Thomas J. Ahrens, the Fletcher Jones Professor of Geophysics, Emeritus, at Caltech, died at his home in Pasadena on November 24. He was 74.
An expert in the behavior and properties of rocks and minerals undergoing shock compression, Ahrens studied the dynamics of high-pressure materials inside Earth and other planets. His research also included planetary impacts and the formation of craters and planets.
"Tom was both a highly productive and broadly knowledgeable scientist and a dedicated mentor to dozens of students, postdocs, and visitors who now fill the ranks of mineral physics positions at universities around the world," says Professor of Geology and Geochemistry Paul Asimow, who credits Ahrens as his most important mentor while Asimow was a junior faculty member at Caltech. Together, they ran the Lindhurst Laboratory of Experimental Geophysics, which Ahrens built in 1974 when the Seismological Laboratory moved to South Mudd. "Our relationship was symbiotic," Asimow says. "Tom wanted to ensure beyond his retirement the ongoing productivity of the remarkable lab that he built, and I wanted to learn from his accumulated wisdom and to carry out experiments in a lab far beyond the scale and expense of anything I would have considered building from scratch."
Some of Ahrens's most important contributions, according to Asimow, were in developing experimental methods for measuring shock temperatures and the density of liquids at high pressure. When applied to iron, the first technique allowed researchers to determine the temperature structure of Earth's core. The second method is so far the only way to measure the density of molten rocks that might form in Earth's mantle at depths greater than a few hundred kilometers. Ahrens's work has led to a basic understanding of how objects—such as meteorites and comets—carrying volatile materials smash into planets. His research has provided insight into the source and origin of water on Earth and into the environmental effects of meteorite collisions such as the one that struck Earth 65 million years ago and likely led to the extinction of the dinosaurs.
Born in Germany, Ahrens received his BS from the Massachusetts Institute of Technology in 1957, his MS from Caltech in 1958, and his PhD from Rensselaer Polytechnic Institute in 1962. He was a geophysicist with the Pan American Petroleum Corporation from 1958 to 1959, worked as a second lieutenant for the U.S. Army in the Ballistics Research Laboratory from 1959 to 1960, and was the head of the geophysics section in the Poulter Laboratory of the Stanford Research Institute from 1962 to 1967. He joined Caltech in 1967 as an associate professor of geophysics. He became professor of geophysics in 1976 and was the W. M. Keck Foundation Professor of Earth Sciences from 1996 to 2001; he was named the Fletcher Jones Professor of Geophysics in 2004 and became Jones Professor, Emeritus, in 2005.
Ahrens published more than 375 papers, held three U.S. patents, and received numerous honors and awards for his research. He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences, and was a Foreign Associate of the Russian Academy of Sciences. He won the 1995 Arthur L. Day Medal of the Geological Society of America, the 1996 Harry H. Hess Medal of the American Geophysical Union, and the 1997 Barringer Medal of the Meteoritical Society, and he had an asteroid named after him.
He is survived by his wife, Earleen; children Earl, Eric, and Dawn; and grandchildren Greta, Violet, Jacqueline, and Samuel.