The Venus Puzzle: Tectonics And Terrestrial Planets

David H. Kime

Venus is Earth's closest relative in the solar system. Rough similarities between Venus and Earth suggest that the two planets not only have a comparable heat content but also should rid their heat with similar methods. However, both older data and the recent Magellan radar images of the surface of Venus show featuers that are unique to Venus and are completely foreign to Earth. This incongruity has puzzled scientists for years. By utilizing what is known about Earth's mantle and data available from the Magellan spacecraft, it is possible to present one solution to the Venus puzzle. Recent computer models have shown that Earth's mantle behaves in a convective manner that consists of cylindrical upwellings and sheet-like downwellings. On Earth these take the form of hot spots and subduction arcs. These models are only dependent on those characteristics that Earth has in common with Venus. Magellan images of the surface of Venus have shown volcanism similar to hot spots and large arc featurs that may be caused by downwelling sheets. Hence, it is the conclusion that the mantles of the two planets behave in very similar manners, and that the difference in tectonic styles is caused solely by surface conditions. These conditions are related to the existence or absence of water on the planet. This characteristic may have been controlled by something as simple and basic as a planet's distance from the sun. Working in combination with this factor is the existence or non-existence of life. These environmental conditions are as important to the development of tectonics on a terrestrial planet as a planet's mass and composition.