Thomas J. Ahrens

1996 Harry H. Hess Medal Winner
California Institute of Technology, Pasadena


Tom Ahrens was awarded the Harry H. Hess Medal at the AGU Fall Meeting Honor Ceremony on December 17, 1996, in San Francisco. The Hess Medal recognizes outstanding achievements in research in the constitution and evolution of Earth and its sister planets. The award citation and Ahrens response are given here.


“Tom’s research career spans more than 30 years and more than 300 research papers, so it is difficult to adequately summarize his work here. It is also difficult for one person to span Tom’s range of interests, so I will concentrate on Tom’s shock wave contributions to our understanding of Earth. He has also done much work on understanding the effects of planetary impacts (with implications ranging from planetary accretion to cometary impacts on major planets) and much basic research in materials science. His work directly impacts (pun intended) our understanding of the origin and evolution of Mars, Moon, Jupiter, Venus, and Earth. Since the Hess medal is given for outstanding achievements on research in the constitution and evolution of Earth and its sister planets, awarding the medal to Tom seems not merely appropriate, but rather inevitable.

“For almost all of geophysical history, Tom has run academia’s premier shock wave laboratory. Given the expense of such experiments, it is no small testament to Tom’s unique abilities that he has kept this laboratory operating, funded, and populated with excellent graduate students and researchers. Many of Tom’s early papers dealt with the fundamental understanding of the response of materials to dynamic loading as well as basic developments in shock wave technology. He extensively studied both solid and molten silicates undergoing shock loading and was the discoverer of major phase transitions in several minerals. His work on structural changes in molten silicates has profound implications for our ideas of planetary evolution. He was the first to conduct shock wave experiments on lunar samples and soon thereafter was the first to do planetary cratering calculations.

“Tom always seems to be doing several things at once, so in the early 1980s we find him writing on the effects of large impacts and extinctions, on shock wave-induced temperatures in silicates, on iron partitioning in the mantle, on electrical measurements in shocked minerals, on the composition of the core, on shock-induced devolatilization and the generation of planetary atmospheres, and much more.

“By the late 1980s, Tom and his coworkers had developed shock wave temperature measurement methods for opaque minerals and soon had the first direct estimates of the melting temperature of iron at core pressures. These difficult experiments gave surprisingly high temperatures for the melting of iron and were extremely controversial. By and large, these results have held up over time and have basically been verified by more extensive experiments by the government laboratories.

Tom has also done many of the fundamental experiments on alloys of iron of relevance to the Earth’s core that give us the information we need to talk intelligently about core composition. Tom continues to develop dynamic loading technology with experiments on preheated specimens to increase the range of pressure-temperature space sampled by the technique and with improved techniques for measuring sound velocities in shock experiments.

“While many of Tom’s papers report basic equation of state data, he also uses these data to make important inferences about the Earth and the properties of minerals at high pressure and temperature. His papers with coworkers address such diverse issues as core composition, thermal expansivity of the mantle, the behavior of volatiles in planets, and sound velocities in mantle materials, and these serve to identify the next important experiments that Tom will surely be the one to do in the future.

“Well, enough of this dry detailing of a stellar academic career. What about Tom the man? After digging around, I came up with some revealing stories. It is well known that Tom has always been an extremist with regard to laboratory safety, and when you make your living blowing things up, this is not a bad idea. However, on a summer afternoon in 1970, his secretary at that time noticed an explosion had occurred close by in the neighborhood.

A cloud of smoke was rising from the original (and still operating) `lower’ Kresge Seismological Laboratory! The Seismological Laboratory offices at that time were about 100 yards up the road from the lower laboratory, so Tom raced down the road to find the doors to the outside flung open with smoke pouring out of the Shock Wave Lab. A laboratory technician, David Johnson, was in the control room, shaken up but unhurt.

However, the gun was in two pieces, neatly separated at the midjoint in the barrel. It was later discovered that David had mistakenly assembled the gun with 0.5-inch hardware store bolts instead of high-strength steel alloy bolts. The hardware store bolts had lasted three shots. Twenty two of the bolts had sheared off, the doors to the outside were flung open by the blast, and some 8 feet of drainpipe next to the door was flattened. The flattened drainpipe remains to this day!

“The upshot was that red lights now adorn all doors to the “new” Shock Wave Laboratory (the Roland and Helen Lindhurst Laboratory of Experimental Geophysics), and you are advised to not stand by them during experiments. Incidentally, the shot that blew the doors open (and took out two ventilators) provided good data for orthoclase, which was published in the Journal of Geophysical Research in May 1973.

“A second story concerns Tom’s first attempts at “recovery” experiments in 1967. The Provost of the California Institute of Technology was concerned that the neighbors of the Kresge Seismological Laboratory who inhabited a row of mansions across from the ritzy Annandale Golf Club might have problems with large gun blasts. An acoustical consultant was hired who suggested that a test gun be fired in the proposed laboratory space while he made measurements outside. Tom went to Sears and bought a single-shot 12-gauge shotgun for $79.95. The test shots were barely audible outside so Tom immediately bolted the gun down and started firing at mineral samples! As sporting shotguns cannot really handle heavy plastic projectiles, several $79.95 shotgun specials were blown up before a proper 12-gauge (now renamed 20 mm to be politically correct) recovery shock apparatus was machined.

“Tom has done much more than blow things up and write papers. He has worked hard for AGU, has supervised a stellar array of graduate students and postdocs, and has probably done more than almost anyone to make mineral physics the important part of AGU that it now is. It is most fitting that AGU now recognize Thomas J. Ahrens with the 1996 Harry H. Hess medal.’

—GUY MASTERS, Scripps Institution of Oceanography, La Jolla, Calif.