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Thomas J. Ahrens dies at 74; Caltech geophysicist

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Thomas J. Ahrens, a Caltech geophysicist who pioneered the academic use of shock waves to study minerals under high temperatures and pressures such as those found at the center of the Earth, died Nov. 24 at his home in Pasadena. He was 74. The cause of death was not revealed.

Today, physicists have several ways to look at materials under high pressures and temperatures. Diamond anvils can be used to exert very high pressures on minerals, and lasers are used to heat them to extreme temperatures. Modern computers can be used to calculate properties directly.

But when Ahrens started his work half a century ago, according to his Caltech colleague Paul Asimow, the only way to obtain the necessary conditions was by slamming two things together. "When he started, it was the only way to obtain information about the deep Earth and to produce a sensible model of the deep Earth," Asimow said.

Government labs here and abroad were doing such studies for nuclear weapons testing, but Ahrens was among the first to do it in an academic setting to provide fundamental geological information about the Earth's core.

He started with $79.95 shotguns from Sears that shot special pellets at targets, but he soon graduated to a 30-foot steel tube loaded with a pound of gunpowder that could fire an 80-gram projectile, mimicking pressures at depths up to about 600 miles. Later, he developed a two-stage gun that could achieve firing speeds of nearly 18,000 mph.

He also developed new instruments for collecting data after impact as well as tools for melting the sample before impact to simulate, for example, a comet hitting magma. His experiments gave a clearer picture of the Earth's core and of how cratering occurred on objects throughout the solar system.

Ahrens' measurements that suggested the temperature of iron in the core of the Earth, obtained in the 1980s, were surprisingly high and quite controversial. By and large, however, they have been replicated and confirmed by research in other laboratories.

His observations on the aftereffects of impacts led to a number of influential papers. One paper, for example, questioned the prevailing theory that Earth arose from the accretion of many impacts. If that had occurred, he said, where did water and other atmospheric gases come from? Each impact would have blown those lighter elements into space. The only possible explanation, he wrote, was that icy comets must have brought water and air after the body of the Earth was formed.

Ahrens and geophysicist Manfred Lange also shot steel bullets into rocks at 4,500 mph to simulate the asteroid that researchers believe caused the extinction of dinosaurs some 65 million years ago. They found that the impact on limestone released tremendous amounts of carbon dioxide that would have warmed the planet by 9 to 36 degrees Fahrenheit for as long as 10,000 years, wiping the creatures out through a massive greenhouse effect.

Based on other experiments, Ahrens and geophysicist John D. O'Keefe of TRW Inc. also concluded that the object that flattened trees in a 20-mile circle in Tunguska, Russia, in 1908 was a low-density comet, not an asteroid as others had believed.

Thomas Julian Ahrens was born April 25, 1936, in Frankfurt, Germany. He earned his undergraduate degree at MIT in 1957, a master's at Caltech in 1958 and a doctorate at Rensselaer Polytechnic Institute in 1962. After two years at the Pan American Petroleum Corp., a year with the Army and five years with the Stanford Research Institute, he spent the rest of his career at Caltech.

"He had a remarkable legacy as a prodigious trainer of students who went out in the world," Asimow said. "Everybody who is anybody worked with Tom at some point."

He published nearly 400 papers and received numerous honors and awards.

Ahrens is survived by his wife, Earleen; two sons, Eric of Pittsburgh and Earl of Los Angeles; a daughter, Dawn of Los Angeles; and four grandchildren.

thomas.maugh@latimes.com

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