Robert Clayton has dedicated his life to the analysis of extraterrestrial material, from lunar rocks to meteorites from Mars and the asteroid belt. Now, the editors of a new book, Oxygen in the Solar System, have dedicated the volume to Clayton, the Enrico Fermi Distinguished Service Professor Emeritus in Chemistry and the Geophysical Sciences at the University of Chicago.
The book contains chapters by 66 authors, the fruits of three Lunar and Planetary Institute workshops conducted in 2004 and 2005. "It covers oxygen everywhere, from the sun to the interstellar medium to the interior of the Earth," said the book's technical editor, Steven Simon, a Senior Research Associate in Geophysical Sciences at the University of Chicago.
Clayton pioneered the use of oxygen isotopes, chemical fingerprints found in meteorites and lunar rocks to understand the processes that formed the planets and asteroids following the birth of the solar system. He "could easily wear the name 'Mr. Oxygen,'" wrote Smithsonian geologist Glenn MacPherson in the book's dedication.
MacPherson highlighted Clayton's discovery with colleagues at the Enrico Fermi Institute in 1973 that the chemistry of oxygen in the Allende meteorite was fundamentally different from that known for Earth rocks. According to MacPherson, "the intense interest generated by the 1973 work revolutionized cosmochemistry in a way that continues to this day."
The editors collected an overview of Clayton's oxygen isotopic data in a frontispiece for the book that they labeled, "The Solar System in Clayton-Mayeda Space." The late Toshiko Mayeda, Senior Research Associate in the Enrico Fermi Institute, was a longtime collaborator of Clayton's.
MacPherson, the book's editor-in-chief, wrote the book's introduction. MacPherson served as a Research Associate in the University of Chicago laboratory of Lawrence Grossman, Professor in Geophysical Sciences, from 1978 until 1984.
The book also includes a chapter by Clayton. Three of his Chicago colleagues contributed additional chapters. With a team of international collaborators, Andrew Davis, Professor in Geophysical Sciences, co-wrote an article on oxygen and the sun. Simon and Grossman co-authored another article with 1986 Chicago Ph.D. alumnus John Beckett on oxygen in meteorites.
The book dedication is the most recent of many honors that Clayton has received, including the National Medal of Science, the Meteoritical Society's Leonard Medal and election to the National Academy of Sciences and the Royal Society of London.
Clayton joined the Chicago faculty in 1958 and officially retired in 2001. Yet in 2002, he published an article in Nature that successfully predicted that the oxygen isotopic composition of the sun would be vastly different from Earth's and the rocky planets. An analysis of data from NASA's Genesis satellite, released in March, verified his prediction.
Most cosmochemists now seem to agree that the compositional differences in oxygen isotopes arise from chemical reactions driven by ultraviolet light that occurred early in the history of the solar system. "The question that is unanswered is: Where did that happen?" Clayton said.
Clayton prefers the idea that these photochemical reactions happened close to the sun. This idea implies that material from which Earth and other planets are made migrated close to the sun, where they were exposed to the ultraviolet light, then moved back out again. "That would be a radical departure from what people think of as how to make planets," he said.
In the other two scenarios, ultraviolet light from other stars primarily sparks chemistry in cold material at distances beyond the known planets, then brings the material into the inner solar system.
Clayton has devised a roundabout way of solving this issue.
"If we have the right explanation for oxygen, then it should probably apply to the isotopes of nitrogen," he said, because of the similarity of the relevant molecules. Cosmochemists already hold some critical measurements of Jupiter's nitrogen isotopes, courtesy of NASA's Galileo spacecraft.
"I have anticipated that the Jupiter number and the solar number will be the same, and the Earth is different, just as we see in oxygen," Clayton said. If so, then the photochemistry involved must have taken place in the inner solar system.
The definitive measurements, based again on samples collected by Genesis, may be available later this year. At that point, cosmochemists will stand better positioned to understand planet formation. "Right now, we're still waiting for more facts," Clayton said.
—Prof. Jacqueline Stewart
