James R. Arnold 1923–2012

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James R. Arnold passed away on January 6, 2012, at age 88 ending a distinguished career in meteoritics, lunar, and space sciences. He has played key roles in the planning of sample return for scientific studies in the Apollo program and advising during the missions, in determining the composition and history of the lunar regolith. Jim was a recipient of several awards, including the 1976 Leonard Medal of the Meteoritical Society, the Atomic Energy Commission’s E. O. Lawrence Award, and a NASA medal for “Exceptional Scientific Achievement,” recognizing his work on lunar surface processes and on cosmic rays. He was elected to the National Academy of Sciences in 1964.

Jim graduated from Princeton University in 1943 and received his Ph.D. there in 1946 and during this time he was associated with the Manhattan Project. He was working with Willard Libby when they demonstrated that cosmic ray-produced radiocarbon abundances on the Earth could be used for dating archaeological and geological samples, work which later won a Nobel Prize to Libby. Jim Arnold was involved in the development of the method of scintillation counting of natural radioactivities. He discovered additional cosmic ray-produced radionuclides, 7Be and 10Be, in the Earth’s atmosphere. The potential usefulness of the latter was soon realized first as a cosmic ray flux monitor and in numerous chronological applications in geophysics and in cosmochemistry. Jim was among the few select scientists who were inducted in La Jolla in 1958 by Roger Revelle to launch the San Diego campus of the University of California. Arnold worked with Roger Revelle in planning the new campus and in attracting Nobel laureates, such as Harold Urey and Maria Mayer, to the UCSD faculty. During the time Jim Arnold served as the first chemistry Department chair he and his research group expanded the list of measured long-lived radionuclides in iron meteorites to four (10Be, 26Al, 36Cl, and 53Mn). This work laid the foundations for studies of the past history of cosmic radiation and for the evaluation of the space histories of meteorites. Following earlier work by Öpik, Jim used a statistical (Monte Carlo) approach to decipher the collisional history of meteorites in space before the capture by the Earth. At this time, dynamical simulations were not very advanced and computers were slow to integrate orbits over millions of years of meteorite orbital travel. In subsequent years Jim was much involved in working with NASA in planning science activities and a lunar sample return during the Apollo missions. In 1966–68 he served on a Panel of the President’s Science Advisory Committee and on several committees of the National Academy of Sciences. Also, he pioneered the development of experimental approaches to determine the composition of the lunar and planetary surfaces by gamma-ray spectroscopy. This technique was then successfully used in orbital measurements on the Moon. The study of cosmic ray-produced radionuclides conducted by Jim and his colleagues and students has shown that the intensity of galactic cosmic radiation has been approximately constant over millions of years. This conclusion could be verified in lunar rocks and soils, and with the additional advantage of the Moon’s known position led to the determinations of long-term averaged fluxes of galactic and solar cosmic radiation. The propagation of secondary particles and their reactions had to be treated in 1-D models; nevertheless, these models in conjunction with depth-dependent concentration measurements of 53Mn and other nuclides permitted tests for the variability of solar particle fluxes and specific regolith histories and gardening rates in the lunar regolith.

There were numerous other cosmochemical studies carried out by the Arnold research group, like condensation and agglomeration studies in a porcelain test chamber with cooling rates of the order of 106 ° s−1. This research, which served the purpose of simulating protoplanetary conditions by using laser vaporization, was later also used to study the trapping of noble gases during condensation processes. Another project was an investigation of Brownlee particles from atmospheric collections and from deep-sea spherule concentrations with the aim of measuring the flux of extraterrestrial material and attempting to find its origin. Furthermore, measurements of concentrations of in-situ produced 10Be and 26Al activities in terrestrial environments were carried out. The geological applications of such data to studies of erosion rates and of quartz transports in different locations turned out to be very useful.

During the early Apollo days Jim’s collegial discussions over lunch covered topics from Harold Urey’s primitive composition of the Moon to surface compositions and differences between the lunar front and backside, to the chemical makeup of hydrogen and water on the Moon. Some studies led Jim to conclude that appreciable amounts of water may exist in the polar regions of the Moon, and in 1979 he published a paper calling attention to the possible existence of deposits of ice at the lunar polar regions. This as well as other documents added weight in planning NASA’s search for water on the Moon. Only recently several reports that minor amounts of water have in fact been found in the extremely dry lunar rocks did stimulate further discussions of lunar water.

Jim was a dedicated educator and a man of vision. He often considered questions from the listeners with the depth due to a scientific peer. He was always interested in sharing and discussing new data, not only in the laboratory or at meetings but also with other interested colleagues. Jim Arnold was always in demand for help in planning and organizational matters. He established the California Space Institute and served as director of this research organization from 1980 to 1989, specifically developing programs for space applications.

In 1980 an 8-mile-wide asteroid discovered by Eleanor Helin and Eugene Shoemaker was named “2143 Jimarnold” in his honor. Jim was closely associated with Harold Urey since the Manhattan Project and it was fitting in 1983 that he was honored as the first holder of the Harold C. Urey Chair in Chemistry. His interest in manned space exploration and the question of availability of resources led him along with Gerard O’Neill, Freeman Dyson, and others, to consider and discuss the future of human exploration and of possible settlements on the Moon, Mars, and on asteroids. Some of Jim’s research and committee experiences have already been published (Marvin 2001). The annual Jim Arnold Lecture at UCSD recognizes Arnold’s contributions where invited speakers discuss novel insights in cosmochemistry and space sciences.

Jim is survived by his wife Louise, their sons Bob, Ted, and Ken, and four grandchildren.