Inge Lehmann (1888–1993)

Born on 13 May 1888 in Copenhagen, Denmark, Inge Lehmann was a seismologist who discovered Earth’s inner core. She studied math at the University of Copenhagen but developed an interest in seismology when she became an assistant to the head of the Royal Danish Geodetic Institute. She went back to school, obtained a master’s in seismology, and in 1928 was appointed head of the seismological department at the Institute. Lehmann’s most influential work started with a major earthquake in New Zealand in 1929. Analyzing seismic data from several stations, she found that the arrival times and intensities of the recorded seismic waves suggested that Earth’s interior was more complex than thought. Lehmann’s 1936 paper, titled P’, theorized that Earth’s core has two layers: a solid inner core and a liquid outer core. It took 34 years to confirm her proposal, but she was right, and the liquid–solid core boundary is now known as the Lehmann discontinuity. Lehmann later studied Earth’s mantle and found another boundary about 200 km below the surface, which is also known as the Lehmann discontinuity. She died in 1993 at age 104. (Photo credit: AIP Emilio Segrè Visual Archives)

Date in History: 13 May 1888