Inside the earth

A nice surprise this morning to see that Google is celebrating the 127th birthday of Inge Lehmann, pioneering seismologist. Two weeks ago, this was a name new to me, picked up from a radio programme. I became interested because Inge Lehmann is a woman who forged a career in science in the 1930s, a time when few women did that, and of those who did, most gave up their career upon marriage.

As well as using seismic data to discover the earth's inner core (its boundary is now called the Bullen  discontinuity, after the geophysicist who in 1942 designated the earth's layers alphabetically), Inge discovered another discontinuity, in the earth's mantle, which is named after her. Most importantly, though, Inge discovered that the earth's core is not a single molten sphere.

After studying mathematics at Copenhagen and briefly, due to ill health, at Cambridge, she worked for some years in an actuarial office. After resuming and completing her studies, in 1923 she became assistant to the professor of actuarial science at Copenhagen University, and then to a geodesist who tasked her with setting up seismological observatories in Denmark and Greenland. In 1928, having passed the geodesy exam, she became head of the department of seismology at the Geodetical Institute of Denmark.

In 1936, in a paper with surely one of the shortest titles ever - "P' " - she interpreted the change in P wave arrival as indicating that the earth had an inner core. This theory was rapidly accepted.

She retired from the Geodetic Institute in 1953, aged 65, then went to the United States, and it was at this time that she discovered "her" discontinuity, which lies 190-250 km down and appears beneath continents but not usually beneath oceans. It 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".

A remarkable woman - and she has an asteroid named after her.

Earth's layers (via); the outer core is molten, and the inner core is solid,
spinning independently of the rest of the planet