and mathematics in two years, graduating in 1920. When she returned to Denmark in 1923, she accepted a position at Copenhagen University as an assistant to J.F. Steffensen, the professor of actuarial science.
Lehmann had a younger sister, Harriet, who became a movie writer and who had family and children in contrast to Lehmann, who lived by herself all her adult life.
Career
In 1925, Lehmann was assigned to be the assistant of seismologist Niels Erik Nørlund. She took an interest in his field, and she began studying it on her own. She was chosen as a delegate for Denmark to attend the International Union of Geodesy and Geophysics in 1927—a role she filled another eight times over the next forty years. By 1928, Lehmann obtained a magister scientiarum in seismology, and she was appointed head the Geodætisk Institut's seismological department the same year. In this position, she was responsible for overseeing the operation of three seismographic observatories, two of which were in Greenland. She personally operated the one in Copenhagen, producing reports based on its readings. Though it was not part of her job, Lehmann also engaged in research at the facility.
In a paper titled P&apos; (1936), Lehmann was the first to interpret P wave arrivals—which inexplicably appeared in the P wave shadow of the Earth's core—as reflections from an inner core, for example from the strong 1929 Murchison earthquake. Other leading seismologists of the time, such as Beno Gutenberg, Charles Richter, and Harold Jeffreys, adopted this interpretation within two or three years, but it took until 1971 for the interpretation to be shown correct by computer calculations. Also in 1936, she co-founded the Danish Geophysical Society, which she chaired twice, in 1941 and 1944. She continued her work during World War II, though international collaboration was limited. She became the first president of the European Seismological Foundation.
When American geologist Maurice Ewing visited her station in 1951, he invited Lehmann to work at the Lamont Geological Observatory that he ran at Columbia University, and she studied there for some of 1952.
She retired from her position as head of the Geodætisk Institut's seismological department in 1953, giving her more time to conduct research over the following decades. Throughout the 1950s and 1960s, Lehmann traveled to North America several times and visited different seismological observatories throughout the United States and Canada. She became a prominent member of the community at the University of California, Berkeley, one of her most frequent stops.
While in the United States, Lehmann collaborated with Maurice Ewing and Frank Press on investigations of Earth's crust and upper mantle. During this work, she discovered another seismic discontinuity, which is a step-change increase in the speed of seismic waves at depths between 190 and 250 km. This discontinuity was named the Lehmann discontinuity after her. Francis Birch noted that the "Lehmann discontinuity was discovered through exacting scrutiny of seismic records by a master of a black art for which no amount of computerization is likely to be a complete substitute."
Lehmann was also involved in the creation of the International Seismological Centre from 1961 to 1967.
Awards, honours, and legacy
Lehmann received many honours for her scientific achievements, among them the Harry Oscar Wood Award in 1960, the Emil Wiechert Medal in 1964, the Gold Medal of the Danish Royal Society of Science and Letters 1965, the Tagea Brandt Rejselegat in 1938 and 1967, her election as a Fellow of the Royal Society in 1969, the William Bowie Medal in 1971, and the Medal of the Seismological Society of America in 1977. She was awarded honorary doctorates from Columbia University in 1964 and from the University of Copenhagen in 1968, as well as numerous honorific memberships.
The asteroid 5632 Ingelehmann was named in Lehmann's honour. A species of beetle was named after her on the hundredth anniversary of women's suffrage in Denmark:Globicornis (Hadrotoma) ingelehmannae.
Because of her contribution to geological science, in 1996, the American Geophysical Union established the annual Inge Lehmann Medal to honour "outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth's mantle and core."
In 2015, on the 127th anniversary of her birth, Google dedicated its worldwide Google Doodle to Lehmann.
A memorial dedicated to Lehmann was installed on Frue Plads in Copenhagen in 2017, designed by Elisabeth Toubro.
Key publications
See also
Richard Dixon Oldham
List of geophysicists
Timeline of women in science
Notes
References
Further reading
External links
Inge Lehmann at CWP at UCLA
Royal Society citation
Inge Lehmann: Discoverer of Earth's Inner Core
Danish geophysicists
Seismologists
1888 births
1993 deaths
Danish women geologists
Alumni of Newnham College, Cambridge
Foreign Members of the Royal Society
University of Copenhagen alumni
Danish centenarians
20th-century Danish physicists
Women geophysicists
Scientists from Copenhagen
20th-century Danish women scientists
Women centenarians