Inge Lehmann The beating of the Earth’s solid heart The invention of the modern seismograph in 1880 caused previous speculation about the Earth’s interior to fall like a house of cards. During that same decade, when science was still hostile territory for women, a scientist was born who would solve the last great unknown: the structure of the Earth’s core. In 1888, a prominent Copenhagen family welcomed (without realising) its most illustrious member: Inge Lehmann. This girl was educated in the egalitarian environment of a mixed and progressive school. This environment, anomalous for the time, pushed Inge to develop to her full potential, but could not prevent her from coming face to face with reality in the coming years. She studied mathematics at the universities of Copenhagen and Cambridge. After graduating, she started working in an insurance office until, in 1925, she was hired to set up the first seismological observatories in Denmark and Greenland. In this fortuitous way, she specialised in seismology in a country that was practically aseismic but, as she would later learn, ideally located for recording earthquakes in geologically active and distant areas of the Earth, such as the South Pacific. In 1927, she spent time at research centres in Germany, France, the Netherlands and Belgium, which brought her into contact with experts of the time such as Beno Gutenberg. After her European sojourn, Inge was appointed the first Head of the Seismology Department of the Royal Danish Geodetic Institute. She was in charge of maintaining and repairing equipment, interpreting seismograms and publishing data bulletins. She worked practically alone, which gave her an unrivalled knowledge of the patterns of seismic waves passing through the Earth. Although it was not her job to do scientific work, Inge was intrigued by the fact that P-waves did not behave in a way that was consistent with the previously known structure of the Earth. She began to suspect that something else was lurking in the Earth’s liquid core as she observed the different patterns of deflection and velocity change experienced by waves that managed to pass through the core. In 1929, a major earthquake in New Zealand gave her the key to the problem. After years of painstaking analysis, in 1936 she published a paper whose brief title (P’) gave no hint of the revolution it would produce. In it, Inge described a new seismic discontinuity in the Earth’s structure, a new boundary separating the well-known liquid outer core from an unknown solid inner core. After this great achievement, Inge continued her research and became one of the world’s leading experts on the Earth’s mantle. Internationally respected, she retired from her position at the observatory in 1953 and travelled the world to help wherever she was needed. She was the first woman to receive the William Bowie Medal, the highest award in geophysics, for being ‘ the master of a black art for which a computer can never be a substitute’ ; this almost esoteric phrase reflected the painstaking analysis and rudimentary means by which Inge answered the great scientific questions of a pre-digital age. She died in 1993, at the age of 104, in aseismic Denmark. I never noticed any difference between the intellect of boys and girls, which caused me some disappointment when I realised that this was not the general attitude. 19
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