The Geology of Venus

The nature of the surface of Venus is one of the keys to answering fundamental questions about the origin and evolution of the terrestrial planets and is of critical significance to comparative planetology. The last 25 years of solar system exploration have provided unprecedented views of the Earth, Moon, Mars, and Mercury. These views have shown that the smaller terrestrial planetary bodies (those one half the radius of the Earth or less: the Moon, Mercury, and Mars) are characterized by globally continuous, unsegmented lithospheres that stabilized very early in the history of the solar system and by ancient surfaces that preserve the severalbillion-year-old record of early heavy bombardment and early heating and volcanism (Head & Solomon 1981). In contrast, the Earth is characterized by a globally segmented, laterally moving lithosphere, which is created at divergent plate boundaries and destroyed at convergent plate boundaries. Movement is measured in centimeters per year, and the average age of the surface of the planet is less than 2 b.y. Plate tectonics and plate recycling are fundamental mechanisms of heat loss for the Earth, in contrast to conduction, which is the dominant mechanism for the smaller, one-plate planetary bodies (Solomon & Head 1982). What are the reasons for these differences? Is size, or perhaps position in the solar system and initial conditions, the key? Venus, which is approximately the same size and density as Earth and is the closest planet to Earth, offers an opportunity to test these ideas.