Astronomy

In size and mass, Venus is a near twin to Earth, but in other characteristics, it shows significant differences. Compared to the other terrestrial planets, Venus' atmosphere is extremely dense, with a surface pressure 90 times that of Earth, equivalent to 1,300 pounds per square inch. Chemically, it is 96 percent carbon dioxide (CO ₂) and slightly less than 4 percent molecular nitrogen (N ₂), with the remainders being argon (Ar) and water vapor (H ₂O), and trace amounts of sulfuric acid (H ₂SO ₄), hydrochloric acid (HCl), and hydrofluoric acid (HF). The enshrouding clouds, which historically prevented Earth‐based study of the planet, are most likely droplets of sulfuric acid (H ₂SO ₄), a product of the planet's active volcanism. The planet's temperature at the cloud level is 240 K (–35°F) as expected for its high albedo (reflectivity) of 0.76; but at the surface, the temperatures are far higher—750 K (890°F) on the daytime side and 550 K (500°F) on the night side. The planet's dense carbon dioxide atmosphere is responsible for an extreme greenhouse effect. The large temperature difference between the planet's day and night sides produces extremely high winds in the upper atmosphere—300 km/hr, which can circle the planet in only four days. (In contrast, on Earth, the solar heat is fairly uniformly spread about planet.) (See Figure 1)

Figure 1

Temperature in Venus' atmosphere.

That Venus is so close to Earth in size and mass makes the difference in atmosphere even more striking. Why should they be so different in this respect? Actually the atmosphere of Venus contains the quantity of carbon dioxide (CO ₂) expected from volcanic processing ( volcanic outgassing) of the rocky material that makes up that body. It is therefore Earth that has the odd atmosphere due to the existence of the oceans and also due to life on the planet. The oceans dissolve CO ₂ and, augmented by processes of living organisms, precipitate this out of a solution in the form of solid limestone. If all CO ₂ were released from the ocean and rocks, the terrestrial atmosphere would be like that of Venus.

The surface of Venus can only be mapped by radar techniques, with low resolution from Earth and with higher resolution (1 km) achieved by the Magellan satellite that was placed in orbit about the planet. This has revealed the existence of highland (“continental”) and lowland (basin) regions; but the elevation contrast is relatively shallow compared to Earth and better described as a rolling terrain. Meteoric impact craters, volcanic caldera, and a number of other surface features quite unlike those known on Earth are found in Venus' topography. For example, coronae, circular bulges marked by radial and concentric stress fractures, appear as the result of the movement of subsurface molten materials. Other regions show patterns of parallel ridges suggesting some compression of these areas.

Active vulcanism was confirmed by the short‐lived Russian Venera spacecraft that landed on the planet and photographed a surface strewn with relatively young angular rocks and boulders of a basaltic composition. Overall, the surface appears generally to be between 300 and 500 million years old. The surface is too hot for oceans to exist, and in its atmosphere the water would be quickly disassociated due to the subsequent loss of hydrogen. The isotopic abundance ratio of the remaining deuterium (heavy hydrogen) and hydrogen, however, shows that Venus originally had some water, but not enough to have formed extensive oceans. Without oceans, there was no dissolution of atmospheric carbon dioxide (CO ₂) and no precipitation out in the form of solid limestone. As the surface temperatures rose, the extreme heating of the surface rocks released even more CO ₂ and the greenhouse temperature increase grew even higher.

Also by comparison with Earth, Venus should still be actively cooling and the features of active plate tectonics phenomena should be found on its surface, but they are not. There are no extended mountain chains equivalent to Earth's mid‐oceanic ridges, no large scale faults associated with sea floor spreading, no subduction troughs, and no marginal mountains caused by crustal compression at the collision of continental plates. The mantle of Venus may once have been convecting, but now it and the crust appear to be frozen solid. The high surface temperature has two consequences for the dynamics of the planet's mantle. First, the temperature difference between core and surface is smaller than on Earth, hence there is a smaller thermal driving force to produce convection. Second, the high outer temperature “cooked” the mantle, driving out water that otherwise would be chemically bonded to silicate materials. Without the water, the mantle is solid, not plastic (in the same way that silly putty placed in an oven will lose its elastic properties).

The rotation of the planet is unusual, being the slowest (244 days) of any planet and opposite in direction (that is, retrograde) to the rotations of most solar system objects. This odd rotation is likely the result of an early impact by a large planetesimal during the planet's formative period; or perhaps Venus was the final result of the merger of two large planetesimals. The length of Venus' solar day, consequently, is 117 Earth days, hence the strong differential heating between its day and night sides. See Table 1 for Venus's physical and orbital data.