Penrose Medal Lecture: PLANETARY EVOLUTION: A GEOLOGICAL PERSPECTIVE

Spacecraft exploration in the last half-century has revealed fundamental themes in the geological and thermal evolution of the terrestrial planets. Smaller terrestrial planetary bodies, less than one-half the diameter of Earth, lose heat very efficiently, and rapidly become one-plate planets, recording and preserving the geologic record of the first half of Solar System history. These records reveal the magmatic transition from primary to secondary crust, and show the importance of impact bombardment at the large-crater and basin scale in setting the stage for evolution of the last half of Solar System history. The Moon reveals the nature of primary and secondary crustal formation on one-plate planets, and records the influence of impact basin formation on subsequent thermal, tectonic and volcanic evolution. Mercury provides insight into different scale-lengths of mantle convection and their role in volcanic eruption style and resurfacing rate, and illustrates the close relationship between volcanism and global thermal evolution. The geological record of Mars reveals the importance of the presence of water on the surface and in the atmosphere in modulating both volcanism and impact cratering, and forming fluvial systems and glacial activity at all latitudes. Impact craters and basins have played key roles in the early geologic and climatic evolution of Mars. These one-plate planet themes provide insight into the diverse and contrasting geological records of the largest terrestrial planets, Venus and the Earth. Earth’s global plate tectonics system operates at high rates of resurfacing, but has erased most of the early geologic record. What caused the onset of Earth plate tectonics? Why is the average age of the surface of Venus comparable to that of the Earth despite the current lack of plate tectonics? What role did impact basins have in the formative years of Earth and Venus history and in their contrasting styles of global tectonics? These are among the significant questions that form the basis for the next 50 years of planetary exploration.