Typically referred to as a “Nobel Prize-winning chemist,” Harold Urey did indeed make invaluable contributions to our understanding of the atom in general and isotopes in particular. But from the very beginning he was drawn to other disciplines. Urey’s Ph.D. in chemistry was preceded by a Bachelor of Science degree in zoology from the University of Montana and was followed by a year in Copenhagen at the Institute for Theoretical Physics, run by the renowned physicist Niels Bohr. Urey “settled down” to teach chemistry, but he continued to invade the traditional domain of physical scientists. The result was a remarkable body of work, not just in the field of physical chemistry, but also in geochemistry, lunar science, and astrochemistry.
In 1929, Urey became a professor of chemistry at Columbia University and wrote Atoms, Molecules, and Quanta (with A. E. Ruark) that was published the next year. It was there that he tackled the problem of “heavy hydrogen.” Other chemists had suggested that there might be a form of hydrogen atom with twice the mass of the ordinary hydrogen atom—a heavy isotope—although if it existed at all, it would only be in a small concentration. (An element is identified by the number of protons in its atomic nucleus. But different nuclei of the same element can have a different number of neutrons. Isotopes are atoms with the same number of protons but with a different number of neutrons.) Urey reasoned that if liquid hydrogen were slowly evaporated, most of the heavy hydrogen would remain in the liquid residue. He was right, and eventually Urey produced a liquid residue that contained enough of the heavy isotope to be detected through a spectroscope. The isotope was named deuterium, and in 1934 Urey received the Nobel Prize in chemistry for his discovery. Refusing to travel to Sweden because his wife was pregnant, he delivered his Nobel lecture the following year. Urey went on to devise a large-scale process for obtaining water containing high proportions of deuterium—so-called “heavy water.”
Urey next applied his energies to separating isotopes of other elements. Deducing that heavy isotopes would have a slightly slower reaction time than their lighter twins, he devised ways in which to build up those differences into mea-surable quantities. By the late 1930s, he was able to create high concentrations of isotopes such as carbon-13 and nitrogen-15, found in minute quantities in natural carbon and nitrogen. The chemist’s work was interrupted by World War II, in which he played a critical role. As director of the Atomic Bomb Project at Columbia University, Urey directed the production of isotopes of boron, hydrogen, and uranium. From uranium-238 he separated the rare isotope uranium-235, essential to the development of the atom bomb. After the war, hydrogen-2 (the deuterium Urey had discovered) was used to make the even more destructive H-bomb.
Urey was deeply concerned with the danger posed by the nuclear weapons he had helped create, and became an extremely active advocate of nuclear arms control. In 1945 he took a position as professor of chemistry at the University of Chicago and turned to an entirely new area of study: geochemistry. Once again, his knowledge of isotopes was put to ingenious use, this time in calculating the temperature of ancient oceans. Because isotopes react more slowly at colder temperatures, the properties of certain isotopes found in fossil shells reflect the temperature of the ocean at the time the shells were formed. Using oxygen isotopes as a “paleothermometer,” Urey and his co-workers analyzed tiny fossil shells from ocean sediment cores to create a history of changing ocean temperatures over long geologic periods.
It was in 1953, while he was still at the University of Chicago, that Urey and graduate student Stanley L. Miller performed a landmark experiment that brought the problem of the origin of life into the laboratory. Complex organic molecules called amino acids are the building blocks of the proteins necessary for life. At the time, they had been found only in living systems. Urey believed that life was common in the universe, and that these building blocks must have spontaneously come into being on the early Earth. He and Miller set out to demonstrate how this could have happened. They filled a flask with methane, hydrogen, ammonia, and steam, which was thought to replicate the early atmosphere, and passed 60,000-volt electric charges through it to simulate lightning. Miller sat by the crackling apparatus for a week, then analyzed the chemicals in the water. They were full of amino acids. Most scientists now believe that the early atmosphere had a different composition than the one tested by Miller and Urey, and that organic molecules originated by other mechanisms. Yet the experiment galvanized the scientific comm-unity to think about how life may have begun.
Urey also studied the chemical make-up of the Sun, Moon and planets, and formulated detailed theories about the origin of the solar system. He believed that planets were built up by the accretion of smaller, mainly metallic fragments at relatively low temperatures, and that the Moon was formed separately. His second book was called The Planets.
A man of deeply-held political convictions, Urey took a courageous early stand against Senator Joseph McCarthy’s anti-Communist campaign. He remained an anti-war activist, and opposed the use of nuclear power. Urey’s legacy is a significant one. The isotope-labeling techniques he introduced have proven immensely valuable for researchers in many fields. He was widely published on many subjects, receiving countless awards and honorary degrees. He died in 1981, at the age of 88.