Venus

Venus was one of the last planets to be explored, despite its position as the closest planet to Earth. This is largely because it is perpetually shrouded in a uniformly bland covering of clouds. The cloud cover made looking at Venus through a telescope about as exciting as staring at a billiard ball. While Mars and the Moon were objects of much attention by early telescopic observation, the surface of Venus remained a mystery. It was even easier to say something about the outer planets, such as Jupiter and Saturn, than it was to make meaningful observations of Venus.

The absence of information about Venus was particularly ironic because Venus is the most like Earth in size and position within the solar system, thus suggesting that it could be more like Earth than any of the other planets. Venus's diameter is only 651 kilometers (404 miles) smaller than Earth's diameter of 12,755 kilometers (7,908 miles). Venus's density is 0.9 times that on Earth, and its surface gravity is 0.8 that of Earth. Venus orbits the Sun in just under one Earth year (224.7 days). When compared to Earth, all of the planets except Venus are much larger or smaller, higher or lower in density, located at much greater or lesser distances from the Sun, or enveloped in atmospheres much thinner or colder. Thus Venus was a cornerstone in scientists' survey of the solar system and offered the chance to see how an Earth-sized planet might have evolved similarly or differently. Planetary geologists now know that it is very different. This fact has revealed that the details of how a planet geologically evolves are probably as important in planetary evolution as differences in fundamental characteristics. Venus and Earth are truly twins separated at birth.

Atmospheric Characteristics

Because of the cloud cover, one of the first things that could be determined about Venus in the early days of planetary astronomy was the characteristics of the visible atmosphere. This was done first through telescopic measurements and early spacecraft flybys . In the nineteenth century, rare transits of Venus across the surface of the Sun were used to prove that Venus was enveloped in an atmosphere. This led to all sorts of early speculation that the clouds were, like clouds on Earth, water vapor clouds, and that the surface was a teeming primordial swamp filled with plants and animals similar to the Paleozoic coal swamps of Earth. This speculation withered under results of early spectroscopic observations, which were able to determine that the atmosphere was largely carbon dioxide, not oxygen and nitrogen as on Earth, and later on, that the clouds appeared to be sulfuric acid fog, not water vapor.

By the 1960s little was still known about Venus, but modern instruments were beginning to reveal more. Early surface temperature estimates were made by observing infrared wavelengths to better determine the temperature. Such observations, using radio telescopes and the first U.S. inter-planetary flyby spacecraft, Mariner 2, in 1962, implied that the surface temperatures were high. Over the next decade, several U.S. atmospheric entry probes (Pioneer-Venus 1 and 2) and Soviet landers (Venera 9, 10, 13, and 14) directly measured the temperature and pressure within the atmosphere. These measurements revealed a surface temperature of 450°C (842°F), or about as hot as the surface of a catalytic converter on an automobile. The surface pressure on Venus was found to be ninety-two times that of Earth (92 bars or 9.2 million pascals). This is equivalent to pressures at about 1 kilometer (0.6 mile) of depth in the sea, or about fifty times greater than a pressure cooker.

The atmosphere is so dense that pressures and temperatures similar to the surface of Earth occur at about 60 to 70 kilometers (37 to 43 miles) of altitude. Most of the atmosphere on Earth lies below 10 kilometers (6.2 miles), and it pretty much peters out before 30 kilometers (18.6 miles). On Venus, a daring future adventurer in a balloon with an oxygen mask (and protection from sulfuric acid clouds) could float in the upper atmosphere at an altitude of 60 to 70 kilometers (37 to 43 miles) in relative comfort. The global trip would be rapid since the atmosphere super-rotates, meaning that it flows from west to east faster than the underlying surface rotates. A balloon traveler in the upper atmosphere of Venus could circumnavigate the planet in only four days, especially near the equator where the speeds are greatest. This is unlike Earth, where the surface spins beneath a relatively sluggishly moving atmosphere that takes several weeks for a complete circuit. The balloon traveler's view would be boring, however, because the surface of Venus would be obscured by the main cloud layer, which occurs at 45 to 70 kilometers (28 to 43 miles) of altitude.

These conditions are the result of early development of a thick atmosphere consisting mostly of carbon dioxide (about 97 percent) through a so-called runaway greenhouse effect. First discovered from the study of Venus, the greenhouse effect is now discussed for Earth, where it is recognized that industrial additions of carbon dioxide to the atmosphere pose potential environmental problems of similar global magnitude.

Surface Features and Geologic Findings

Although the surface of Venus has been seen locally around a few Russian landers with optical cameras, a true picture of the global surface was obtained only with the advent of radar that could penetrate the dense obscuring clouds and create radar images. Early results were obtained through Earth-based radio telescopes at Goldstone (in California) and Arecibo (in Puerto Rico), which emitted tight beams of radar and built up images showing differences in the radar reflecting properties of the surfaces. These images were low in resolution, but they enabled large areas of unusually radar-reflective terrain to be detected. These also allowed the first estimates of the rotation to be made, and they showed that Venus rotates backwards, or west to east, and slowly. It takes about 243 days to do so. Oddly, this is a little longer than its year (224.7 days). Stranger still, at closest approach to Earth (a distance of just under 40 million kilometers [24.8 million miles]), or opposition, Venus presents the same hemisphere to Earth. The origin of this unusual set of rotation conditions is not known.

The first truly global maps of Venus was made by the Pioneer-Venus orbiter using radar altimetry . This showed the surface elevations over the globe resolved at scales of about 100 kilometers (62 miles) and revealed a relatively flat surface, with the absolute range of elevations much less than that found on Earth. More than 80 percent of the surface area lies within a kilometer of the mean planetary radius (6,051.8 kilometers [3,752.1 miles]). A few highland regions rise from one to several kilometers above the mean planetary radius, but these cover only about 15 percent of the surface of Venus. Whereas Earth has two common elevations, seafloors and continents, Venus has one most common elevation, broad plains.

Radar images of the surface, somewhat similar to photographs, were made later for large areas of the globe by the Russian Venera 15 and 16 orbiters, for the northern quarter of Venus, and, several years later, by the U.S. Magellan orbiter, for about 98 percent of the surface. These efforts obtained images of the surface at scales of 2 kilometers (1.2 miles) and 0.3 kilometers (0.2 miles), respectively, thus generating the first true images of what the surface of Venus really looks like and permitting the first geologic analysis.

The radar images show that the surface of Venus is a complex of plains, mountains, faults , ridges, rift valleys , volcanoes, and a few impact craters , a surface more complex and geologically modified than any of the other planets seen previously. The highland regions seen first in low-resolution Earth-based radar images and Pioneer-Venus radar altimetry are among the most complex surfaces and consist of a terrain that is complexly faulted in orthogonal patterns. These regions are known as tessera after the Greek word for mosaics of tiles. The sequence of geologic surfaces suggests that tesserae (plural of "tessera") also represent the oldest preserved surfaces on Venus. One of the highlands is surrounded by ridgelike mountain belts that rise from 6 to 11 kilometers (3.7 to 6.8 miles) above the mean planetary radius and appear to have formed from compression and buckling of the surface, similar to mountain belts on Earth. Low ridges of possibly similar origin, in a range of sizes, occur singly or in belts throughout the plains areas.

Faults, fractures , and immense rift valleys are present in abundance. One rift valley, Diana Chasma, is similar in size to the great East African rift valley and Rio Grande rift valley of Earth. Like those on Earth, it probably formed from the stretching and pulling apart of the surface. On Earth, erosion and sedimentation quickly obscure all but the latest structures associated with such rifts. But on Venus the absence of erosion means that all of the structural details are perfectly preserved as a complex mass of faults.

Large volcanoes up to several hundreds of kilometers across but only a few kilometers high are common, as are long lava flow fields, extensive lowlying regions of lava plains, and lava channels. One lava channel is longer than the largest rivers on Earth. Low-relief domical volcanoes, many less than several kilometers in diameter, are globally abundant, numbering in the hundreds of thousands. Some volcanoes appear similar to those formed from eruption of thick, viscous lavas on Earth. Additional volcanic features include calderas similar to those on Earth, although generally much larger; complex topographic annular spider-and-web-shaped features known as arachnoids; and circular structural patterns up to several hundred kilometers across with associated volcanism known as coronae. Many of these are generally thought to represent local formation of large and deep magma reservoirs. Radial patterns of fractures associated with volcanoes are common and may represent the surface deformation associated with radial-dike-like magma intrusions.

Impact craters are about as numerous on Venus as they are on continental areas of Earth, and are thus not as common as they are on most other planets. Only about 900 have been identified on Venus. Meteors smaller than a certain size disintegrate on entering the atmosphere. As a result, impact craters smaller than 2 kilometers (1.2 miles) are infrequent. Morphologically, craters on Venus resemble those on other planets with several exceptions related to the interaction of the crater ejecta with the dense atmosphere. These include extensive parabola-shaped halos much like fallout from plumes associated with volcanic eruptions on Earth. These open to the west and possibly record the interaction of the upward expanding cloud of crater ejecta with the strong global easterly winds. Many craters are characterized by large lava-flow-like features that may represent molten ejecta flowing outward from the crater after the impact.

Impact craters also appear nearly uniformly distributed, unlike most planets where large areas of different crater abundance indicate variations in age of large areas of their surfaces. Based on estimates of their rates of formation on surfaces in the inner solar system, impact crater statistics indicate an average surface age on Venus of about 500 million years. Either most of the surface was formed over 500 million years ago in a catastrophic resurfacing event and volcanism has been much reduced since that time, or continual, widespread, and evenly spaced volcanism and tectonism remove craters with a rate that yields an average lifetime of the surface of 500 million years. The rate of volcanism on Venus is estimated to be less than 1 cubic kilometer per year, somewhat less than the 20 cubic kilometers associated largely with seafloor spreading on Earth. The surface of Venus appears to be dominated by volcanic hot spots rather than spreading and subduction associated with plate tectonics.

Another spacecraft observation method allowed something to be determined about the interior of Venus. By carefully tracking spacecraft orbits, variations in gravitational acceleration associated with differences in mass on and beneath the surface can be detected. On Venus, this technique reveals that the strength of gravity is mostly proportional to the surface topography, in contrast to Earth, where mass associated with topography is generally compensated underneath by lower density roots. This means that many large topographic features on Venus are supported either by strong lithosphere without a low-density root, or by topography originating from the dynamical uplift of the surface through convective processes in the deep interior. If the first type is assumed, it may indicate that the lithosphere is strong and that a low-strength layer at the base of the lithosphere (called the asthenosphere on Earth) is not present. The second type may be attributed to upwelling associated with volcanic hot spots.

Several Venera landers of the Russian space program returned both optical images of the surface and chemical information about the rocks at several sites. Early landers had searchlights in case the cloud cover made it too dark to see anything. Despite the dense cloud cover, enough light gets through that the surface is illuminated to the equivalent of a cloudy day on Earth. But the sky as seen from the surface is probably a bland fluorescent yellow-white, rather than mottled gray. The relatively rocky surroundings appeared to be volcanic lava flow surfaces or associated rubble. The measured chemical compositions are indistinguishable for the most part from tholeiitic and alkali basalts typical of ocean basins and hot spots on Earth.

The low number of impact craters scattered over the surface implies that only the last 20 percent of the history of Venus appears to be preserved, and little is known about the earlier surface geologic history. The geological complexity and young surface ages of both Venus and Earth relative to smaller terrestrial planets can be attributed to their larger sizes and correspondingly warmer and more mobile interiors, extensive surface deformation (tectonism), and mantle melts (volcanism) over a greater period of geological time.

see also Exploration Programs (volume 2); Government Space Programs (volume 2); NASA (volume 3); Robotic Exploration of Space (volume 2); Planetary Exploration, Future of (volume 2).

Larry S. Crumpler

Bibliography

Cattermole, Peter. Venus: The Geological Story. Baltimore, MD: Johns Hopkins University Press, 1994.

Cooper, Henry S. F., Jr. The Evening Star. New York: Harper Collins, 1993.

"Magellan at Venus" (special issues on results of Magellan mission to Venus). Journal of Geophysical Research 97, no. E8 (1992):13,063-13,675; 97, no. E10 (1992): 15,921-16,382.