Inge Lehmann (May 13, 1888 – February 21, 1993), Foreign Member of the Royal Society of London, was a Danish seismologist who, in 1936, argued that the Earth's core is not one single molten sphere, but that an inner core exists which has physical properties that are different from those of the outer core.
Inge Lehmann was born and grew up in Østerbro, a part of Copenhagen, as daughter of the experimental psychologist Alfred Georg Ludvik Lehmann (1858-1921). She received her school education at a pedagogically progressive high school led by Hanna Adler, an aunt of Niels Bohr. According to Lehmann, her father and Adler were the two most significant influences for her intellectual development. After having finished school, she studied, with some interruptions due to poor health, mathematics at the universities of Copenhagen and Cambridge. After a few years of work in the insurance business she became an assistant to the geodesist Niels Erik Nørlund, who assigned her the task of setting up seismological observatories in Denmark and Greenland. The beginning of her interest in seismology dates back to this time. In 1928 she passed her exam in geodesy and accepted a position as state geodesist and head of the department of seismology at the Geodetical Institute of Denmark, which was led by Nørlund.
In a paper with the unspectacular title P', she was the first to interpret P wave arrivals which inexplicably appeared in the P wave shadow of the Earth's core as reflexions at an inner core. This interpretation was adopted within two to three years by other leading seismologists of the time, such as Beno Gutenberg, Charles Richter, and Harold Jeffreys. The Second World War and the occupation of Denmark by the German army hampered Lehmann's work and her international contacts significantly during the following years.
In the last years until her retirement in 1953 the relations between her and other members of the Geodetical Institute deteriorated, partly probably because she had little patience with less competent colleagues. After 1953, Inge Lehmann moved to the USA for several years and collaborated with Maurice Ewing and Frank Press on investigations of the Earth's crust and upper mantle. During this work, she discovered another seismic discontinuity, which lies at depths between 190 and 250 km and is usually referred to as "Lehmann discontinuity" in honor of its discoverer. 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..."
She received many honors for her outstanding scientific achievements, among them the Harry Oscar 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 (1969), the William Bowie Medal (1971, as the first woman), and the Medal of the Seismological Society of America (1977). Furthermore, she was awarded honorific doctorates of Columbia University, New York, in 1964 and of the University of Copenhagen in 1968 as well as numerous honorific memberships. The asteroid 5632 was named Ingelehmann in her honour. In Aventura, Florida, there is a stretch of U.S. 1 and a bridge named in her honour.
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