Inge Lehmann

Inge Lehmann
Born	(1888-05-13)13 May 1888 Copenhagen, Denmark
Died	Copenhagen, Denmark
Resting place	Hørsholm Cemetery
Fields	seismology, geophysics
Institutions	Geodetical Institute of Denmark
Alma mater	University of Copenhagen, University of Cambridge
Notable awards	William Bowie Medal (1971)

Inge Lehmann ForMemRS (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 the Earth having a single molten core. 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.

Early life and education

Inge Lehmann was born and grew up in Østerbro, a part of Copenhagen. Her mother was Ida Sophie Tørsleff; her father was experimental psychologist Alfred Georg Ludvik Lehmann (1858–1921). She received her school education at a pedagogically progressive high school led by Hanna Adler, Niels Bohr's aunt. According to Lehmann, her father and Adler were the most significant influences for her intellectual development.

She studied mathematics at the University of Copenhagen and University of Cambridge, 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 candidatus magisterii degree in physical science and mathematics in two years. 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.

Career

A modern understanding of the Lehmann discontinuity: velocity of seismic S-waves in the Earth near the surface in three tectonic provinces: TNA = Tectonic North America SNA = Shield North America and ATL = North Atlantic.

In 1925 Lehmann became an assistant to the geodesist Niels Erik Nørlund, who assigned her the task of setting up seismological observatories in Denmark and Greenland. Based on her studies in seismology, in 1928 she earned the magister scientiarum degree (equivalent to an MA) in geodesy and accepted a position as state geodesist and head of the department of seismology at the Geodetical Institute of Denmark led by Nørlund.

In a paper titled P' (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. Other leading seismologists of the time, such as Beno Gutenberg, Charles Richter, and Harold Jeffreys, adopted this interpretation within two or three years. Lehmann was significantly hampered in her work and maintaining international contacts during World War II and the German occupation of Denmark. In 1971 the interpretation was shown correct by computer calculations.

In 1952, Lehmann was considered for a professorship in geophysics at Copenhagen University, but was not appointed. In 1953, she retired from her position at the Geodetic Institute. She moved to the US for several years and 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 lies at depths between 190 and 250 km and was named for her, the Lehmann discontinuity. 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."

Awards and honors

Lehmann received many honors for her outstanding scientific achievements, among them the Gordon Wood Award (1960); the Emil Wiechert Medal (1964); the Gold Medal of the Danish Royal Society of Science and Letters (1965); the Tagea Brandt Rejselegat (1938 and 1967); the election as a Fellow of the Royal Society in 1969; the William Bowie Medal (1971, as the first woman); 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 her honor.

In 1997, the American Geophysical Union established the Inge Lehmann Medal to honor "outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core."

On the 127th anniversary of her birth, Google dedicated its worldwide Google Doodle to her.

Bibliography

• Kousholt, Bjarne. "Inge Lehmann og Jordens kerne" ISBN 8750209574, 9788750209577

See also

• Richard Dixon Oldham
• List of geophysicists

External links

• Inge Lehmann at CWP at UCLA
• Royal Society citation
• Inge Lehmann: Discoverer of the Earth's Inner Core