Venus

In terms of size and density Venus is the planet that is most similar to the Earth. An obvious working hypothesis would be that the tectonics of Venus would be similar to the tectonics of the Earth and there would be plate tectonics. We now know that this is not the case and that mantle convection on Venus has a different surface expression than mantle convection on the Earth. The cloud cover on Venus has prevented optical observations. However, Pioneer Venus radar, Earth-based radar observations, Venera 15–16 orbital imaging radar, and the Magellan radar images have provided clear views of the surface of Venus. These views, along with topography and gravity data, indicate that Earth and Venus are in fact quite different. On Earth the global oceanic rift system and the arcuate ocean trenches are the primary surface manifestations of plate tectonics. The almost total absence of these features on Venus has led to the conclusion that active plate tectonics is not occurring on that planet at this time. Clearly, any comprehensive understanding of tectonism and volcanism on Venus requires an understanding of how heat is transported in the absence of plate tectonics. There are other ways in which Venus is strikingly different. It rotates in a retrograde sense with a period of 243 days; a Venusian day is 117 Earth days.

Venus has a massive, mostly CO2 atmosphere with a surface temperature of about 750 K and a surface pressure of nearly 10 MPa. Sulfuric acid clouds 106 Plate Tectonics blanket the planet and prevent us from directly viewing the surface. Because of its earthlike size and mass, Venus most likely has a metallic core similar to Earth’s. However, magnetic field measurements by the Pioneer Venus orbiter during 1979 and 1980 revealed that Venus does not have an intrinsic global magnetic field; these observations determined that if Venus had an intrinsic magnetic field, its dipole moment would have to be much less than 1019 A

m2. Studies of the surface of Venus during the Magellan mission have provided a wealth of data on its tectonic and volcanic processes. The radar images of the surface are complemented by global topography and gravity data. The planet is remarkably smooth; 64% of the surface comprises a plains province with elevation differences of 2 km or less; highland areas stand as much as 10 km above the plains but they constitute only about 5% of the surface; lowlands are 2 to 3 km below the plains and occupy the remaining 31% of the surface. Figure 1–73 shows the distribution of these topographic provinces. Although local elevation extremes on Venus and Earth are roughly comparable, global topographic variations are much smaller on Venus; the planet’s surface is at a nearly uniform level. There are tectonic features on Venus that resemble major tectonic features on the Earth. Beta Regio has many of the features of a continental rift on Earth. It has a domal structure with a diameter of about 2000 km and as well amplitude of about 2 km. It has a well-defined central rift valley with a depth of 1–2 km and there is some evidence of a three-armed planform (aulacogen). It is dominated by two shieldlike features, Theia Mons and Rhea Mons, which rise about 4 km above the mean level. The U.S.S.R. Venera 9 and 10 spacecraft, which landed just east of Beta Regio, measured a basaltic composition and a density of 2800 kg m−3 for the surface rocks. These observations substantiate the identification of Theia Mons and Rhea Mons as shield volcanoes. Atla, Eistla, and Bell Regiones have rift zone characteristics similar to Beta Regio.