Citation for Ho-kwang Mao

The Lehmann Medal is awarded in recognition of outstanding contributions to understanding of the structure, composition, and dynamics of the Earth's mantle and core. When Ho-kwang (Dave) Mao completed his Ph.D. in 1968 at the University of Rochester and moved to the Geophysical Laboratory, a revolution in how we could achieve that understanding was in the making.

Over the following four decades, Dave Mao redefined solid earth geophysics, demonstrating that the mantle and core are accessible in the laboratory through direct study of their components at high pressures and temperatures. Through diverse, ingenious experimentation, this journey to the center of the planet has revealed the many dramatic ways that Earth materials transform under extreme conditions and established a materials-based understanding of the deep interior.

Dave's first paper in J.G.R., based on his M.S. thesis, broke new ground in studies of the core by demonstrating polymorphism in iron alloys. This theme has continued throughout his career, leading to diverse studies of the phase diagram, equation of state, and optical, elasticity, and texture measurements - all at pressures of the core. Work on oxides and silicates that commenced with his Ph.D. thesis addressed the complete range of upper and lower mantle minerals. Dave and his group established the full suite of properties of silicate perovskites, including phase relations with accessory oxides and volatile components. His papers 30 years ago proposed that magnesium-rich silicate perovskite is the most abundant mineral in the planet, a paradigm that remains intact to this day. Recent work is revealing the nature of silicate post-perovskite and the core-mantle boundary region.

Dave accomplished this by focusing on the development of instrumentation that could first, recreate the entire range of pressures and temperatures of the deep interior and second, allow accurate measurements under those conditions. His revolutionary refinements of the diamond anvil cell with Peter Bell established that static megabar pressures could be achieved in the laboratory, and steadily advanced the range of accessible pressures to the inner core. A remarkable array of techniques were developed and applied, from lasers to synchrotron radiation to neutron methods. These accomplishments in experimental geophysics have been a windfall for other disciplines, leading to numerous discoveries beyond geoscience.

During my first experiments with Dave, I realized his key to success: an uncanny ability to distill a scientific problem into an experiment, to develop an appropriate technique, and go into the lab and solve *the problem*, all with infectious enthusiasm and a generous spirit. Nearly every leading group in the field of static high-pressure research worldwide has benefited from contact with Dave, or from the people he has trained.

This is Dave's first medal from AGU, which is surprising given his numerous other accolades, including membership in not one but three National Academies, his many other medals, and the fact that Dave identifies himself as a geophysicist. Since that first article in J.G.R., Dave has published some 690 papers with 420 different co-authors; countless others he has influenced directly or indirectly, all testaments to a body of work that has profoundly impacted our understanding of the mantle and core. Moreover, this productivity and impact show no sign of deceleration. I present Ho-kwang Mao, the Lehmann Medalist for 2007.

Russell J. Hemley, Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20815