Harold C. Urey: Science, Religion, and Cold War Chemistry
Stanley Miller demonstrates the Miller-Urey experiment, which simulated the chemical conditions of early Earth. The experiment showed that amino acids and sugars could, with the addition of electrical discharges, form in an environment of water, methane, ammonia, and hydrogen. Many of Urey’s postwar interests focused on the origin of things. (Roger Ressmeyer/CORBIS)
Mass Spectrometers, Isotopes, and Earth History
Rather than change his name, Urey went back to the lab and changed his research subject. His new work would help define an entirely new field of isotope geochemistry.
After the war Urey and several of his Columbia University colleagues—including Enrico Fermi and Joseph Mayer and Maria Goeppert-Mayer—had moved to the University of Chicago, where they established the Institute for Nuclear Studies (later renamed the Fermi Institute). The university’s president, Robert M. Hutchins, was committed to a large-scale postwar research program in nuclear physics and chemistry.
Because of his newfound aversion to isotope separation and a calendar filled with speaking engagements, Urey spent his first several months at the institute adrift with no active research projects to speak of. The institute turned out to be a good fit for Urey’s aimlessness since it emphasized applying atomic expertise to a wide range of problems. By the end of 1946 Urey had hit on a new and promising research direction—using isotope ratios in nature to answer questions from Earth’s deep past.
Oxygen-isotope ratios were the first to draw Urey’s attention. Sea creatures use oxygen atoms in the surrounding water to build their calcium carbonate (CaCO3) shells. Most oxygen is oxygen-16, but as water temperatures decrease, the ratio of oxygen-18 to oxygen-16 in shells increases minutely. Thus, shells formed in colder temperatures contain more oxygen-18 than those formed in warmer waters.
Oxygen isotopes trapped in shells would make possible, in Urey’s words, “a new thermometer of great durability . . . buried in the rocks for hundreds of millions of years after recording the temperature of some past geological epoch.” For the first time, fluctuations in ancient climates would be open to direct study.
Because the differences in isotope ratios being measured were so tiny, Urey needed a cutting-edge instrument. As it happened, he knew exactly what he wanted. During the war Urey had overseen the development of highly precise mass spectrometers designed to the specifications of his colleague Alfred O. Nier at the University of Minnesota. Urey estimated that Nier’s new mass spectrometers would allow researchers to determine the temperature at which seashells were formed to within 6°C.
This new research program invigorated Urey. As Samuel Epstein, one of Urey’s first postdoctoral researchers at the institute remembered, “He never walked up a set of stairs one step at a time, always two steps at a time. His enthusiasm for his research was contagious.”
Urey and Epstein took two years to get their spectrometers built and operating reliably, longer than Urey had initially expected. He had thought the army would sell him one of the instruments General Electric had built during the war for the Oak Ridge uranium isotope–separation facility; instead, Urey had to build his own instruments with schematics and key components from Nier’s laboratory. By February 1949 Urey’s lab had constructed the two mass spectrometers they would use in developing their “oxygen thermometer.”
Origin of Life in the Cold War
Urey always seemed content to allow his students and junior colleagues to take full credit for the work they did with him. They would be the ones to carry on this work once they left his lab, while he would move on to a new question. This was certainly the case for Urey’s graduate student Stanley Miller.
By the early 1950s Urey had moved from geochemistry to the chemistry of the solar system and the origins of Earth—an emerging field that came to be known as cosmochemistry. In 1951 Urey worked on a paper positing that early Earth had a highly reducing atmosphere (one with a low concentration of oxygen) consisting primarily of hydrogen, ammonia, methane, and water. Electrical discharges within this atmosphere, such as lightning, formed the first carbon compounds, establishing the prebiotic conditions within which life could emerge.
But how did life first start in such a harsh, hydrogen-rich environment? At the institute’s weekly seminar Urey speculated on an experiment that might shed light on the origin of life: mix water and methane, add electric discharges, and see what organic compounds formed. Back at his office Urey found a young graduate student, Stanley Miller, at his door. Miller asked if he could use this experiment as the subject of his dissertation.
At first Urey tried to talk Miller out of the project, worried the experiment might be a dead end. Miller persisted, and to Urey’s surprise the experiment produced results within a matter of weeks. The mixture of water, methane, ammonia, and hydrogen with which they had begun the experiment now contained organic compounds, including those typical of life: amino acids and sugars. In early 1953 Miller and Urey sent the results to Science, with Miller listed as the sole author. Urey was content to be thanked in a footnote.
Urey’s own work on the origins of Earth and the solar system, which included the publication of his book The Planets: Their Origin and Development, led to another diversion: lunar and planetary exploration. Insiders at NASA credit Urey’s theory of the Moon’s origins and significance as having been chief among the scientific motivations for the American lunar program.