During World War II, Urey turned his knowledge of isotope separation to the problem ofuranium enrichment. He headed the group located at Columbia University that developed isotope separation usinggaseous diffusion. The method was successfully developed, becoming the sole method used in the early post-war period. After the war, Urey became professor of chemistry at theInstitute for Nuclear Studies, and later Ryerson professor of chemistry at theUniversity of Chicago.
Urey speculated that the early terrestrialatmosphere was composed ofammonia,methane, and hydrogen. One of his Chicago graduate students wasStanley L. Miller, who showed in theMiller–Urey experiment that, if such a mixture were exposed to electric sparks and water, it can interact to produceamino acids, commonly considered the building blocks of life. Work withisotopes of oxygen led to pioneering the new field ofpaleoclimatic research. In 1958, he accepted a post as a professor at large at the newUniversity of California, San Diego (UCSD),[2][3] where he helped create the science faculty. He was one of the founding members of UCSD's school of chemistry, which was created in 1960. He became increasingly interested in space science, and whenApollo 11 returnedMoon rock samples from the Moon, Urey examined them at theLunar Receiving Laboratory. Lunar astronautHarrison Schmitt said that Urey approached him as a volunteer for a one-way mission to the Moon, stating "I will go, and I don't care if I don't come back."[4]
Harold Clayton Urey was born on April 29, 1893, inWalkerton, Indiana, the son of Samuel Clayton Urey,[5][6] a school teacher and a minister in theChurch of the Brethren,[7] and his wife, Cora Rebecca née Reinoehl.[8] Of mostly German ancestry, the family name had English origins.[9] He had a younger brother, Clarence, and a younger sister, Martha. The family moved toGlendora, California, after Samuel became seriously ill withtuberculosis, in hopes that the climate would improve his health. When it became clear that he would die, the family moved back to Indiana to live with Cora's widowed mother. Samuel died when Harold was six years old.[10][6]
Urey was educated in anAmish grade school, from which he graduated at the age of 14. He then attended high school inKendallville, Indiana.[8] After graduating in 1911, he obtained a teacher's certificate fromEarlham College,[11] and taught in a small school house in Indiana. He later moved to Montana, where his mother was then living, and continued to teach there.[7]
Urey entered theUniversity of Montana inMissoula in the autumn of 1914.[12] Unlike Eastern universities of the time, the University of Montana was co-educational in both students and teachers.[6] Urey earned aBachelor of Science (BS) degree inzoology there in 1917.[13]
As a result of theUnited States entry into World War I that same year, there was strong pressure to support the war effort. Urey had been raised in a religious sect that opposed war. One of his professors suggested that he support the wartime effort by working as a chemist. Urey took a job with theBarrett Chemical Company inPhiladelphia, makingTNT, rather than joining the army as a soldier.[6] After the war, he returned to theUniversity of Montana as an instructor inchemistry.[14][11]
On returning to the United States, Urey received an offer of aNational Research Council fellowship toHarvard University, and also received an offer to be a research associate atJohns Hopkins University. He chose the latter. Before taking up the job, he traveled toSeattle, Washington, to visit his mother. On the way, he stopped byEverett, Washington, where he knew Dr. Kate Daum, a colleague from the University of Montana.[20] Dr. Daum introduced Urey to her sister, Frieda. Urey and Frieda soon became engaged. They were married at her father's house inLawrence, Kansas, in 1926.[14] The couple had four children:Gertrude Bessie (Elizabeth), born in 1927; Frieda Rebecca, born in 1929; Mary Alice, born in 1934; and John Clayton Urey, born in 1939.[21]
At Johns Hopkins, Urey andArthur Ruark wroteAtoms, Quanta and Molecules (1930), one of the first English texts on quantum mechanics and its applications to atomic and molecular systems.[19] In 1929, Urey became an associate professor of chemistry atColumbia University, where his colleagues includedRudolph Schoenheimer,David Rittenberg, andT. I. Taylor.[22]
In the 1920s,William Giauque andHerrick L. Johnston discovered the stableisotopes of oxygen. Isotopes were not well understood at the time;James Chadwick would not discover theneutron until 1932. Two systems were in use for classifying them, based on chemical and physical properties. The latter was determined using themass spectrograph. Since it was known that theatomic weight of oxygen was almost exactly 16 times as heavy as hydrogen,Raymond Birge, andDonald Menzel hypothesized that hydrogen had more than one isotope as well. Based upon the difference between the results of the two methods, they predicted that only one hydrogen atom in 4,500 was of the heavy isotope.[23]
In 1931, Urey set out to find it. Urey and George M. Murphy (1903–1968)[24][25] calculated from theBalmer series that the heavy isotope should have linesblueshifted (correspondingly the light isotoperedshifted) by 1.1 to 1.8ångströms (1.1×10−10 to 1.8×10−10metres). Urey had access to a 21-foot (6.4 m)gratingspectrograph, a sensitive device that had been recently installed at Columbia and was capable of resolving the Balmer series. With a resolution of 1 Å per millimetre, the machine should have produced a difference of about 1 millimetre.[26] However, since only one atom in 4,500 was heavy, the line on the spectrograph was very faint. Urey therefore decided to delay publishing their results until he had more conclusive evidence that it was heavy hydrogen.[23]
Urey and Murphy calculated from theDebye model that the heavy isotope would have a slightly higher boiling point than the light one. By carefully warming liquid hydrogen, 5 litres of liquid hydrogen could be distilled to 1 millilitre, which would be enriched in the heavy isotope by 100 to 200 times. To obtain five litres of liquid hydrogen, they traveled to the cryogenics laboratory at theNational Bureau of Standards in Washington, D.C., where they obtained the help ofFerdinand Brickwedde, whom Urey had known at Johns Hopkins.[26]
The first sample that Brickwedde sent was evaporated at 20 K (−253.2 °C; −423.7 °F) at a pressure of 1 standard atmosphere (100 kPa). To their surprise, this showed no evidence of enrichment. Brickwedde then prepared a second sample evaporated at 14 K (−259.1 °C; −434.5 °F) at a pressure of 53 mmHg (7.1 kPa). On this sample, the Balmer lines for heavy hydrogen were seven times as intense.[23] The paper announcing the discovery of heavy hydrogen, later nameddeuterium, was jointly published by Urey, Murphy, and Brickwedde in 1932.[27] Urey was awarded theNobel Prize in Chemistry in 1934 "for his discovery of heavy hydrogen".[28] He declined to attend the ceremony in Stockholm, so that he could be present at the birth of his daughter Mary Alice.[29] He was elected to both theAmerican Philosophical Society and the United StatesNational Academy of Sciences the following year.[30][31]
Working withEdward W. Washburn from the Bureau of Standards, Urey subsequently discovered the cause of the anomalous sample. Brickwedde's hydrogen had been separated from water byelectrolysis, resulting in a depleted sample. Moreover,Francis William Aston had reported that his calculated value for the atomic weight of hydrogen was wrong, thereby invalidating Birge and Menzel's original reasoning. The discovery of deuterium stood, however.[23]
Urey and Washburn attempted to use electrolysis to create pureheavy water. Their technique was sound, but they were beaten to it in 1933 by Lewis, who had the resources of the University of California at his disposal.[32] Using theBorn–Oppenheimer approximation, Urey andDavid Rittenberg calculated the properties of gases containing hydrogen and deuterium. They extended this to enriching compounds of carbon, nitrogen, and oxygen. These could be used as tracers inbiochemistry, resulting in a whole new way of examining chemical reactions.[33] He founded theJournal of Chemical Physics in 1932, and was its first editor, serving in that capacity until 1940.[34]
At Columbia, Urey chaired the University Federation for Democracy and Intellectual Freedom. He supportedAtlanticistClarence Streit's proposal for a federal union of the world's majordemocracies, and therepublican cause during theSpanish Civil War. He was an early opponent of GermanNazism and assisted refugee scientists, includingEnrico Fermi, by helping them find work in the United States, and to adjust to life in a new country.[35]
By the time World War II broke out in Europe in 1939, Urey was recognized as a world expert on isotope separation. Thus far, separation had involved only the light elements. In 1939 and 1940, Urey published two papers on the separation of heavier isotopes in which he proposed centrifugal separation. This assumed great importance due to speculation by Niels Bohr thaturanium 235 wasfissile.[36] Because it was considered "very doubtful whether a chain reaction can be established without separating 235 from the rest of the uranium,"[37] Urey began intensive studies of how uranium enrichment might be achieved.[38] Apart from centrifugal separation,George Kistiakowsky suggested thatgaseous diffusion might be a possible method. A third possibility wasthermal diffusion.[39] Urey coordinated all isotope separation research efforts, including the effort to produce heavy water, which could be used as aneutron moderator innuclear reactors.[40][41]
In May 1941, Urey was appointed to theCommittee on Uranium, which oversaw the uranium project as part of theNational Defense Research Committee (NDRC).[42] In 1941, Urey andGeorge B. Pegram led a diplomatic mission to England to establish co-operation on development of the atomic bomb. The British were optimistic about gaseous diffusion,[43] but it was clear that both gaseous and centrifugal methods faced formidable technical obstacles.[44] In May 1943, as theManhattan Project gained momentum. Urey became head of the wartime Substitute Alloy Materials Laboratories (SAM Laboratories) at Columbia, which was responsible for the heavy water and all the isotope enrichment processes exceptErnest Lawrence's electromagnetic process.[45]
Early reports on the centrifugal method indicated that it was not as efficient as predicted. Urey suggested that a more efficient but technically more complicated countercurrent system be used instead of the previous flow-through method. By November 1941, technical obstacles seemed formidable enough for the process to be abandoned.[46] Countercurrent centrifuges were developed after the war, and today are the favored method in many countries.[47]
The gaseous diffusion process remained more encouraging, although it too had technical obstacles to overcome.[48] By the end of 1943, Urey had over 700 people working for him on gaseous diffusion.[49] The process involved hundreds of cascades, in which corrosiveuranium hexafluoride diffused through gaseous barriers, becoming progressively more enriched at every stage.[48] A major problem was finding proper seals for the pumps, but by far the greatest difficulty lay in constructing an appropriate diffusion barrier.[50] Construction of the hugeK-25 gaseous diffusion plant was well under way before a suitable barrier became available in quantity in 1944. As a backup, Urey championed thermal diffusion.[51]
Worn out by the effort, Urey left the project in February 1945, handing over his responsibilities to R. H. Crist.[52] The K-25 plant commenced operation in March 1945, and as the bugs were worked out, the plant operated with remarkable efficiency and economy. For a time, uranium was fed into theS50 liquid thermal diffusion plant, then the K-25 gaseous, and finally theY-12 electromagnetic separation plant; but soon after the war ended the thermal and electromagnetic separation plants were closed down, and separation was performed by K-25 alone. Along with its twin, K-27, constructed in 1946, it became the principal isotope separation plant in the early post-war period.[53][51] For his work on the Manhattan Project, Urey was awarded theMedal for Merit by the Project director,Major GeneralLeslie R. Groves, Jr.[52]
After the war, Urey became professor of chemistry at theInstitute for Nuclear Studies, and then became Ryerson professor of chemistry at theUniversity of Chicago in 1952.[11] He did not continue his pre-war research with isotopes. However, applying the knowledge gained with hydrogen to oxygen, he realized that thefractionation betweencarbonate and water foroxygen-18 andoxygen-16 would decrease by a factor of 1.04 between 0 and 25 °C (32 and 77 °F). The ratio of the isotopes could then be used to determine average temperatures, assuming that the measurement equipment was sufficiently sensitive. The team included his colleagueRalph Buchsbaum. Examination of a 100-million-year-oldbelemnite then indicated the summer and winter temperatures that it had lived through over a period of four years. For this pioneeringpaleoclimatic research, Urey was awarded theArthur L. Day Medal by theGeological Society of America, and the Goldschmidt Medal of theGeochemical Society.[54] While at the University of Chicago, Urey contributed to theUrey–Bigeleisen–Mayer equation, a model of stable isotope fractionation.
Miller–Urey experiment
Urey actively campaigned against the 1946May-Johnson bill because he feared that it would lead to military control of nuclear energy, but supported and fought for the McMahon bill that replaced it, and ultimately created theAtomic Energy Commission. Urey's commitment to the ideal ofworld government dated from before the war, but the possibility ofnuclear war made it only more urgent in his mind. He went on lecture tours against war, and became involved in Congressional debates regarding nuclear issues. He argued publicly on behalf ofEthel and Julius Rosenberg, and was called before theHouse Committee on Un-American Activities.[55]
In later life, Urey helped develop the field ofcosmochemistry and is credited with coining the term. His work on oxygen-18 led him to develop theories about theabundance of the chemical elements on Earth, and of their abundance and evolution in the stars. Urey summarized his work inThe Planets: Their Origin and Development (1952). Urey speculated that the early terrestrialatmosphere was composed ofammonia,methane, and hydrogen. One of his Chicago graduate students,Stanley L. Miller, showed in theMiller–Urey experiment that, if such a mixture is exposed to electric sparks and to water, it can interact to produceamino acids, commonly considered the building blocks of life.[56]
Urey also researched theisotopic abundance of cosmic bodies, particularly the contribution of radioisotopes tointernal heating.[57] In tribute to his contributions, scientists refer to the ratio of a planet's internal heat generation to surface heat flux as theUrey ratio.[58]
Urey spent a year in the United Kingdom as a visiting professor atOxford University in 1956 and 1957.[59] In 1958, he reached the University of Chicago's retirement age of 65, but he accepted a post as a professor at large at the newUniversity of California, San Diego (UCSD), and moved toLa Jolla, California. He was subsequently made aprofessor emeritus there from 1970 to 1981.[60][61][11] Urey helped build up the science faculty there. He was one of the founding members of UCSD's school of chemistry, which was created in 1960, along with Stanley Miller,Hans Suess, andJim Arnold.[60][62]
In the late 1950s and early 1960s, space science became a topic of research in the wake of the launch ofSputnik 1. Urey helped persuadeNASA to make uncrewed probes to the Moon a priority. WhenApollo 11 returnedMoon rock samples from the Moon, Urey examined them at theLunar Receiving Laboratory. The samples supported Urey's contention that the Moon and the Earth shared a common origin.[60][62] While at UCSD, Urey published 105 scientific papers, 47 of them about lunar topics. When asked why he continued to work so hard, he joked, "Well, you know I'm not on tenure anymore."[63]
^Urey, Harold (March 3, 1965)."Harold Urey's Interview".Voices of the Manhattan Project (Interview). Interviewed by Stephane Groueff.Atomic Heritage Foundation. RetrievedJanuary 20, 2024.The name is English. All the rest of my grandparents are German. Their names are Hofstettler. Hofstettler is a corruption. It was Hochstettler or something. And Eckhart and Reinoehl, very German, you see.
