Alex Halliday was awarded the 2016 Harry H. Hess Medal at the AGU Fall Meeting Honors Ceremony, held on 14 December 2016 in San Francisco, Calif. The medal is for “outstanding achievements in research on the constitution and evolution of the Earth and other planets.”  

Alex Halliday is a pioneer in the application of isotopic techniques in geochemistry. The past few decades have seen remarkable developments that constrain the timing and nature of events in the early history of the solar system, and Alex has been one of the principal leaders in this effort. His ­­­wide-­­­ranging efforts have also contributed to our understanding of weathering, paleoclimatology, mantle geochemistry, and basalt and granite petrogenesis.

Leadership can come from pioneering new measurements and from putting together a story. Alex has done both. He was a major player in the development and use of multicollector inductively coupled plasma mass spectrometry (ICPMS) for isotope geochemistry. Most significant, working with Der-Chuen Lee, he recognized and developed use of the ­­­hafnium-­­­tungsten system. This made it possible to say something about the timing of core formation, a centrally important process in planetary formation and evolution. The relevant ­­half-­life, 9 million years, is conveniently similar to the timescale of planet and core formation, and the measurement can be done to sufficient precision to see the small amount that results from decays after multiple ­­half-­lives. In proposing the big picture, Alex sought to place his results in the context of our evolving view of how planets (and Earth in particular) formed. Core formation is contemporaneous with accretion, with some (perhaps most) taking place in the first few million years, but with a substantial tail extending out to as much as 100 million years. Alex placed his results in the context of the giant impact, the prevailing view of lunar formation. He named the impactor Theia. This singular event and its isotopic consequences are widely accepted, although several aspects remain imperfectly understood. The story embraces Earth formation, the origin of the Moon, the relative timing of Mars, and the context provided by meteorites.

Alex also led the way with ­­­high-­­­precision silicon isotope measurements; these have had multiple applications, including the isotopic similarity of Earth and the Moon. Alex has made other important contributions, including influential earlier work that interprets the incompatible trace elements in oceanic island basalt and ­­­mid-­­­ocean ridge basalt. He has had many successful students, and he has also contributed great service to the scientific community, including leadership at Oxford and the Royal Society of London, and as a section president (Volcanology, Geochemistry and Petrology) at AGU. He is an outstanding recipient of the Hess Medal.

—David Stevenson, California Institute of Technology, Pasadena

I should start by thanking Dave and others who were involved in my nomination, and AGU, for this great honor. The Hess Medal is not special just because of the previous recipients and the science it recognizes. Hess himself was special. Ron Oxburgh, a former Hess student, who opened my labs in Michigan, describes him as one of the most relaxed, courteous, and considerate heads of department ever. He was highly entrepreneurial. Apparently, during World War II he commanded a troop transporter in the Pacific and, while under fire, the devoted coxswains of the landing craft would leap out of the boat and grab rocks for him! He also left his ­­­echo-­­­sounder running continuously, which was not “normal” but facilitated the modern bathymetric chart of the Pacific, delineating deep ocean trenches associated with subduction. My Ph.D. had little to do with deep planetary interiors. However, at Newcastle I got to hear talks by several big thinkers in the field, such as a younger Dave Stevenson. It was later, at Scottish Universities Research and Reactor Centre and then at the University of Michigan, that I was able to pursue my fascination first with the deep Earth and then cosmochemistry. Like Hess, I have had excellent postdocs and students including ­­­Der-­­­Chuen Lee mentioned by Dave, but also many others. There is one in particular I should mention who helped me get started at Michigan; Jon Davidson is being made an AGU Fellow this year but sadly died of leukemia at the end of September. Jon, like Hess, was an inspiration to many. In the spirit of Hess, isotope geochemistry has benefited from innovative approaches to big problems like the origins of the Earth and Moon. When I saw the prototype ­­­MC-­ICPMS it was clear what it could achieve, but it has required the support of program directors and university leaders at Michigan, ETH, and Oxford. As we developed tungsten and other systems, and explored new archives, there were also many supportive colleagues such as Claude Allègre, Keith O’Nions, and the late Jerry Wasserburg. We have all had the privilege of working at times of great scientific discoveries. It’s brilliant. However, my biggest thank you has to be to Christine and our boys, Jamie and Ross, who have put up with a busy Alex and three major relocations while I have fun being paid to do something I love. Thank you all so much. —Alex Halliday, University of Oxford, U.K.