Alfred O. C. Nier, Regents' Professor of Physics Emeritus at the University of Minnesota, died on 16 May 1994 from injuries suffered two weeks earlier in an auto accident. He would have been 83 on May 28. His first paper, "A Device to Compensate for Magnetic Field Fluctuations in a Mass Spectrograph," appeared in 1935; his last, on noble gases in lunar dust grains, in 1994. The 59 years and 212 publications between these two record his remarkable contributions to the birth and growth of what today are major disciplines in the physical, chemical, and biological study of nature.
More than any other one individual, AI was the founder of the modem fields of isotope geology, isotope geochemistry, and geochronology. In just his second paper, written a year before receiving his Ph.D. in 1936, he used his own design of the forerunner of today's mass spectrometers to confirm the existence of38 Ar and discover the radioactive potassium isotope40 K. Thirteen years later he and L. T. Aldrich found excess40A r in potassium-bearing minerals and established the K-Ar chronometer. At Harvard from 1936-1938 as a postdoctoral fellow with K. T. Bainbridge (his only position away from Minnesota, except for a two-year service in the Manhattan Project, during his entire career), AI made another seminal contribution to the measurement of geologic age. From his studies of the isotopic composition of lead, and his determination of the then unknown238 U/235 U abundance ratio, came the Pb-Pb geochronometer. The impact of his "Pb-Pb equation" on geochronology and cosmochronology was profound: by the mid-1950s it had yielded the first accurate estimates of the ages of the Earth and meteorites, the origins and correlations of ores and minerals, and the ages of the oldest crust.
Al's most famous achievement, to the public at large, was probably his mass spectrometric separation of238 U and235 U in 1940, leading to the demonstration that235 U was the slow-neutron-fissioning uranium isotope. But in 1940, with his 30th birthday still ahead, his scientific and instrumental mastery was just beginning to flower. Over the next five decades AI and his many students and collaborators launched pioneering investigations in atomic and nuclear physics, atmospheric physics and the planetary sciences. Most magnetic spectrometers in use today (including the helium leak detector) are direct descendants of his inventions and designs of the 1940s and 1950s. Enormous, ultra-high resolution spectrometers developed and built at Minnesota yielded atomic mass measurements of unprecedented accuracy.
In the 1950s AI and still another group of colleagues had begun their work in the then new field of isotope cosmochemistry, which expanded over the years at Minnesota to include studies of the abundances, compositions, and origins of volatile elements in extraterrestrial matter. Many of the 44 papers AI wrote after his retirement in 1980 describe exquisite mass spectrometric analyses of gases in tiny individual lunar and interplanetary dust particles.
At the other end of the instrumental size range, the first reliable data on the compositional, thermal and velocity profiles of ionic and molecular constituents in the Earth's upper atmosphere were returned from miniaturized spectrometers flown on balloons and sounding rockets. These studies of the near-space environment, and the instrumental innovations that made them possible, led naturally to his next steps outward in the 1970s, to Mars as leader of the Viking Entry Science team and as a central member of the Molecular Analysis team, and to Venus as a co-investigator on Pioneer Venus. By the time of Viking and Pioneer Venus, Al was already deeply involved in other aspects of space and planetary research.
In all of these many pursuits, Al's enthusiastic curiosity and instrumental mastery were unfailingly coupled with scholarly interpretations that reflected his deep respect for the data and the scientific truths they embodied. He had a lifelong interest in forging scientific collaborations across disciplinary lines; together with scientists in biology, chemistry, and medicine, he continued throughout his career to play a central role in originating and instrumenting new areas of research which, after his eventual departure to pursue some other interest, have generally blossomed today into major fields of inquiry and methodology. His profound influence and lasting intellectual legacy across an astonishing range of scientific fields is recorded in his own papers, as well as in thousands more by others that stemmed from his research and by the honors awarded him over the years, including his election to the National Academy of Sciences, the American Philosophical Society, the American Academy of Arts and Sciences, and the Royal Swedish Academy of Science. Al was also recipient of the Geological Society of America's Arthur L. Day Medal, the Viktor M. Goldschmidt Medal of the Geochemical Society, the American Chemical Society's Field and Franklin Award, and the American Geophysical Union's William Bowie Medal, among many others. He will be remembered by all who knew him as an exceptional scientist, a superb teacher and a gentle and modest man.
Al's papers have been deposited at the University of Minnesota and are housed in the University Archives, in the care of Archivist Penelope Krosch. The 48 cartons of papers, photographs and tapes dating from 1932 to 1994 include field notes, laboratory notebooks, professional correspondence and manuscripts of published and some unpublished writings; sketches, drawings and designs of laboratory equipment, and many photographs and lantern slides of laboratory activities and instruments which supported his experimental research. In the opinion of the collection's appraiser " ... no history of 20th century American physics could call itself definitive without study of this archive ... "
The fullest account of his life and work, with a bibliography of his published writings, is in the transcript of a 1989 oral history interview also housed in the university archives. A shorter oral history was taken as part of the National Air and Space Museum's Space Astronomy Oral History Project. Full obituaries have appeared in Meteoritics 29, 747-749 (1994) and EOS 76, No.7, 66-67 (1995); another currently in preparation will be published as a National Academy of Sciences Biographical Memoir.