Thomas J. Ahrens

December 13, 2010

Thomas J. Ahrens received his B.S. degree in Geology-Geophysics from MIT in 1957, the same year that Sputnik 1 was launched. Sputnik's launch on October 4, 1957, marked not only man's conquest of space, but the expansion of geology and geophysics beyond this terrestrial sphere. Ahrens, who was co-Investigator on the Cosmic Dust Analyzer Experiment for the Cassini–Huygens Mission to Saturn and a major participant in this new wave of geology, died on November 24, 2010, at age 74.[1-3]

Thomas J. Ahrens (California Institute of Technology Photo)

After MIT, Ahrens continued in geophysics, earning an M.S. from the California Institute of Technology in 1958, and a Ph.D. from Rensselaer Polytechnic Institute in 1962. After his Ph.D., he became head of the Geophysics Section of the Poulter Laboratory, Stanford Research Institute, continuing there through 1967. In 1967 he became an Associate Professor at the California Institute of Technology and continued there his entire professional career. Ahrens was the Fletcher Jones Professor of Geophysics, Emeritus, from 2005 until his death. One of his research interests was the investigation of the behavior of rocks and minerals during impact and shock excitation when they're subjected to high temperature and pressure, and he discovered shock induced phase transitions in carbonates. Such experiments illuminated the process of impact crater formation on the moon and planets, and they also gave clues to the origin of planetary atmospheres and Earth's ocean.

Ahrens' shock excitation experiments may have been aided by his experience as a Second Lieutenant at the U.S. Army Ballistics Research Laboratory from 1959-60. The modern method for subjecting materials to high pressure, the diamond anvil cell, was not available at the time of his early research. According to his obituary in the Los Angeles Times,[2] Ahrens bought inexpensive shotguns from Sears to shoot specially crafted pellets at targets. Eventually, he scaled this shotgun approach to a thirty foot tube that used a pound of gunpowder to fire an eighty gram projectile. His apparatus eventually would fire projectiles at 18,000 miles per hour.

Ahrens' experiments on iron at high temperature and pressure in the 1980s indicated that the temperature of Earth's core was higher than believed. His estimate for a core temperature of 6900±1000 K was confirmed by a group at Lawrence Livermore National Laboratory.[4] Ahrens argued also that Earth's water and atmosphere came from a later bombardment of icy comets and not from its initial accretion of matter. His work on the chemical consequences of the terrestrial impact event that caused the Chicxulub Crater in Mexico led to the theory that the extinction of dinosaurs 65 million years ago was caused by a meteor's impact on limestone (CaCO₃) that released 10¹⁶ kilograms of carbon dioxide into the atmosphere. This would have heated the earth by 5-10 ^oC for 10,000 years.[4] Looking at the conditions surrounding the Tunguska, Russia, impact event of 1908, Ahrens and John D. O'Keefe of TRW Inc. concluded that the impactor was a low density comet, and not a high density meteor.[2]

Ahrens published more than 375 papers. He served as Associate Editor, Review of Scientific Instruments, 1972-74; Associated Editor, Journal of Geophysical Research, 1972-74; and Editor, Journal of Geophysical Research (Solid Earth and Planets) 1979-1982. Among his honors were a Fellowship with the American Geophysical Union, 1982; the Newcomb-Cleveland Prize of the American Association for the Advancement of Science, 1984; Membership in the U.S. National Academy of Sciences, 1992; and Fellowship with the American Association for the Advancement of Science, 1994. Ahrens was named a Foreign Associate of the Russian Academy of Sciences in 1999. [3] A Main Belt Asteroid, discovered by E. Bowell of Flagstaff Observatory on October 15, 1985, was named after him in 1991 (4739, Tomahrens; 1985 TH1).
 

Location of asteroid 4739 Tomahrens (1985 TH1) on November 24, 2010.
The Main Belt Asteroids, of which 4739 Tomahrens is a member, lie between the orbits of Mars and Jupiter. The positions of the Sun's first four planets and their orbits are indicated.

References:

1. Thomas J. Ahrens, California Institute of Technology Press Release, December 1, 2010.
2. Thomas H. Maugh II, "Thomas J. Ahrens dies at 74; Caltech geophysicist," Los Angeles Times, December 10, 2010.
3. Curriculum Vitae - Thomas J. Ahrens, California Institute of Technology.
4. Research - Thomas J. Ahrens, California Institute of Technology.