THE ENDURING MYSTERY OF VENUS

The mapping of Venus by radar in the early 1990s revealed for the first time the planet's harsh, rugged volcanic surface and defined an enduring geologic puzzle. To frame this puzzle, recall that collisions of cometary and asteroidal debris provide clues about the geology of the planets and moons. From crater counts planetary geologists demonstrated that major geologic terranes on the Moon differ in relative age. With spacecraft exploration of Mercury and Mars, a broad range of surface ages was established. Like the Moon, these planets had surfaces that recorded the earliest events in the Solar System and younger cratering and volcanic events continuing to the present. Because of its thick cloud cover and dense atmosphere, the exploration of Venus posed a difficult task. Hints of surface features were observed by Earth-based radio telescopes and a Soviet orbital radar mission, but we knew little about the geologic nature of Earth's sister planet. In the early 1990s as NASA's Magellan began mapping long, narrow image strips, it was widely anticipated that the science team would see a range of surface types with differing numbers of impact craters; in fact, the first few orbits revealed spectacular impact craters. After the first few weeks, all the craters appeared to be relatively young, with fresh crisp ejecta features, and they were randomly distributed across the surface. After one complete mapping cycle of the planet virtually all the craters seemed young and randomly distributed over all of Venus. The dominant terrane types had been identified from earlier data as relatively level plains and rugged highlands. We had expected the highlands to be more cratered, but they are not distinguishable in age from the plains, based solely on the numbers of impact craters. Thus a scientific debate began. One side argued that the surface of Venus that we see today is relatively young. They concluded that about 500 million years ago there was a geologic catastrophe and the entire planet was resurfaced by volcanic and tectonic overturn. The other side maintained that continuous volcanic activity could have destroyed craters about as fast as they form and equilibrium was established which caused the surface to appear about the same as we see it now for billions of years. This debate continues today in various forms with increasing sophistication.