THOMAS H. JORDAN is Director of the Southern California Earthquake Center, University Professor and W. M. Keck Foundation Professor of Earth Sciences at the University of Southern California, USA. As SCEC's principal investigator since 2002, he has overseen all aspects of its program in earthquake system science, which currently involves over 600 scientists at more than 60 universities and research institutions worldwide (http://www.scec.org). Jordan currently chairs the International Commission on Earthquake Forecasting for Civil Protection (appointed by the Italian government), is a member of the California Earthquake Prediction Evaluation Council and has served on the Scientific Earthquake Studies Advisory Committee of the U.S. Geological Survey. Jordan's research is focused on system-level models of earthquake processes, earthquake forecasting and forecast-evaluation, and full-3D waveform tomography. He is an elected member of the National Academy of Sciences, the American Academy of Arts and Sciences, and the American Philosophical Society. JOHN GROTZINGER is the Fletcher Jones Professor of Geology at the California Institute of Technology, USA. He currently works as a geologist on the Mars Exploration Rover team, the first mission to conduct ground-based exploration of the bedrock geology of another planet, which has resulted in the discovery of sedimentary rocks formed in aqueous depositional environments. His research addresses the chemical development of the early oceans and atmosphere, the environmental context of early animal evolution, and the geologic factors that regulate sedimentary basins. He is a member of the American Academy of Arts & Sciences and the U.S. National Academy of Sciences.