Harold Urey and the Moon (1961)

Nobel Laureate and Moon fan Harold Urey. Image credit: NASA.
Harold Clayton Urey as born in the small town of Walkerton, Indiana, on 29 April 1893. He taught school in Indiana and Montana, then earned Bachelor's degrees in biology and chemistry from the University of Montana. After a stint at a chemical plant in Philadelphia, he earned a PhD in chemistry at the University of California at Berkeley in 1923.

Following a fellowship in theoretical physics at the Bohr Institute in Copenhagen, he joined the Chemistry faculty at Johns Hopkins University in Baltimore, then moved to Columbia University in New York. On Thanksgiving Day in 1931, Urey discovered the hydrogen isotope deuterium, a feat that earned him the Nobel Prize in Chemistry in 1934.

By most accounts, Urey was a generous and humble man. For example, he shared credit for his deuterium discovery with the scientist who manufactured the five liters of liquid hydrogen he used for his research.

Urey left Columbia for the University of Chicago in 1945. While in Chicago, he read Ralph Baldwin's 1949 book The Face of the Moon, which made the case for the impact hypothesis; that is, that the Moon's many craters are not volcanic calderas, as was widely believed, but are instead scars left by asteroid impacts. Baldwin's book changed Urey's professional life.

In 1952, Urey published The Planets, which launched the science of geochemistry as applied to extraterrestrial bodies. He christened this new discipline "cosmochemistry." In his book, Urey espoused the "cold Moon" theory; that is, that the Moon never became hot enough internally for its rocks to melt. The cold Moon hypothesis and the impact hypothesis went hand in hand; Urey expected that a cold, quiescent moon would be necessary to preserve ancient impact craters.

Earth's natural satellite was, he argued, little changed from the time it had formed. If humans one day could collect a piece of the Moon, it followed, then they would have in hand a "Rosetta Stone" for deciphering the Solar System's early history.

What turned out to be the first steps toward lunar sample return occurred shortly after Urey's book saw print. In late July 1955, the United States announced that it would launch a civilian scientific Earth satellite during the International Geophysical Year (IGY), an 18-month worldwide science campaign that would begin on 1 July 1957. A little more than a month later, in early September 1955, the Soviet Union announced that it, too, would launch a satellite into Earth orbit during the IGY.

President Dwight Eisenhower had little enthusiasm for rockets and satellites except insofar as they had defense applications. The U.S. IGY satellite, though civilian in nature, received his support because it had a hidden military agenda. It was intended to assert the international legal principle of the "Freedom of Space," which was meant to be analogous to the long-established principle of the Freedom of the Seas. The new principle would, Eisenhower hoped, quell Soviet protests when the United States began to launch surveillance satellites into orbits that carried them over Soviet territory.

The Eisenhower Administration believed that the Soviet Union did the United States a "good turn" by launching Sputnik 1, the first Earth satellite, on 4 October 1957. The Soviet satellite, which passed over U.S. territory several times each day, made unnecessary American assertion of the Freedom of Space principle.

Sputnik 1 soon turned into a liability for the Eisenhower Administration, however. The old General tried to downplay its significance, but neither an American public fearful of apparent Soviet technological superiority nor Democratic Senate Majority Leader (and Presidential hopeful) Lyndon B. Johnson would stand for it.

One result of Sputnik 1 was the creation of the civilian National Aeronautics and Space Administration (NASA), which opened its doors on 1 October 1958. By then, both U.S. and Soviet rocketeers had begun to launch small probes toward the Moon.

During 1958, Urey retired from the University of Chicago and went to work at the University of California, San Diego. On 29 October 1958, at the Lunar and Planetary Exploration Colloquium held at the Jet Propulsion Laboratory in Pasadena, California, he famously predicted that new lunar discoveries would give him a "very red face" in only a few years; that is, that spacecraft would soon collect data that would disprove many of his favorite lunar theories. "Nature can always be more complicated than we imagine," he added.

In November 1958, Urey met newly hired NASA scientist Robert Jastrow, whom he quickly converted to the cause of lunar exploration. The following month, Urey and Jastrow met with NASA Deputy Director for Space Flight Programs Homer Newell at NASA Headquarters in Washington, DC.

At the time, scientists interested in space physics — the study of particles and fields in space — dominated NASA space science. Urey and Jastrow sought to convince Newell that NASA should apply some of its scientific energies (and funds) to the exploration of the Moon's geology.

On 5 February 1959, the NASA Working Group on Lunar Exploration, chaired by Jastrow, met for the first time. Urey was an enthusiastic member. He also became a founding member of the influential National Academy of Sciences Space Science Board, which displayed its backing for lunar exploration by forming a "Lunar Committee." The group strongly supported President John F. Kennedy's 25 May 1961 call for a man on the Moon by 1970.

Three weeks after Kennedy's "Moon Speech," Urey responded to an informal request from Newell that he recommend landing sites on the Moon. In a 19 June 1961 letter, the polymath Nobel Laureate told Newell that "we should attempt to. . . get as great a variety of objectives as possible in as few landings as possible." In acknowledging that the Apollo landings might be few in number he was ahead of many of his colleagues. Urey then listed six classes of sites he felt should be explored.

The numbers in the image above are explained in the post text. Image credit: NASA/David S. F. Portree.
The first class took in sites at high latitudes (that is, close to the lunar poles) (1 on the image above). Urey explained that Harrison Brown, a fellow member of the Working Group on Lunar Exploration, had "presented evidence that water may exist close to the surface in certain high latitude areas." This was, of course, in keeping with Urey's "cold Moon" hypothesis.

Urey then called for landings on two of the lunar maria ("seas"), the smooth, relatively dark-hued plains that mottle the Moon's Earth-facing Nearside hemisphere. One of these, he explained, should be "of the deep type" — that is, it should be an obvious giant impact basin such as "the great collision area just before Sinus Iridium in Mare Imbrium" (2 on the image) or Mare Serenitatis (3). Seismic instruments emplaced on a deep mare would, Urey believed, enable determination of the depth to which the giant impactors that formed them had penetrated the Moon's crust.

The other mare landing should occur on a "shallow" mare, Urey wrote. In the shallow category he listed Oceanus Procellarum (4) and Mare Tranquillitatis (5), neither of which displays the distinctive round outline of Mare Imbrium and Mare Serenitatis. Urey told Newell that NASA would probably want to land first on Oceanus Procellarum in any case because it was a wide plain with few mountains or other obstructions to imperil descending spacecraft.

Next on Urey's wishlist was the interior of a large impact crater. He suggested Alphonsus (6), an old crater partly filled with "gray material." Soviet scientist Nikolai Kozyrev claimed to have observed there in 1958 a short-lived white cloud. Urey noted also that geologist Eugene Shoemaker, founder and first chief of the U.S. Geological Survey's Branch of Astrogeology in Menlo Park, California, was hard at work studying the young crater Copernicus (7) in "very great detail," and that his work might pave the way for a landing there.

Fourth on Urey's list was one of the "great wrinkles in the maria." He told Newell that the wrinkle ridges, as they are known, might be places where water had escaped from the Moon's icy cold interior. He added that Gerard Kuiper, founder of the Lunar and Planetary Laboratory in Tucson, Arizona, had observed deposits of white material atop the ridges. Urey interpreted these as salts left behind as water boiled away in the lunar vacuum.

A Moon lander dispatched to Mare Imbrium near Sinus Iridium could, Urey added, explore both a deep mare and prominent wrinkle ridges (8). Similarly, a landing near Copernicus could explore both the great crater and nearby "little volcano-like things" (9) that Urey believed were related in some way to the wrinkle ridges.

Number five on Urey's list was a mountainous area. His chief candidate was the Haemus Mountains on the south edge of Mare Serenitatis (10), which he believed constituted a mass of material blasted out during the formation of Mare Imbrium.

Finally, Urey listed features that were of interest to him personally. These included an unusual dark gray line in Mare Serenitatis, which he had theorized in the early 1950s was a streak of carbon-rich material similar to that found in primitive carbonaceous chondrite meteorites (11). He also suggested the Aristarchus-Herodotus region (12), which Kozyrev had found to be "luminous," and Lacus Mortis (13), which Urey believed was a graben; that is, a sunken block of lunar crust.

Urey ended his letter by asking Newell to share with him any landing site suggestions he received from other scientists. He argued that site selection was an important matter that "should be considered by many of us."

In his reply of 29 June 1961, Newell told Urey that he had forwarded his suggestions to NASA's Office of Lunar and Planetary Programs and to "the special study groups who have been working out plans for the manned lunar landing." Newell also urged Urey to share with him "any ideas that the Lunar Committee of the Academy's Space Science Board might have."

Urey remained active in lunar exploration throughout the 1960s. In early 1962, he joined the 12-member ad hoc working group NASA's Office of Space Science created to outline the Apollo science program. He participated in the Ranger (1961-1965) and Surveyor (1966-1968) automated missions, as well as the piloted Apollo 11 (July 1969) and Apollo 12 (November 1969) missions, which sampled Mare Tranquillitatis and Oceanus Procellarum, respectively. As he predicted, he had occasion to become red in the face: the Moon, the Apollo samples and surface experiments showed, was molten throughout its first 1.5 billion years of existence, probably experienced surface volcanism as recently as a billion years ago, and today has a molten inner mantle and outer core.

Urey continued his lunar studies until he was well into his 80s. Among his last scientific papers was one on lunar iron chemistry published in 1977. He died in La Jolla, California, on 5 January 1981.


Letter, Harold C. Urey to Dr. Homer E. Newell, Deputy Director, Space Flight Programs, NASA Headquarters, 19 June 1961.

Letter, Homer E. Newell to Dr. Harold C. Urey, School of Science and Engineering, University of California-San Diego, 29 June 1961.

"The Chemistry of the Moon," Harold C. Urey, Proceedings of the Lunar and Planetary Exploration Colloquium, 29 October 1958, Publication 513W3, Vol. 1, No. 3, Missile Division, North American Aviation, 1958.

"Harold Urey and the Moon," Homer E. Newell, The Moon, Volume 7, pp. 1-5, 1973.

NASA's Origins and the Dawn of the Space Age, Monographs in Aerospace History #10, David S. F. Portree, NASA History Division, September 1998.

More Information

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