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6.10 Tectonics on Venus

Venus is dotted with impact craters, but they are rare, indicating that the surface is relatively young. In addition, the thick atmosphere stops small asteroids from hitting the surface, so there are no small craters. Erosion by water or wind is insignificant; there is no liquid water on Venus, and although the atmosphere is extremely dense, wind speeds are very slow near the surface. This leaves the related processes of volcanism and tectonism to shape the surface of Venus.

Venus as seen by the Hubble Space Telescope (false color image). Click here for original source URL.

Radar imagery of Venus revealed abundant tectonic features, some of which had never been seen before, and some of which are still not understood. For example, coronae are large round or oval mounds, hundreds of kilometers across, which are surrounded by trenches and concentric cracks. They are believed to be formed when an upwelling mantle plume pushes the lithosphere up in one spot. Venus also has areas of parallel ridges and troughs where the crust has been pulled apart. While scientists have identified other mysterious Venusian features such as “arachnoids” (Greek for spider), “anemones” and “ticks,” they still don’t know for certain how these distinctive patterns formed.

What caused all this deformation of Venus’s crust? The number and variety of tectonic features indicates that the surface has experienced a great deal of stress. However, there’s no evidence for the type of large-scale plate movement that shapes our planet. Volcanoes on Venus are not concentrated along plate boundaries, they are scattered randomly over the planet. The highlands regions of Venus may appear similar to Earth’s continents, but there is no evidence they were formed in the same way. Earth’s plate tectonics seem to be unique in our solar system.

Scientists think that Venus’s lack of water may be crucial in explaining the absence of plate tectonics. On Earth, water plays a large part in “softening” mantle rocks, making them flexible enough to flow. But Venus is so dry that rocks are much more brittle, thwarting plate tectonics. Also, gravity data collected by orbiting probes suggest that Venus may have a thicker lithosphere than the Earth. This would also impede plate tectonics by making it more difficult for the lithosphere to break into separate plates.

If plate tectonics don’t occur on Venus, some other process must be creating all the surface features we see. One model has large plumes of material rising in the mantle and creating stress on the crust. It’s also possible that plate tectonics has occurred in the past on Venus, but the resurfacing event that occurred 500 to 800 million years ago erased all evidence of it. Studies of the Venusian surface and interior may eventually shed more light on the structure and history of both Venus and the Earth. The explanation for the different styles of tectonics and continent growth on the two planets remains to be discovered.

We have been stressing some of the differences between Venus and the Earth, but apart from Venus being closer to the Sun they are very similar in mass, and mass dictates of lot of planet properties. If intense episodes of volcanism pumped lots of carbon dioxide into the atmosphere and led to a runaway greenhouse effect on Venus, it"s interesting to rewind the history. The best models suggest that 3 billion years ago, Venus may well have been a temperate place, with oceans and rivers — a habitable planet! In fact, a total inventory of carbon dioxide on Venus and the Earth finds similar numbers. On slightly warmer Venus, the carbon dioxide stayed in the atmosphere and was added to by volcanism. On the slightly cooler Earth, the carbon dioxide dissolved in seawater, became trapped in carbonate rocks, and the small amount that remained in the atmosphere was not enough to cause a big greenhouse effect. This is an important lesson in planetary properties: we see them at an instant in time, but planets can change dramatically!

Composite radar map of Venus from Magellan. Click here for original source URL.