Magellan and comparative planetology

Abstract

Magellan data have been widely used to provide explanations for the tectonics and volcanism on Venus. However, it is also important to compare our understanding of these processes on Venus with the Earth, Moon, and Mars. One essential question concerning the behavior of these bodies is, How does the heat get out? Internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism. On the Earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells. The subduction of these cold plates at ocean trenches contributes about 75% of the mantle heat flux. It is now quite clear that active plate tectonics is not occurring on Venus. The surface of Venus is a single rigid plate, as is the case for the Moon, Mars, and Mercury. Prior to Magellan, the general consensus was that Venus was in a near‐uniformatarian state, with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet. However, crater studies from Magellan images provide direct evidence for a catastrophic or episodic evolution of Venus. One explanation is that Venus died tectonically 500 m.y. ago; plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased. An alternative explanation is that Venus experiences episodic global subduction events. Two major questions concerning the Earth remain largely unanswered, particularly when compared with Venus. (1) Why does the Earth have continents? (2) Why does the Earth have plate tectonics? A major difference between the Earth and Venus is the presence of liquid water (oceans) during much of the evolution of the Earth. It has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the Earth, and the absence of well‐defined continental masses on Venus supports this hypothesis. It has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates. Plate evolution in time results in geometrical incompatibilities that require overlaps or gaps. On the Earth these incompatibilities lead to broad zones of continental deformation, i.e., the western United States and China. The absence of plate tectonics on Venus suggests that rheologically soft continents are necessary for plate tectonics. A reasonable conclusion is that the Earth is the remarkable planet in terms of tectonics and volcanism, not Venus.