#inge lehmann

Inge Lehmann: Discovering the In’s and Out’s of the Core

Earth’s interior is another mysterious place on our planet. We can’t see it; we can’t travel to it, we can’t drill to it, so how do we know what’s down there? Part of that answer is seismic data and very smart people. One of those very smart people was Inge Lehmann, a Danish geophysicist who first differentiated between the outer and inner core.

Before we get to Inge, here’s some background information about seismic waves that will help us understand her discovery: Earthquakes release a lot of energy in the form of seismic waves; these waves can be detected worldwide. One type of wave released is called P-waves, or “primary” waves so named because they travel quickly and arrive seismic stations first (as opposed to their slower counterpart, the S-wave). These waves refract and change speed as they go through different layers of Earth, which can result in “shadow zones” where particular seismic waves are not detected.

Inge Lehmann studied mathematics and physics at the University of Copenhagan and Cambridge. She actually worked as an actuary for a while before devoting her mathematical talent to geological pursuits. In 1929, Inge was at the Geodetical Institute of Denmark studying the P-waves of a particularly large earthquake that had just rocked New Zealand. She noticed something a bit odd (always the harbinger of a great scientific discovery...) The P-waves were not behaving like she’d expected them to—instead of being deflected by the core, they seemed to be…traveling through it. According to what she knew so far, the core (which was believed to be entirely liquid) should have deflected the P-waves, thus creating a shadow zone in certain areas of the world. But as it turns out, the P-waves were still making it to seismic stations previously thought off-limits.

Her explanation? The core must not be entirely liquid. P-waves did indeed travel through the core, but not at uniform speed, suggesting the core is made of varying material. She concluded that the outer core is liquid but the inner core is solid. The speed of P-waves changed dramatically between the two layers—a phenomenon called a “seismic discontinuity”. Later in 1970, when more precise seismic data was available, this theory was confirmed.

The seismic boundary between the outer core and inner core is now called the Lehmann discontinuity in Inge’s honor. She was as a scrupulous and dedicated worker—in a world before computers and Internet, Inge would keep track of her data on the back of meticulously organized oatmeal boxes. Inge lived a long life, contributing crucial work to the geophysics field the whole time. She died in 1993 at the age of 105.

-CM

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Inge Lehmann – Scientist of the Day

Inge Lehmann, a Danish seismologist, was born May 13, 1888.

Inge Lehmann was a Danish seismologist and geophysicist who is known for her discovery in 1936 of the solid inner core that exists within the molten outer core...

10 years ago - Inge Lehmann’s 127th Birthday

“She 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. Other seismologists tested then accepted Lehmann’s explanation. Lehmann was also one of the longest-living scientists, having lived for over 104 years.”