Obituaries
Thomas J. Ahrens
1936 – 2010
Thomas J. Ahrens (MS ’58), the Jones Professor of Geophysics, Emeritus, died at his home in Pasadena on November 24. He was 74.
Ahrens, who worked in the U.S. Army’s Ballistics Research Laboratory from 1959 to 1960 en route to earning his doctorate, was among the first to take shock-compression techniques developed by government labs for testing nuclear weapons and apply them to the academic study of conditions deep within the earth. These methods subjected materials to extremely high temperatures and pressures by smashing two samples together at very high speeds—in other words, by loading one into a cannon and shooting it at the other. (Nowadays, researchers reach these conditions routinely by squeezing a sample between diamond anvils and heating it with a laser.)
Ahrens’s first “cannon” was a shotgun from Sears, but over the years larger and larger pieces of ordnance found their way into the subbasement of South Mudd—culminating in three 20-foot-long barrels cut from six-inch-caliber naval guns and joined end-to-end to form the pump section of a two-stage “light gas gun” that was in use until 1991. The first stage of such a gun uses a conventional smokeless-powder charge to fire a piston down the barrel, which is filled with highly compressible hydrogen. Halfway to the muzzle, the gun abruptly necks down into a second, smaller-diameter barrel containing the sample projectile and separated from the pump barrel by a thin metal plate. The supercompressed hydrogen bursts through the plate, gains additional velocity from being forced into the smaller second stage, and shoots the projectile at velocities of up to 7.5 kilometers per second—more than the minimum impact speed of a slow asteroid hitting Mars, and two-thirds the minimum impact velocity with Earth.
In the 1980s, Ahrens’s team used a single-stage, 40-millimeter gun capable of achieving pressures of 400,000 times Earth’s atmosphere—sufficient to melt an 80-gram iron projectile on impact—to estimate the temperature profile of Earth’s core. Other studies looked at the effects of meteor strikes. Ahrens concluded from these that our water (and much of our atmosphere) must have arrived from the outer reaches of our solar system via icy comets after the protoplanets that formed Earth had finished crashing into one another—otherwise, each fresh impact would have blasted such volatile substances into space.
In 1986, Ahrens and former postdoc Manfred Lange published a calculation of the amount of carbon dioxide that would have been released into the atmosphere when a 10-kilometer asteroid splashed into shallow seas just off the Yucatán peninsula 65 million years ago. Ahrens and Lange used bullets of steel and targets of limestone, a common sedimentary rock made of calcium carbonate, and concluded that enough of the greenhouse gas would have been generated to raise Earth’s average surface temperature between 5 and 20°C for up to 10,000 years. If this didn’t kill the dinosaurs, it would have made them mighty uncomfortable.
“Tom was a highly productive scientist and a dedicated mentor to dozens of students, postdocs, and visitors who now fill the ranks of mineral physicists at universities around the world,” says Professor of Geology and Geochemistry Paul Asimow (MS ’93, PhD ’97), an Ahrens protégé who now runs the Lindhurst Laboratory of Experimental Geophysics, as Ahrens’s gun collection is formally known.
Born in Frankfurt, Germany, on April 25, 1936, Ahrens received his BS from MIT in 1957 and his PhD from Rensselaer Polytechnic Institute in 1962. He headed the Poulter Laboratory’s geophysics section at the Stanford Research Institute from 1962 to 1967 before joining Caltech as an associate professor. 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 Jones Professor in 2004, and went emeritus in 2005.
Ahrens published nearly 400 papers and held three U.S. patents. 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. His professional honors included the Geological Society of America’s Day Medal, the American Physical Society’s Duvall Award, the American Geophysical Union’s Hess Medal, and the Meteoritical Society’s Barringer Medal. The asteroid 4739 Tomahrens (1985 TH1) is named after him.
Ahrens is survived by his wife, Earleen; children Earl, Eric, and Dawn; and grandchildren Greta, Violet, Jacqueline, and Samuel. —DS/MW
