Lecture Title: Geodetic Clues in Planetary Treasure Hunts

Maria T. Zuber was awarded the 2012 Harry H. Hess Medal at the AGU Fall Meeting Honors Ceremony, held on 5 December 2012 in San Francisco, Calif. The medal is for “outstanding achievements in research on the constitution and evolution of Earth and other planets.”  

For two decades, Maria Zuber – the E. A. Griswold Professor of Geophysics at the Massachusetts Institute of Technology – has led the geophysical exploration of the Moon, Mars, Mercury, and the asteroids. That she is AGU’s 2012 Harry H. Hess Medalist is because of her leadership of multiple spacecraft experiments and an entire mission; her rigorous analysis of the observations from those experiments to advance our understanding of the form, internal structure, and evolution of solar system objects; and her development of mathematical and numerical models of planetary deformational and interior dynamical processes.

Her principal tools have been laser altimetry and planetary gravity fields. Zuber led the analysis of laser ranging data from the lunar Clementine mission to produce the first global topographic map of the Moon. With that map, and a newly determined lunar gravity field, she produced the first global model of lunar crustal structure, which changed our understanding of the nature of isostatic compensation early in lunar history and elucidated for the first time the strongly aspherical nature of internal temperature and melt production in the lunar mantle. The Lunar Orbiter Laser Altimeter on the Lunar Reconnaissance Orbiter, an experiment for which Zuber is Deputy Principal Investigator, and the dual-spacecraft Gravity Recovery and Interior Laboratory mission, for which she is Principal Investigator, are now revolutionizing our understanding of the lunar topography and gravity fields, respectively, and their implications for lunar magmatism, tectonics, impact cratering, and interior differentiation and dynamics.

The Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft, an experiment for which Zuber served as Deputy Principal Investigator, produced the first high-resolution global topographic map of Mars and stimulated a new understanding of all phenomena that affect the Martian surface, from cratering and deformation, to volcanism and atmospheric circulation, to the erosional and depositional action of water and ice. From the first global map of crustal thickness on Mars, Zuber showed that the planet can be divided into two approximately hemispherical provinces, a southern province dominated by a progressive thinning of the crust from south to north and a northern province of uniformly thinner crust, key constraints on the planet’s differentiation and early impact history. Zuber’s group also determined the density and inferred ice content of the south polar layered deposits and documented that seasonal variations in polar topography correlate with variations in the planet’s gravitational oblateness and changes expected from models of atmospheric circulation and CO2 exchange.

Moreover, Zuber led the analysis of laser altimetry observations of Mercury made by the MESSENGER spacecraft that yielded the first maps of crustal thickness in Mercury’s northern hemisphere. She led the determination of the first detailed three-dimensional shape model for an asteroid (433 Eros) and determined that body’s mean density and porosity. She championed the idea that many tectonic features on the inner planets arise from instabilities in the lithosphere induced by in-plane or basal shear stress, and with this idea she inferred mechanical properties of the lithosphere in extensional and contractional regimes on Earth, Venus, and Mars.

–Sean C. Solomon, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York

Thank you, Sean, and other colleagues who took the time to nominate me for this extraordinary honor. It is deeply meaningful to be recognized by the AGU, the primary scientific society in which I’ve participated throughout my career. The ever-expanding reach of AGU provides an outlet for our research much beyond our immediate field, important because, as I tell my students: “It doesn’t matter how good your work is if no one knows about it.”Viewing the list of distinguished past Hess recipients, many of whom have influenced me greatly, it is impossible to overstate how humbled I am. I’ve always had a passion for studying space and I was just plain lucky to have been born at the dawn of the age of space exploration. When I was starting my career, data from robotic spacecraft revealed planetary surfaces that displayed tectonic patterns beyond imagination.In graduate school at Brown University, Marc Parmentier provided me with the guidance I needed to understand what some of these fascinating features could tell us about the internal structures and stress histories of these bodies.Subsequently, an NRC post doc at the Goddard Space Flight Center sent me in an exciting new direction. Long the go-to place for space geodetic measurements of tectonic motions and satellite-based models of Earth’s gravity field, Dave Smith and his group were beginning to look towards the planets. At the time I was impatient because I didn’t have the data I needed to test my models, and while at Goddard I came to realize that I myself could be part of the solution to that problem. By taking advantage of advances in military laser technology, we realized that we could build an altimeter with the requisite lifetime and performance to map planets. In parallel, we extended Goddard’s orbit determination capability to allow us to produce the first high-resolution planetary gravity fields. Our goal was to make great maps, and also to interpret them.Because we designed the experiments and performed the precise corrections to the measurements, we were well positioned to understand what we could conclude with confidence. Along with Dave Smith, Sean Solomon, Roger Phillips, Greg Neumann and many others, we applied this philosophy to Mars, Mercury, the Moon and several asteroids, and along the way we have learned a thing or two about how solid planetary bodies evolved. The key to progress has always been collaboration. I have been fortunate to have scientific colleagues, post docs, and students who challenge me, outstanding analysts who bring the rigor of terrestrial geodesy to the planets, engineers who design, build, and operate the spacecraft and instruments that allow scientific dreams to be realized, and family who have always been there to support me. One of the joys of my life is assembling a group of like-minded people to pursue a lofty goal. I appreciate this medal so much because it is a celebration of the remarkable teams with whom I’ve been fortunate to work. –Maria T. Zuber, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts.