2012 Leonard Medal for Donald S. Burnett


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Dear colleagues and friends of the Meteoritical Society, it is a great pleasure to present Professor Donald S. Burnett of the California Institute of Technology as the recipient of the 2012 Leonard Medal. About 20 years ago, I was a junior member of a NASA panel looking at ideas for a comet sample return mission. During the introductions of approximately 35 people, everyone took turns explaining what their particular expertise was that (presumably) qualified them to have an informed opinion about comet samples and the potential importance of obtaining one. When it was his turn, Don Burnett simply piped up that he was “a card-carrying cosmochemist” without any further elaboration. I recall being amused by his brevity and a bit puzzled by the overall characterization, but I have since come to appreciate what an apt description this is, given the astonishing breadth of contributions that Don has made in cosmochemistry over a long and highly productive career. Don Burnett has been, and is, an extraordinary nuclear chemist, lunar investigator, planetary scientist, petrologist, and meteoriticist, and I can now think of no scientist more deserving of our society’s highest honor. In a culmination of a nearly half-century-long effort to characterize the chemical and isotopic abundances of a wide variety of solar system materials, this man has even managed to grab a bit of the Sun and bring it back to Houston (of all places)! With apologies to Gilbert and Sullivan, Don Burnett is indeed the very model of the Modern Major General cosmochemist, although I must note that I have yet to see him produce this card that he speaks of carrying. Of course, from this point forward, he need not worry about showing his union card to practice the art of cosmochemistry; instead, he can just point to his medal with Frederick Leonard’s likeness on it.

Donald Burnett was born in 1937, during the first Great Depression, in Dayton, Ohio. He attended the University of Chicago, graduating in 1959 with a degree in chemistry. His Ph.D. work was done in the “hot” field of nuclear chemistry at UC Berkeley under the direction of Stanley Thompson and Nobel laureate Glenn Seaborg, the discoverer of plutonium. Following his Ph.D. (awarded in 1963), Burnett was an NSF postdoctoral fellow working for some other fellows named Wasserburg and Fowler in the Laboratory Astrophysics Group at Caltech. He joined the faculty as an assistant professor in 1965 and ascended the ranks in the Division of Geological and Planetary Sciences, attaining the status of Emeritus Professor of Nuclear Geochemistry in 2006. Although Don and his wife, Mary Louise, retreat to their cabin at Big Bear Lake in the San Bernardino Mountains of southern California on most weekends, this has been anything but a calm “retirement.” In fact, one could say that it’s been more exciting and topsy-turvy than, I don’t know, a spacecraft returning to Earth.

And so, we should speak of Genesis. It has long been recognized that knowledge of the starting composition of the solar system provides the baseline required to decipher the complex chemical and physical processes that accompanied planetary formation. Historically, this has been approached by utilizing the compositions of “primitive” meteorites, so-called because they are thought to have undergone little chemical fractionation during formation in the proto-solar nebula and subsequent residence on asteroids. Indeed, the elemental compositions of chondrites match abundances deduced from photospheric absorption lines tolerably well, lending credence to this approach. However, basing our understanding of the Sun (and hence the average solar system composition) on analyses of enigmatic rocks of obscure origin that fall randomly from the sky has two significant flaws: the photospheric match fails for volatile elements and, perhaps more importantly, isotopic compositions of the Sun are virtually unknown and cannot be inferred for the abundant light elements from analyses of chondrites because of large variability produced by poorly understood processes.

For more than two decades, Don Burnett has championed the view that solar system abundances need to be based on analyses of solar material and that only then can we hope to sort out the processes by which isotopic anomalies and chemical fractionations were imparted to meteoritic and planetary materials. And so, with Roger Wiens, Marcia Neugebauer, Amy Jurewicz, Dotty Woolum, and a team of dedicated engineers and (perhaps naively optimistic) mass spectrometrists, Don pushed for a new sample return of the solar wind, which became the Genesis mission.

It is unfortunate, but understandable, that the Genesis mission is best known among the public as that spacecraft that crashed spectacularly into the desert floor in Utah. Lesser motivated persons may have allowed themselves to be discouraged by the contamination of the painstakingly assembled ultra-pure targets of the solar wind collector arrays, but this setback did not deter the indefatigable Burnett who pointed out that “the scientific mission begins once the samples are back—and the samples are back.” (He also noted, correctly, that “if you’re going to crash into a planet . . . Earth is the best one.”) Following the “hard landing,” Don worked tirelessly with the science team to find methods for sample cleaning and to support the development of new technologies for isotopic analysis of the precious solar atoms locked in the near surface regions of the collector materials. These efforts have showcased an inherent strength of sample return, in that scientific results do not need to be obtained at a specific time and place, but can be accomplished in the normal, iterative manner in which contingencies are dealt with by creative modification of the experiment. Although this is true of any sample return mission, it is no exaggeration to say that without the enthusiasm, steady perseverance, and knowledgeable leadership of Burnett, who is deeply involved in every detail of data analysis and interpretation, it is doubtful that much science would have been accomplished from Genesis. As it now stands, the highest priority scientific questions have been answered but, because we have the samples, there are many additional questions that will be addressed for years to come. And we will know our star like never before.

I’m sure that some in my home country will think it improper to speak of things “pre-Genesis,” but I will nonetheless. The reason is straightforward: with the benefit of hindsight, it is now easy to see how a continuous intellectual thread binds together Don’s remarkable career and prepared him for his Genesis discoveries. From his early days at Caltech, Don has been concerned with isotopic abundances in the solar system, first considering irradiation effects on D, Li, Be, and B with Fowler and Hoyle, then working out Rb-Sr ages of iron meteorites with Wasserburg, and, moving up the periodic table, actinide abundances in meteoritic phosphates with Murrell and Woolum (including 244Pu). Thinking about chronology and nuclear irradiation naturally leads one to the noble gases, and Don contributed to many important papers with Bogard, Huneke, Podosek, and others. The need to understand irradiation histories led Don to isotopes of Sm and Gd in lunar core samples and to become PI on his first return sample, the Apollo 17 Lunar Neutron Probe Experiment, the only experiment to directly measure the neutron flux as function of depth on the lunar surface. Other studies of lunar rocks included measurements of noble gases produced by cosmic rays and implanted by the solar wind, determination of exposure ages and erosion rates by micrometeorite bombardment, and the study of solar wind sputtering effects. The foreshadowing of Genesis is obvious. Additional important threads, left untold here, are Don’s work on CAIs and on instrument development, and particularly his leadership in getting NASA to recognize that not all mission hardware needs to leave Earth’s surface (if the samples are brought back).

Finally, I would be remiss if I did not remark on a prominent theme, which threads several of the letters nominating Don for this award: his generosity to colleagues in the selfless and open pursuit of knowledge. Like many others, I consider myself very fortunate to have worked with Don. In his quiet, thoughtful, and un-assuming way, Don Burnett personifies the joy of simply doing science with no personal agenda or egotism. By his example, Don reminds me, and all those around him, of how lucky we are to be able to work together in trying to solve some interesting problems about our origins.

In summary, it is altogether fitting that the Meteoritical Society recognize an extraordinary scientist and his decades-long effort to measure the composition of the solar system by awarding the Leonard Medal to Don Burnett. This is not the first time that our society’s highest award is given for the analysis of stellar matter, but this time it is for the analysis of the isotopic and elemental compositions of our own star. The Genesis data are enabling new insights into the composition and formation of the terrestrial planets—a fundamental goal of meteorite science and of card-carrying cosmochemists everywhere. Mr. President, I am honored to present to you Don Burnett for the 2012 Leonard Medal.