Venus
Rotation and Orbital Motion The interior orbit of Venus means that it never strays far from the Sun in the sky. Because of its highly reflective cloud cover, Venus is brighter than any star in the sky, as seen from Earth. It is so bright that it can be seen even in the daytime. Radar observers announced that the Doppler broadening of their returned echoes implied a sluggish 243-day rotation period. Furthermore, Venus s spin was found to be retrograde that is, in a sense opposite that of Earth and most other Solar System objects, and opposite that of Venus s orbital motion.the planet s rotation is slow and retrograde, most likely because of a collision between Venus and some other Solar System body during the late stages of the planet s formation. Because Venus, of all the other planets, most nearly matches Earth in size, mass, and density, and because its orbit is closest to us, it is often called Earth s sister planet. But unlike Earth, Venus has a dense atmosphere and thick clouds that are opaque to visible radiation, making its surface completely invisible from the outside at optical wavelengths. Surface Features and Geological Activity Venus s surface has been thoroughly mapped by radar from Earth-based radio telescopes and orbiting satellites. The most recent and most thorough survey has been carried out by the U.S. Magellan satellite. The planet s surface is mostly smooth, resembling rolling plains with modest highlands and lowlands. Two elevated continent-sized regions are called Ishtar Terra and Aphrodite Terra. There is no evidence for plate tectonic activity as on Earth. Many lava domes and shield volcanoes have been found by Magellan on Venus s surface, but none of the volcanoes has yet been proven to be currently active. The planet s surface shows no sign of plate tectonics. Features called coronae are thought to have been caused by an upwelling of mantle material that, for unknown reasons, never developed into full convective motion. The surface of the planet appears to be relatively young, resurfaced by volcanism every few hundred million years. Some craters on Venus are due to meteoritic impact, but the majority are volcanic in origin. The evidence for currently active volcanoes on Venus includes surface features resembling those produced in Earthly volcanism, fluctuating levels of sulfur dioxide in Venus s atmosphere, and bursts of radio energy similar to those produced by lightning discharges that often occur in the plumes of erupting volcanoes on Earth. However, no actual eruptions have been seen.
(a) This image of the surface of Venus was made by a radar transmitter and receiver on board the Pioneer spacecraft, which is still in orbit about the planet but now inoperative. The two continentsized landmasses are named Ishtar Terra (upper left) and Aphrodite (lower right). Colors represent altitude: blue is lowest, red highest. The spatial resolution is about 25 km. (b) A planetwide mosaic of Magellan images, colored in roughly the same way as part (a). Aphrodite Terra is at the center of this image. (NASA) Soviet spacecraft that landed on Venus photographed surface rocks with sharp edges and a slablike character. Some rocks on Venus appear predominantly basaltic in nature, implying a volcanic past. Other rocks resemble terrestrial granite and are probably part of the planet s ancient crust. Many areas of Venus have extensive volcanic features. Figure 12 shows a series of seven pancakeshaped lava domes, each about 25 km across. They probably formed when lava oozed out of the surface, formed the dome, and then withdrew, leaving the crust to crack and subside. Lava domes such as these are found in numerous locations on Venus. (a)
(b) (a) These dome-shaped structures resulted when viscous molten rock bulged out of the ground and then retreated, leaving behind a thin solid crust that subsequently cracked and subsided. Magellan has found features like this in several locations on Venus. (b) A three-dimensional representation of four of the domes. The computer view is looking toward the right from near the center of the image in part (a). Color is based on data returned by Soviet Venera landers. (NASA) Most volcanoes on the planet are of the type known as shield volcanoes. Shield volcanoes, such as the Hawaiian Islands on Earth, are not associated with plate boundaries. Instead, they form when lava wells up through a hot spot in the crust, and are built up over long periods of time by successive eruptions and lava flows. A characteristic of shield volcanoes is the formation of a caldera, or crater, at the summit when the underlying lava withdraws and the surface collapses. The distribution of volcanoes over the surface of Venus appears random quite different from the distribution on Earth, where volcanic activity clearly traces out plate boundaries strongly supporting the idea that plate tectonics is absent on Venus. Two pieces of indirect evidence suggest that volcanism on Venus continues today. First, the level of sulfur dioxide above Venus s clouds shows large and fairly frequent fluctuations. It is quite possible that these variations result from volcanic eruptions on the surface. If so, volcanism may be the primary cause of Venus s thick cloud cover. Second, both the Pioneer Venus and the Venera orbiters observed bursts of radio energy from Aphrodite and other regions of the planet s surface. These bursts are similar to those produced by lightning discharges that often occur in the plumes of erupting volcanoes on Earth, again suggesting ongoing activity. However, while these pieces of evidence are quite persuasive, they are still only circumstantial. No smoking gun (or erupting volcano) has yet been seen, so the case for active volcanism is not yet complete. Not all the craters on Venus are volcanic in origin. Some craters on Venus, like Cleopatra, were formed by meteoritic impact. Large impact craters on Venus are generally circular, but those less than about 15 km in diameter can be quite asymmetric in appearance. Venus s atmosphere is sufficiently thick that small meteoroids do not reach the ground, so there are no impact craters smaller than about 3 km across. Atmospheric effects probably also account for the observed scarcity of impact craters less than 25 km in diameter. Overall, the rate of formation of largediameter craters on Venus s surface seems to be only about one-tenth that in the lunar maria. Applying the same crater-age estimates to Venus as we do to Earth and the Moon suggests that much of the surface of Venus is quite young less than a billion years old. Although erosion by the planet s atmosphere may play some part in obliterating surface features, the main agent is volcanism, which appears to have resurfaced much of the planet about 500 hundred million years ago.
Atmosphere The extremely thick atmosphere of Venus is nearly opaque to visible radiation, making the planet s surface invisible from the outside. Spectroscopic examination of sunlight reflected from the planet s cloud tops shows the presence of large amounts of carbon dioxide. Venus s atmosphere is nearly 100 times denser than Earth s. The temperature of the upper atmosphere is much like that of Earth s upper atmosphere, but the surface temperature is 730 K. Venus is comparable in both mass and radius to Earth, suggesting that the two planets started off with fairly similar surface conditions. However, the atmospheres of Earth and Venus are now very different. The total mass of Venus s atmosphere is about 90 times greater than Earth s. The greenhouse effect stemming from the large amount of carbon dioxide in Venus s atmosphere is the basic cause of the planet s current high temperatures. Almost all the water vapor and carbon dioxide initially present in Earth s early atmosphere quickly became part of the oceans or surface rocks. Because Venus orbits closer to the Sun than does Earth, surface temperatures were initially higher, and the planet s greenhouse gases never left the atmosphere. On Venus, the runaway greenhouse effect caused all the planet s greenhouse gases carbon dioxide and water vapor to end up in the atmosphere, leading to the extreme conditions we observe today. The greenhouse effect on Venus was even more extreme in the past, when the atmosphere also contained water vapor, another greenhouse gas. By intensifying the blanketing effect of the carbon dioxide, the water vapor helped the surface of Venus reach temperatures perhaps twice as hot as at present. At those high temperatures, the water vapor was able to rise high into the planet s upper atmosphere so high that it was broken up by solar ultraviolet radiation into its components, hydrogen and oxygen. The light hydrogen rapidly escaped, the reactive oxygen quickly combined with other atmospheric gases, and all water on Venus was lost forever. To understand the runaway greenhouse effect, imagine that we took Earth from its present orbit and placed it in Venus s orbit, some 30 percent closer to the Sun. At that distance from the Sun, the amount of sunlight striking Earth s surface would be about twice its present level, so the planet would warm up. More water would evaporate from the oceans, leading to an increase in atmospheric water vapor. At the same time, the ability of both the oceans and surface rocks to hold carbon dioxide would diminish, allowing more carbon dioxide to enter the atmosphere. As a result, the greenhouse heating would increase, and the planet would warm still further, leading to a further increase in atmospheric greenhouse gases, and so on. Once started, the process would run away, eventually leading to the complete evaporation of the oceans, restoring all the original greenhouse gases to the atmosphere. Although the details are quite complex, basically the same thing would have happened on Venus long ago, ultimately leading to the planetary inferno we see today.
The atmosphere of Venus is about 90 times more massive than Earth s, and it extends to a much greater height above the surface. On Earth, 90 percent of the atmosphere lies within about 10 km of sea level. On Venus the corresponding (90 percent) level is found at an altitude of 50 km instead. The surface temperature and pressure of Venus s atmosphere are much greater than Earth s. However, the temperature drops more rapidly with altitude, and the upper atmosphere of Venus is actually colder than our own. Venus s troposphere extends up to an altitude of nearly 100 km. The reflective clouds that block our view of the surface lie between 50 and 70 km above the surface. Data from the Pioneer Venus multiprobe indicate that the clouds may actually be separated into three distinct layers within that altitude range. Below the clouds, extending down to an altitude of some 30 km, is a layer of haze. Below 30 km, the air is clear. Above the clouds, a high-speed jet stream blows from west to east at about 300 400 km/h, fastest at the equator and slowest at the poles. This high-altitude flow is responsible for the rapidly moving cloud patterns seen in ultraviolet light. Infrared observations carried out in the 1970s showed that the clouds (or at least the top layer of clouds) are actually composed of sulfuric acid, created by reactions between water and sulfur dioxide. Sulfur dioxide is an excellent absorber of ultraviolet radiation and could be responsible for many of the cloud patterns seen in ultraviolet light. Spacecraft observations confirmed the presence of these compounds in the atmosphere. They also indicated that there may be particles of sulfur suspended in and near the cloud layers, which may account for Venus s characteristic yellowish hue. Carbon dioxide is the dominant component of the atmosphere, accounting for 96.5 percent of it by volume. Almost all of the remaining 3.5 percent is nitrogen. Trace amounts of other gases, such as water vapor, carbon monoxide, sulfur dioxide, and argon, are also present. However, there is no sign of the water vapor that we might expect to find if a volume of water equivalent to Earth s oceans had evaporated. If Venus started off with Earthlike composition, something has happened to its water it is now a very dry planet. Magnetic Field Venus has no detectable magnetic field, almost certainly because the planet s rotation is too slow for any appreciable dynamo effect to occur. To some planetary geologists, Venus s interior structure suggests that of the young Earth, before convection became established in the mantle. Having no magnetosphere, Venus has no protection from the solar wind. Its upper atmosphere is continually bombarded by high-energy particles from the Sun, keeping the topmost layers permanently ionized. However, the great thickness of the atmosphere prevents any of these particles from reaching the surface.