Venus

Basic properties

The rotation rate of Venus

Venusian surface detail

Venusian surface processes

Venusian internal structure

The venusian atmosphere

The greenhouse effect

Impact craters on venus

Venus geologic history

Current and future missions to Venus

Resources

Venus is the second planet from the sun and is in some superficial geological features like Earth, which is why it is sometimes called Earth’s sister planet. In many important geochemical features, however, it is quite different. Next to the sun and the moon, Venus can be the brightest object in the sky. At its most brilliant, Venus is sixteen times brighter than Sirius (Canis Majoris), the brightest star in the night sky. The extreme brilliance of Venus is partly due to its occasional closeness to Earth, and partly due to it having a highly reflective atmosphere. After the moon, Venus is the brightest object in the night sky, with an apparent magnitude of about -4.6. It is named after the Roman goddess of love (Venus), and this pattern of using mythological women has continued when naming most of its surface features.

Basic properties

Venus orbits the sun on a near circular orbit at mean distance of 0.723 astronomical units (AU). At aphelion the planet is at a maximum distance of 0.728 AU from the sun, while at perihelion Venus it is 0.718 AU away. Coming as near as 0.272 AU, no other planet approaches Earth as close as Venus does.

Since it circles the sun in an orbit smaller than that of Earth’s, Venus is never very far away from the sun for terrestrial viewers (which is why it is considered an inferior planet). The greatest angular distance between the sun and Venus, as seen from Earth, is 47 degrees. This means that even under the most favorable of conditions Venus will set at most three hours after the sun, or rise no earlier than three hours before the sun. Observed since the most ancient of times, Venus is often called the morning star, if it rises before dawn or the evening star if it sets after the sun. The Greek philosopher Homer referred to Venus in his Illiad as “the most beautiful star set in the sky.”

The time required for Venus to complete one orbit about the sun, its sidereal period, is 224.701 Earth-days, whereas the time for Venus to repeat alignments with respect to the sun and Earth is 584 Earth-days. The best times for viewing Venus are when it is near greatest eastern, or greatest western elongation. Greatest western elongation follows about five months after greatest eastern elongation, and greatest eastern elongation’s repeat about every 19 months.

As it circles the sun, Venus shows phases just like the moon, and the planet Mercury. The phase changes of Venus were first recorded by Italian astronomer and physicist Galileo Galilei (1564–1642) in 1610. Galileo’s observations of Venusian phase changes were important since they strengthened his belief in the heliocentric model of the solar system, which had been proposed by Copernicus in his now famous book, De Revolutionibus, published in 1543. Galileo reasoned that if Venus circled Earth, as the then accepted geocentric model of the universe decreed, it would not show the full range of phases that were observed. Likewise, the correlation between angular size and phase would also be different to that observed if Venus orbited Earth.

Transits of Venus across the disk of the sun, as seen from Earth, are not common. If a transit is to occur, however, it will take place in either June or December, when Earth is at the line along which Venus’s orbit cuts the ecliptic—the line of nodes for Venus. A very precise geometrical alignment of Earth, Venus, and the sun is required for a Venusian transit to take place. The next-to-last Venusian transit occurred on December 6, 1882, and the last transit took place on June 8, 2004. The next transit of Venus is June 5, 2012. In general, Venusian transits are seen in pairs. Each of the transits in a pair are eight years apart, and the pair-cycle repeats on an alternating cycle of 121.5 and 105.5 years.

The rotation rate of Venus

The Venusian atmosphere is both optically thick and highly reflective. The upper cloud deck has an albedo of 0.76, meaning that it reflects 76% of the sunlight that falls on it. In addition, the low-lying Venusian clouds are so dense that they completely obscure any optical view of the planet’s surface. Not being able to monitor variations in surface detail has meant that astronomers have only recently discovered the true rotation rate of Venus.

While the Venusian atmosphere presents an impenetrable shroud against optical observations, it is transparent to radio and microwave radiation. Using the giant radio telescope at the Arecibo Observatory in Puerto Rico, astronomers were able to bounce microwave signals off the surface of Venus. By analyzing the Doppler shift in the returned signals, the astronomers were then able to determine the planet’s rotation rate. The results were a complete surprise.

The microwave measurement of Venus’s rotation rate showed that the planet was spinning on its axis in the opposite sense to which it orbits the sun. If one could stand on the surface of Venus, sunrise would be in the west and sunset would be in the east, the exact opposite to that seen on Earth. When viewed from above, all the planets in the solar system orbit the sun in a counter clockwise direction. Most of the planets also rotate about their spin axes in a counter clockwise sense. The only exceptions to this rule are the planets Venus, Uranus, and dwarf planet Pluto. When a planet spins on its axis in the opposite sense to its orbital motion, the rotation is said to be retrograde.

Venus takes 243.01 days to spin once on its axis. With this slow rotation rate it actually takes 18.3 days longer for Venus to spin once on its axis than it takes for the planet to orbit the sun. The Venusian day, that is the time from one noon to the next, is 116.8 terrestrial days long. A curious relationship exists between the length of the Venusian day and the planet’s syndic period. The syndic period of Venus, that is, the time for the planet to repeat the same alignment with respect to Earth and the sun, is 584 days, and this is five times the Venusian day (584 = 5 × 116.8). It is not known if this result is just a coincidence, or the action of some subtle orbital interaction. The practical consequence of the relationship is that, should a terrestrial observer make two observations of Venus that are 584 days apart, then they will see the same side of the planet turned towards Earth.

Venusian surface detail

At its closest approach, the planet Venus can have an angular diameter just slightly larger than 1/60th of a degree. This angular size translates to a physical diameter of 7,520 mi (12,104 km), making the planet

about 95% the size of Earth. Given the near similar sizes of Earth and Venus, it might be expected that the same geological processes that operate on Earth have shaped the surface of Venus. This expectation is only partly true.

While radar maps of the surface of Venus were constructed during the 1970s, the most detailed topographic maps obtained to date are those from the Magellan spacecraft mission. The Magellan spacecraft was placed into Venusian orbit by NASA in 1990, and a powerful on-board imaging radar system was used to map the entire Venusian surface to a resolution of a few hundred meters.

The Magellan radar data showed that Venus is remarkably flat, and that some 80% of the planet’s surface is covered by smooth volcanic plains, the result of many lava out-flows. The altitude map constructed from Magellan data has revealed the existence of two large continentlike features on Venus. These features are known as Ishtar Terra (named after the Babylonian Goddess of love), and Aphrodite Terra (named after the Greek Goddess of love). Ishtar terra, which measures some 621 mi by 931 mi (1,000 by 1,500 km), lies in Venus’s northern hemisphere, and has the form of a high plateau ringed with mountains. The largest mountain in the region, Maxwell Montes, rises to a height of 7 mi (11 km). Aphrodite Terra is situated just to the south of the Venusian equator and is some 9,936 mi long by 1,242 mi (16,000 km by 2,000 km) wide. It is a region dominated by mountainous highlands and several large volcanoes.

Venus has three major terrains, based mainly upon elevation (above mean planet radius). The lowlands are rolling volcanic plains (about 60% of the planet) with under 1,600 ft (500 m) of relief. The uplands are intermediate in elevation and represent a transition between lowlands and highlands. Elevations range from 0 to 1.2 mi (0 to 2 km) in the uplands. The highlands have up to 3 mi (5 km) relief and are quite mountainous in some places. The highlands have compression ridges and fractured rocks and comprise about 15% of the planet’s surface.

Given the similarity in size of Venus and Earth, geologists had speculated on the possibility that plate tectonics, which is the primary agent for re-shaping Earth’s surface, might operate on Venus. The Magellan probe found no evidence, however, for large-scale tectonic activity on Venus. The reasons underlying the absence of any large-scale tectonic activity on Venus are presently unclear, but it may be indicative that the planet has a thinner and weaker lithosphere than Earth. The ridges and folds that cover many of the plain regions of Venus is indicative, however, of a certain amount of local tectonic activity on the planet.

The Magellan maps revealed many large craters on the Venusian surface. There is an apparent cut-off for craters with diameters less than a few kilometers. This is a selection effect imposed by the dense Venusian atmosphere. The atmosphere is in fact such a good filter of incoming meteoroids, that only those objects larger than a few kilometers in diameter survive their passage through the atmosphere, with sufficient mass, to produce a crater at the Venusian surface.

The relative age of different regions on a planet’s surface can be gauged by counting the number of craters that appear per unit area. This method of crater counting has proved very useful for dating the various regions on our moon. The essential idea being exploited in crater count dating is that, assuming the cratering rate is the same over the whole planet, if one region has fewer craters per unit area than another, it implies that some resurfacing, e.g., by a lava flow, has taken place in the region with fewer craters. The re-surfacing has in effect erased the older craters and reset the cratering clock.

The number of craters in the Venusian plains is typically about 15% of the crater counts for the lunar maria. This observation indicates that the Venusian plains have an age roughly equivalent to 15% the age of the lunar maria. From the lunar rock samples that were returned from NASA’s Apollo Moon landing missions, scientists know that the lunar maria are about 3.2 billion years old, and consequently the likely age of the Venusian plains is about 500 million years.

The main agent for re-surfacing the Venusian plains is believed to be aperiodic but widespread volcanism. Certainly, many (apparently) extinct volcano’s were mapped by Magellan during its five-year survey. Some lava flow regions observed by Magellan are believed to be no more than ten millions years old, and they may be much younger.

Some of the more remarkable surface features discovered by Magellan were the pancake-shaped volcanoes. These flat-topped, circular volcanoes are unique to Venus, and it is thought that they are probably formed through the surface extrusion of a very thick and viscous lava.

In recognition of Venus being named in honor of the Greek Goddess Aphrodites, whom the Romans called Venus, the International Astronomical Union (IAU, which officially names celestial bodies) assigns only female names to the planet’s surface features. Craters, for example, can be named after any famous women; linear features are named after Goddesses of War, while plains are named in honor of mythological heroines.

Venusian surface processes

Processes affecting Venus include impact cratering, lava flows, solid-state creep (viscous flow of rocks at the surface due to high temperatures and pressures), and eolian (wind) effects. The latter include deflation (blowing away fine particles), wind erosion (forming yardangs), and wind deposition (formation of streaks, transverse dunes, and wind-shadow dunes). Weathering on the surface is due to interaction between carbon dioxide and sulfur dioxide at high temperature with silicate rocks. At elevations above about 2.2 mi (3.5 km) above mean planet radius, weathering seems to favor formation of iron-sulfur compounds and below that level, iron-oxide and calcium-sulfate compounds, tend to form. The latter was inferred from the nature of the reflected radar signal off surficial materials.

Venusian internal structure

Venus has no natural satellites (although asteroid 2002 VE₆₈ does orbits the planet on an irregular basis, and is considered a quasi-moon of Venus) and, consequently, its mass has only been determined through the gravitational effect that the planet has on passing space probes. A mass equivalent to 82% that of Earth’s, or 4.9 × 10²⁴ kg, has been found for Venus. The bulk density of Venus is 5240 kg/m³, slightly smaller than that of Earth’s.

The similarity between the mass, radius, and bulk density of Venus and Earth suggests that the two planets have similar internal structure. Venus most probably has, therefore, a thin rocky crust, a large iron- and magnesium-silicate mantle, and an inner nickel-iron alloy core (about 25% of the planet’s mass).

One Venusian anomaly that defies present-day theory relates to the planet’s magnetic field, or more correctly to the complete lack of any detectable magnetic field. It is believed that the Earth’s magnetic field is created by a dynamo effect that operates in its hot, liquid nickel-iron alloy core. If, as has been previously argued, Venus has an internal structure similar to that of Earth, why does it not have a similar magnetic field? The answer may lie with the slow Venusian rotation rate. One of the key ingredients of the dynamo theory is that the conducting, liquid core is rotating. Since Venus rotates much more slowly than Earth, by a multiplicative factor of 1/243, it may be that the dynamo effect cannot operate in the planet’s core.

The venusian atmosphere

While Venus is often referred to as Earth’s twin on the basis that the two planets have similar physical characteristics (radius, mass, density, composition, etc), it is far from being Earth’s twin when atmospheric characteristics are compared.

The many spacecraft that have flown past, or landed on, the Venusian surface—such as the NASA Galileo spacecraft, which was launched in October 1989, that did a flyby of Venus on its way to the planet Jupiter and its moons; and the NASA Cassini spacecraft, which was launched in October 1997, that performed two flybys of Venus on April 1998 and June 1999—have found that the uppermost cloud tops, which obscure Earth-based observers’ view of the planet, are about 40 mi (65 km) above the surface. For comparison, on Earth, the highest clouds are about 10 mi (16 km) high. Observations taken of ultraviolet wavelengths reveal that the upper Venusian clouds follow a jet streamlike pattern and circle the planet once every four days, or so. The circulation velocity of the upper cloud deck is much greater than the rotation rate of the planet, and it is believed that this is the result of extensive atmospheric convection driven by solar heating.

The upper cloud deck is about 3 mi (5 km) thick. At about 31 mi (50 km) altitude there is a second much more dense cloud deck. Below about 18 mi (30 km) in altitude the Venusian atmosphere is clear of clouds. The upper cloud deck has been found to contain substantial amounts of sulfur, which give the clouds their dark yellow to yellow-orange color. The lower cloud deck has been found to contain large concentrations of sulfur dioxide, hydrogen sulfide compounds, and droplets of sulfuric acid. It has been suggested that the presence of atmospheric sulfides is indicative of very recent volcanic activity on the planet’s surface.

The greenhouse effect

Astronomers began to suspect that the surface of Venus was a decidedly inhospitable place when radio telescope measurements, made in the 1950s, indicated surface temperatures as high as 750K (891° F; 477° C). It is believed that a greenhouse effect is responsible for maintaining the high surface temperature on Venus.

A greenhouse effect occurs whenever incoming sunlight warms the planetary surface, but the atmospheric gases do not allow the infrared radiation emitted by the heated surface to escape back into space. The net result of the atmospheric trapping of infrared radiation is that the atmosphere heats up, and the surface temperature continues to rise.

That a strong greenhouse effect can operate at Venus is not surprising given that its primary atmospheric component is carbon dioxide. This gas has long been recognized as a problematic “greenhouse” gas on Earth.

Building a spacecraft to land on the Venusian surface has proved to be a major engineering challenge. Not only must a lander be able to operate at temperatures that exceed the melting point of lead, but it must also withstand an atmospheric pressure some 90 times greater than that experienced at sea level on Earth. The pressure exerted by the Venusian atmosphere, at the planet’s surface, is equivalent to that exerted by a 0.6 mi (1 km) column of water in Earth’s oceans. The first of only four spacecraft to soft-land, and successfully transmit images of the surface of Venus back to Earth, was the former Soviet Union-built spacecraft Venera 7. The lander, which set-down on August 17, 1970, managed to transmit data for 23 minutes.

Impact craters on venus

Venus’ atmosphere protects the surface from smaller objects that would otherwise impact the surface if the atmosphere were thinner. Smaller objects, especially those under 0.6 mi (1 km) in diameter are largely broken up in the atmosphere and do not directly impact the surface. For this reason and due to Venus’ high volcanic resurfacing rate, rather few impact craters on Venus are known (fewer than 1,000). Impact craters on Venus are frequently attended by flows of impact ejecta that look much like lava flows. This is due to the effect of the thick atmosphere on ejecta behavior during impact. Meade Patera is the largest impact crater basin on Venus at 174 mi (280 km) in diameter. There are only six known multi-ring impact basins on Venus, of which Meade is one. There are also large splotches (radar bright spots) about 6 to 44 mi (10 to 70 km) in diameter thought to be due to the effect of near-surface atmospheric detonation of incoming objects.

Venus geologic history

Venus has two alternative histories, depending upon how one views the age of the extensive resurfacing volcanic lavas. In the catastrophic model of Venus history, there was a huge resurfacing event in Venus’ history about 200 to 700 million years ago, probably due to rather sudden solidification of the interior of the planet. Vast volcanic features of about the same age favor this interpretation. In the gradualistic model of Venus history, global resurfacing has occurred gradually over the whole of Venus’ history and the rate of this activity has been rather high so no older surface areas still exist. The mechanism for this is random and continuous volcanic activity. This model does not explain the lack of magnetic field (explained in the other model by the internal solidification event). Future studies of Venus will hopefully help scientists understand which of these (or perhaps some other) model is correct about Venus’ past.

Current and future missions to Venus

The Magellan mission was ended in 1994 when it was deliberately sent into the atmosphere of Venus to gather additional information about its density. Since then, the Venus Express was built by the European Space Agency (ESA) and launched by the Russian Federal Space Agency (RFSA), and is currently in polar orbit about the planet, as of April 11, 2006. The probe is making a detailed study of the atmosphere and clouds of Venus. It is also making a comprehensive map of the plasma environment and the surface characteristics of the planet. It will remain orbiting Venus for two Venusian years (around 500 Earth-days). The probe has already discovered an large double atmospheric vortex about the southern pole of Venus.

KEY TERMS

Albedo —The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Doppler effect —The apparent change in the wavelength of a signal due to the relative motion of the source and the observer.

Dynamo effect —A model for the generation of planetary magnetic fields: the circulation of conducting fluids within a planet’s hot, liquid inner-core results in the generation of a magnetic field.

Greenhouse effect —The phenomenon that occurs when gases in a planet’s atmosphere capture radiant energy radiated from a planet’s surface thereby raising the temperature of the atmosphere and the planet it surrounds.

Lithosphere —The solid outer layer, or crust, of a planet’s mantle.

Mantle —The major portion of a terrestrial planet’s interior, made of plastic rock.

Retrograde rotation —Axial spin that is directed in the opposite sense to that of the orbital motion.

Tectonic activity —The theory of crustal motion.

The NASA MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) made a flyby of Venus on October 24, 2006, and will perform another flyby in June 6, 2007, on its way to the planet Mercury. The Bepi Colombo mission, named after Italian engineer and mathematician Giuseppe (Bepi) Columbo (1920–1984) to the planet Mercury is expected to lift off in August 2013 by the ESP and the Japan Aerospace Exploration Agency (JAXA). Although still in the planning stages, the probe is expected to flyby the planet Venus on its way to Mercury.

See also Planetary atmospheres.

Resources

BOOKS

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Sobel, Dava. The Planets. New York: Viking, 2005.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

PERIODICALS

Goldman, Stuart. “Venus Unveiled.” Sky and Telescope 83 (March 1992).

Luhmann, Janet, J. Pollack, and C. Lawrence. “The Pioneer Mission to Venus.” Scientific American (April 1994).

Paul, Jeffrey. “Venus in 3-D.” Sky & Telescope 86, (August 1993).

Saunders, Stephen. “The Surface of Venus.” Scientific American (December 1990).

OTHER

Arnett, B. SEDS, University of Arizona. “The Eight Planets, a multimedia tour of the solar system.” (August 25, 2006) <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html> (accessed November 6, 2006).

Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Welcome to the Planets: Venus.” <http://pds.jpl.nasa.gov/planets/choices/venus1.htm> (accessed November 6, 2006).

SpaceKids, National Aeronautics and Space Administration. “Tour the Solar System and Beyond.” <http://spacekids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 14, 2006).

TeachNet-lab.org. “A Tour of the Planets.” <http://www.teachnet-lab.org/miami/2001/salidoi2/a_tour_of_the_planets.htm> (accessed October 14, 2006).

Martin Beech David T. King, Jr.

Venus

Venus is the second planet from the Sun and is in some superficial geological features like Earth . In many important geochemical features, however, it is quite different. Next to the Sun and Moon , Venus can be the brightest object in the sky. At its most brilliant, Venus is sixteen times brighter than Sirius (α Canis Majoris), the brightest star in the night sky. The extreme brilliance of Venus is partly due to its occasional closeness to Earth, and partly due to it having a highly reflective atmosphere.

Basic properties

Venus orbits the Sun on a near circular orbit at mean distance of 0.723 Astronomical Units (AU). At aphelion the planet is at a maximum distance of 0.728 AU from the Sun, while at perihelion Venus it is 0.718 AU away. Coming as near as 0.272 AU, no other planet can approach the earth as close as Venus does.

Since it circles the Sun in an orbit smaller than that of Earth's, Venus is never very far away from the Sun for terrestrial viewers. The greatest angular distance between the Sun and Venus, as seen from Earth, is 47 degrees. This means that even under the most favorable of conditions Venus will set at most three hours after the Sun, or rise no earlier than three hours before the Sun. Observed since the most ancient of times, Venus is often called the "morning star," if it rises before dawn or the "evening star" if it sets after the Sun. The Greek philosopher Homer referred to Venus in his Illiad as "the most beautiful star set in the sky."

The time required for Venus to complete one orbit about the Sun, its sidereal period, is 224.701 days, whereas the time for Venus to repeat alignments with respect to the Sun and Earth is 584 days. The best times for viewing Venus are when it is near greatest eastern, or greatest western elongation. Greatest western elongation follows about five months after greatest eastern elongation, and greatest eastern elongation's repeat about every 19 months.

As it circles the Sun, Venus shows phases just like the Moon, and the planet Mercury. The phase changes of Venus were first recorded by Galileo Galilei in 1610. Galileo's observations of Venusian phase changes were important since they strengthened his belief in the heliocentric model of the solar system which had been proposed by Copernicus in his now famous book, De Revolutionibus, published in 1543. Galileo reasoned that if Venus circled Earth, as the then accepted geocentric model of the Universe decreed, it would not show the full range of phases that were observed. Likewise, the correlation between angular size and phase would also be different to that observed if Venus orbited Earth.

Transits of Venus across the disk of the Sun, as seen from Earth, are not common. If a transit is to occur, however, it will take place in either June or December, when Earth is at the line along which Venus's orbit cuts the ecliptic—the line of nodes for Venus. A very precise geometrical alignment of Earth, Venus and Sun is required for a Venusian transit to take place. The last Venusian transit occurred on December 6, 1882. The next transit will take place on June 8, 2004. In general, Venusian transits are seen in pairs. Each of the transits in a pair are eight years apart, and the pair-cycle repeats on an alternating cycle of 121.5 and 105.5 years.

The rotation rate of Venus

The Venusian atmosphere is both optically thick and highly reflective. The upper cloud deck has an albedo of 0.76, meaning that it reflects 76% of the sunlight that falls on it. In addition, the low-lying Venusian clouds are so dense that they completely obscure any optical view of the planet's surface. Not being able to monitor variations in surface detail has meant that astronomers have only recently discovered the true rotation rate of Venus.

While the Venusian atmosphere presents an impenetrable shroud against optical observations, it is transparent to radio and microwave radiation . Using the giant radio telescope at the Arecibo Observatory in Puerto Rico, astronomers were able to bounce microwave signals off the surface of Venus. By analyzing the Doppler shift in the returned signals the astronomers were then able to determine the planet's rotation rate. The results were a complete surprise.

The microwave measurement of Venus's rotation rate showed that the planet was spinning on its axis in the opposite sense to which it orbits the Sun. If one could stand on the surface of Venus, sunrise would be in the west and sunset would be in the east, the exact opposite to that seen on Earth. When viewed from above, all the planets in the solar system orbit the Sun in a counter clockwise direction. Most of the planets also rotate about their spin axies in a counter clockwise sense. The only exceptions to this rule are the planets Venus, Uranus , and Pluto . When a planet spins on its axis in the opposite sense to its orbital motion , the rotation is said to be retrograde.

Venus takes 243.01 days to spin once on its axis. With this slow rotation rate it actually takes 18.3 days longer for Venus to spin once on its axis than it takes for the planet to orbit the Sun. The Venusian day, that is the time from one noon to the next, is 116.8 terrestrial days long. A curious relationship exists between the length of the Venusian day and the planet's syndic period. The syndic period of Venus, that is, the time for the planet to repeat the same alignment with respect to Earth and Sun, is 584 days, and this is five times the Venusian day (584 = 5 × 116.8). It is not known if this result is just a coincidence, or the action of some subtle orbital interaction. The practical consequence of the relationship is that, should a terrestrial observer make two observations of Venus that are 584 days apart, then they will "see" the same side of the planet turned towards Earth.

Venusian surface detail

At is closest approach the planet Venus can have an angular diameter just slightly larger than 1/60th of a degree. This angular size translates to a physical diameter of 7,520 mi (12,104 km), making the planet about 95% the size of Earth. Given the near similar sizes of Earth and Venus, it might be expected that the surface of Venus has been shaped by the same geological processes that operate on Earth. This expectation is only partly true.

While radar maps of the surface of Venus were constructed during the 1970s, the most detailed topographic maps obtained to date are those from the Magellan spacecraft mission. The Magellan spacecraft was placed into Venusian orbit by NASA in 1990, and a powerful on-board imaging radar system was used to map the entire Venusian surface to a resolution of a few hundred meters.

The Magellan radar data showed that Venus is remarkably flat, and that some 80% of the planet's surface is covered by smooth volcanic plains, the result of many lava out-flows. The altitude map constructed from Magellan data has revealed the existence of two large continent-like features on Venus. These features are known as Ishtar Terra (named after the Babylonian Goddess of love), and Aphrodite Terra (named after the Greek Goddess of love). Ishtar terra, which measures some 621 mi (1,000 km) by 931 mi (1,500 km), lies in Venus's northern hemisphere, and has the form of a high plateau ringed with mountains . The largest mountain in the region, Maxwell Montes, rises to a height of 7 mi (11 km). Aphrodite Terra is situated just to the south of the Venusian equator and is some 9,936 mi (16,000 km) long by 1,242 mi (2,000 km) wide. It is a region dominated by mountainous highlands and several large volcanoes.

Venus has three major terrains, based mainly upon elevation (above mean planet radius). The lowlands are rolling volcanic plains (~ 60% of the planet) with under 1,600 ft (500 m) of relief. The uplands are intermediate in elevation and represent a transition between lowlands and highlands. Elevations range from 0 to 1.2 mi (0 to 2 km) in the uplands. The highlands have up to 3 mi (5 km) relief and are quite mountainous in some places. The highlands have compression ridges and fractured rocks and comprise about 15% of the planet's surface.

Given the similarity in size of Venus and Earth, geologists had speculated on the possibility that plate tectonics , which is the primary agent for re-shaping Earth's surface, might operate on Venus. The Magellan probe found no evidence, however, for large scale tectonic activity on Venus. The reasons underlying the absence of any large-scale tectonic activity on Venus are presently unclear, but it may be indicative that the planet has a thinner and weaker lithosphere than Earth. The ridges and folds that cover many of the plain regions of Venus is indicative, however, of a certain amount of local tectonic activity on the planet.

The Magellan maps revealed many large craters on the Venusian surface. There is an apparent cut-off for craters with diameters less than a few kilometers. This is a selection effect imposed by the dense Venusian atmosphere. The atmosphere is in fact such a good filter of incoming meteoroids, that only those objects larger than a few kilometers in diameter survive their passage through the atmosphere, with sufficient mass , to produce a crater at the Venusian surface.

The relative age of different regions on a planet's surface can be gauged by counting the number of craters that appear per unit area. This method of crater counting has proved very useful for dating the various regions on our Moon. The essential idea being exploited in crater count dating is that, assuming the cratering rate is the same over the whole planet, if one region has fewer craters per unit area than another, it implies that some resurfacing, e.g., by a lava flow, has taken place in the region with fewer craters. The re-surfacing has in effect erased the older craters and re-set the cratering clock.

The number of craters in the Venusian plains is typically about 15% of the crater counts for the lunar maria. This observation indicates that the Venusian plains have an age roughly equivalent to 15% the age of the lunar maria. From the lunar rock samples that were returned from the Apollo Moon landing missions we know that the lunar maria are about 3.2 billion years old, and consequently the likely age of the Venusian plains is about 500 million years.

The main agent for re-surfacing the Venusian plains is believed to be aperiodic but widespread volcanism. Certainly, many (apparently) extinct volcano's were mapped by Magellan during its five-year survey. Some lava flow regions observed by Magellan are believed to be no more than ten millions years old, and they may be much younger.

Some of the more remarkable surface features discovered by Magellan were the pancake-shaped volcanoes. These flat-topped, circular volcanoes are unique to Venus, and it is thought that they are probably formed through the surface extrusion of a very thick and viscous lava.

In recognition of Venus being named in honor of the Greek Goddess Aphrodites, whom the Romans called Venus, the International Astronomical Union assigns only female names to the planet's surface features. Craters, for example, can be named after any famous women; linear features are named after Goddesses of War, while plains are named in honor of mythological heroines.

Venusian surface processes

Processes affecting Venus include impact cratering, lava flows, solid-state creep (viscous flow of rocks at the surface due to high temperatures and pressures), and eolian (wind ) effects. The latter include deflation (blowing away fine particles), wind erosion (forming yardangs), and wind deposition (formation of streaks, transverse dunes, and wind-shadow dunes). Weathering on the surface is due to interaction between carbon dioxide and sulfur dioxide at high temperature with silicate rocks. At elevations above about 2.2 mi (3.5 km) above mean planet radius, weathering seems to favor formation of iron-sulfur compounds and below that level, iron-oxide and calcium-sulfate compounds, tend to form. The latter was inferred from the nature of the reflected radar signal off surficial materials.

Venusian internal structure

Venus has no natural satellites and consequently its mass has only been determined through the gravitational effect that the planet has on passing space probes. A mass equivalent to 82% that of Earth's, or 4.9x1024 kg, has been found for Venus. The bulk density of Venus is 5240 kg/m3, slightly smaller than that of Earth's.

The similarity between the mass, radius, and bulk density of Venus and Earth suggests that the two planets have similar internal structure. Venus most probably has, therefore, a thin rocky crust, a large iron- and magnesium-silicate mantle, and an inner nickel-iron alloy core (~ 25% of the planet's mass).

One Venusian anomaly that defies present-day theory relates to the planet's magnetic field, or more correctly to the complete lack of any detectable magnetic field. It is believed that the earth's magnetic field is created by a dynamo effect that operates in its hot, liquid nickel-iron alloy core. If, as has been previously argued, Venus has an internal structure similar to that of Earth, why does it not have a similar magnetic field? The answer may lie with the slow Venusian rotation rate. One of the key ingredients of the dynamo theory is that the conducting, liquid core is rotating. Since Venus rotates much more slowly than Earth, by a multiplicative factor of 1/243, it may be that the dynamo effect cannot operate in the planet's core.

The Venusian atmosphere

While Venus is often referred to as Earth's twin on the basis that the two planets have similar physical characteristics (radius, mass, density, composition, etc), it is far from being Earth's twin when atmospheric characteristics are compared.

The many spacecraft that have flown past, or landed on, the Venusian surface have found that the uppermost cloud tops, which obscure Earth-based observers' view of the planet, are about 40 mi (65 km) above the surface. For comparison, on Earth, the highest clouds are about 10 mi (16 km high.) Observations taken of ultraviolet wavelengths reveal that the upper Venusian clouds follow a jet stream-like pattern and circle the planet once every four days, or so. The circulation velocity of the upper cloud deck is much greater than the rotation rate of the planet, and it is believed that this is the result of extensive atmospheric convection driven by solar heating.

The upper cloud deck is about 3 mi (5 km) thick. At about 31 mi (50 km) altitude there is a second much more dense cloud deck. Below about 18 mi (30 km) in altitude the Venusian atmosphere is clear of clouds. The upper cloud deck has been found to contain substantial amounts of sulfur , which give the clouds their dark yellow to yellow-orange color . The lower cloud deck has been found to contain large concentrations of sulfur dioxide, hydrogen sulfide compounds, and droplets of sulfuric acid . It has been suggested that the presence of atmospheric sulfides is indicative of very recent volcanic activity on the planet's surface.

The greenhouse effect

Astronomers began to suspect that the surface of Venus was a decidedly inhospitable place when radio telescope measurements, made in the 1950s, indicated surface temperatures as high as 750K (891°F; 477°C). It is believed that a greenhouse effect is responsible for maintaining the high surface temperature on Venus.

A greenhouse effect occurs whenever incoming sunlight warms the planetary surface, but the atmospheric gases do not allow the infrared radiation emitted by the heated surface to escape back into space. The net result of the atmospheric trapping of infrared radiation is that the atmosphere heats up, and the surface temperature continues to rise.

That a strong greenhouse effect can operate at Venus is not surprising given that its primary atmospheric component is carbon dioxide. This gas has long been recognized as a problematic "greenhouse" gas on Earth.

Building a spacecraft to land on the Venusian surface has proved to be a major engineering challenge. Not only must a lander be able to operate at temperatures that exceed the melting point of lead , but it must also withstand an atmospheric pressure some 90 times greater than that experienced at sea level on Earth. The pressure exerted by the Venusian atmosphere, at the planet's surface, is equivalent to that exerted by a 0.6 mi (1 km) column of water in Earth's oceans. The first of only four spacecraft to soft-land, and successfully transmit images of the surface of Venus back to Earth, was the former Soviet Union-built spacecraft Venera 7. The lander, which set-down on August 17, 1970, managed to transmit data for 23 minutes.

Impact craters on Venus

Venus' atmosphere protects the surface from smaller objects that would otherwise impact the surface if the atmosphere were thinner. Smaller objects, especially those under 0.6 mi (1 km) in diameter are largely broken up in the atmosphere and do not directly impact the surface. For this reason and due to Venus' high volcanic resurfacing rate, rather few impact craters on Venus are known (less than 1000). Impact craters on Venus are frequently attended by flows of impact ejecta that look much like lava flows. This is due to the effect of the thick atmosphere on ejecta behavior during impact. Meade Patera is the largest impact crater basin on Venus at 174 mi (280 km) in diameter. There are only six known multi-ring impact basins on Venus, of which Meade is one. There are also large "splotches" (radar bright spots) about 6–44 mi (10–70 km) in diameter thought to be due to the effect of near-surface atmospheric detonation of incoming objects.

Venus geologic history

Venus has two alternative histories, depending upon how one views the age of the extensive resurfacing volcanic lavas. In the "catastrophic" model of Venus history, there was a huge resurfacing event in Venus' history about 200 to 700 million years ago, probably due to rather sudden solidification of the interior of the planet. Vast volcanic features of about the same age favor this interpretation. In the "gradualistic" model of Venus history, global resurfacing has occurred gradually over the whole of Venus' history and the rate of this activity has been rather high so no older surface areas still exist. The mechanism for this is random and continuous volcanic activity. This model does not explain the lack of magnetic field (explained in the other model by the internal solidification event). Future studies of Venus will hopefully help us understand which of these (or perhaps some other) model is correct about Venus' past.

See also Planetary atmospheres.

Resources

books

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge University Press, 1999.
de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

periodicals

Goldman, Stuart. "Venus Unveiled." Sky and Telescope 83 (March 1992).

Luhmann, Janet, J. Pollack, and C. Lawrence. "The Pioneer Mission to Venus." Scientific American (April 1994). Paul, Jeffrey. "Venus in 3-D." Sky & Telescope 86 (August 1993). Saunders, Stephen. "The Surface of Venus." Scientific American (December 1990).

other

Arnett, B. SEDS, University of Arizona. "The Nine Planets, A Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2003] <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.

Martin Beech

David T. King, Jr.

KEY TERMS

Albedo

—The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Doppler effect

—The apparent change in the wavelength of a signal due to the relative motion of the source and the observer.

Dynamo effect

—A model for the generation of planetary magnetic fields: the circulation of conducting fluids within a planet's hot, liquid inner-core results in the generation of a magnetic field.

Greenhouse effect

—The phenomenon that occurs when gases in a planet's atmosphere capture radiant energy radiated from a planet's surface thereby raising the temperature of the atmosphere and the planet it surrounds.

Lithosphere

—The solid outer layer, or crust, of a planet's mantle.

Mantle

—The major portion of a terrestrial planet's interior, made of plastic rock.

Retrograde rotation

—Axial spin that is directed in the opposite sense to that of the orbital motion.

Tectonic activity

—The theory of crustal motion.

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.

Venus

Venus was an ancient native Italic goddess who originally was associated with flourishing vegetation and whose sphere of influence quickly expanded—perhaps through her association with the Semitic goddess Astarte, the Etruscan Turan, and the Greek Aphrodite—to include erotic relations of all types. She was concerned with both prostitution and marriage. From almost her earliest appearance, Venus has been the symbol par excellence of sexual love in Western culture. The most important elements of the iconography of the goddess (the female nude, winged cupids, roses, and doves) are immediately recognizable today and still carry erotic overtones. The standard poses in which Venus appears in ancient art, such as the nude rising from the sea while standing on a shell (Venus Anadyomene) and the naked goddess shielding her modesty from the prying eyes of the viewer (Venus Pudica), have been depicted frequently by artists from the Renaissance to the present.

THE WORSHIP AND INFLUENCE OF VENUS IN ANCIENT ROME

Though not part of the original ancient Roman pantheon, Venus was worshiped in that city from an early period in several different incarnations, including Venus Obsequens ("favorable" or "gracious"), who received a temple in 295 bce, and Venus Verticordia ("turner of hearts"), who was worshiped at the annual festival of the Veneralia on April 1 by women of respectable status; women of lesser rank (perhaps prostitutes) worshiped another goddess, Fortuna Virilis. She also was connected with two wine festivals, the Vinalia Priora and Vinalia Rustica, observed on April 23 and August 19, respectively, though they actually were festivals of the god Jupiter. The nature of her association with those two celebrations is not clear.

In addition to her influence in private erotic matters, Venus was relevant to Roman political life. During the Second Punic War the Romans built a temple on the Capitoline hill, the most sacred place in the city, to Venus Erycina (the goddess worshiped at Eryx in Sicily). That undertaking was part of efforts to appease the gods after they had manifested their anger with the Romans by allowing Hannibal and his Carthaginian forces to defeat the Roman army at Lake Trasimene in 217 bce. Venus Erycina was famous in the ancient world for the sacred prostitution practiced at her temple in Eryx. There is no certain evidence that that practice came to Rome along with the goddess, though sources indicate that Roman magistrates and generals were happy to participate in such activities when visiting the goddess's sanctuary in Sicily.

In the first century bce, Venus was taken up by the dictator Sulla as his personal protective deity: He adopted the nickname Epaphroditus, meaning "favored by Aphrodite/Venus." Her political value continued to be recognized after Sulla's death in 79 bce. She was taken up by the dictator's younger ally, Pompey the Great, who built a temple to Venus Victrix ("the conqueror") in the year 55. The most lasting association of the goddess, however, was with Pompey's sometime friend and later rival Julius Caesar, whose family claimed descent directly from her. Caesar reinforced awareness of his divine lineage by dedicating a temple to Venus Genetrix ("the ancestor") in 46 bce in his new Forum in Rome. The remains of that temple still can be seen. Venus maintained her political prominence in later centuries under the emperors of Rome. For example, around 121 ce the emperor Hadrian dedicated the largest temple ever built in the city to a pair of goddesses: Venus and the deified city of Rome.

VENUS IN ROMAN LITERATURE

Venus appears frequently in Roman literature, most famously in two epic poems of the first century bce. The earlier of the two poems is Lucretius' De Rerum Natura [On the nature of the universe], which was completed in the first half of the century. In that poem the goddess appears in an opening hymn as the embodiment of the procreative aspect of nature. Venus has the power to calm the violent forces of the universe, and the poet asks the goddess to persuade her lover, Mars, the god of war, to bestow peace on the Roman people.

Venus also plays a prominent role in Vergil's Aeneid, an epic poem in twelve books that was published posthumously in 19 bce at the behest of the emperor Augustus, the great-nephew and adoptive son of Julius Caesar. The Aeneid tells the tale of Aeneas, Venus's mortal son fathered by the Trojan Anchises, and his long and difficult journey to establish the remnants of the Trojan people in Italy after the conclusion of the Trojan War. Once settled in Italy, the Trojans were destined to merge with the native Latins to become a single people, the Romans. It is through Aeneas and his son, Iulus, that the Roman people generally, and Julius Caesar and his successor Augustus specifically, claimed descent from Venus. Throughout the Aeneid, Venus appears both as a national deity and as a love goddess, especially in the poem's fourth book, in which she and Aeneas's half brother Cupid are responsible for the tragic love affair between Aeneas and Dido, the queen of Carthage.

VENUS IN THE MIDDLE AGES

In the late antique and medieval periods Venus appeared commonly in mythographies: collections of classical myths provided with allegorical and metaphorical interpretations that generally were in keeping with Church teachings. The association of Venus with sexual love made her a difficult subject for Christian authors, but mythographers attempted to extract moral lessons from stories about her by finding deeper meaning in the traditional tales. For instance, in the Mythologies of Fulgentius (late fifth to early sixth centuries ce) the story of the birth of Venus from the severed testicles of the god Saturn becomes a lesson in the way overindulgence leads to lustful behavior: It is presented as an allegory for how crops are grown, harvested, and consumed and then produce wantonness. Elsewhere in the same text Venus also stands for devotion to pleasure in the story of the judgment of Paris, for lust in the tale of Cupid and Psyche, and for the corruption of manly virtue in the tale of her adultery with Mars. The last episode was particularly popular among medieval authors, who were consistent in their accounts of the details of the story. There is greater variation in medieval depictions of the relationship of Venus to Cupid: She is sometimes his mother (as in ancient texts), sometimes his wife, and sometimes his companion.

Venus also appears in literary contexts other than the traditional myths associated with her. In the thirteenth-century Romance of Rose, which was begun by Guillaume de Loris and continued by Jean de Meun, Venus is a character in the tale of a young man's adventures in the Garden of Love; she stands as an allegory for unbridled female desire. Elsewhere she is associated with the voluptuous life, that is, a life shamefully spent in the pursuit of pleasure and idleness, as in several of the Canterbury Tales by Geoffrey Chaucer (for example, the "Knight's Tale" and the "Wife of Bath's Tale") and his House of Fame, John Ridewall's Fulgentius Metaphored, and Christine de Pizan's Epistle of Othea.

In addition to her association with illicit desire, Venus could be linked to a more chaste love and with nature's bounty by later authors, as she was by the Romans in the classical period. For example, in Boccaccio's influential Genealogie Deorum Gentilicium, published in the late fourteenth century, two Venuses are identified: one who is "the mother of all fornications" and another who is synonymous with the harmony of the world.

VENUS IN THE RENAISSANCE

The goddess's more positive associations come to the fore during the Renaissance of the fifteenth and sixteenth centuries. The perfection of her physical appearance and the preeminence of her beauty linked her closely with artistic creation in that period. In art and literature Venus was invoked most often as the embodiment of a higher love that inspires the lover to virtue and victory. After nearly disappearing from European art in the medieval period, Venus is such a recurrent motif in painting and sculpture of the Renaissance that her nude form sometimes is identified as a symbol of the Renaissance itself.

Two of the most famous paintings of her were produced by Sandro Botticelli in the mid-1480s: The Birth of Venus, now in the Uffizi Gallery in Florence, Italy, and Venus and Mars, which hangs in the National Gallery in London. Both works are thought to have been inspired by portions of the Stanze written by Angelo Poliziano (Politian) in 1475–1476. That incomplete poem in two books celebrates the chaste and improving love of Julio (Giuliano de' Medici) for the married nymph Simonetta (Simonetta Cattaneo), a love brought about by the workings of Cupid and Venus. The goddess was also the subject of numerous paintings by Titian in the first half of the sixteenth century, including The Venus of Urbino, Venus Anadyomene, The Worship of Venus, and Sacred and Profane Love.

see also Aphrodite; Eros, Cupid; Erotic Art; Greco-Roman Art; Literature: I. Overview; Magic.

BIBLIOGRAPHY

Arscott, Caroline, and Katie Scott, eds. 2000. Manifestations of Venus: Art and Sexuality. Manchester, UK: Manchester University Press.

Freedman, Luba. 2003. The Revival of the Olympian Gods in Renaissance Art. Cambridge, UK: Cambridge University Press.

Lawrence, Marion. 1967. "The Birth of Venus in Roman Art." In Essays in the History of Art Presented to Rudolf Wittkower, ed. Douglas Fraser, Howard Hibbard, and Milton J. Lewine. London: Phaidon.

Schilling, Robert. 1954. La Religion Romaine de Vénus depuis les Origines jusqu'au temps d'Auguste. Paris: De Boccard.

Schreiber, Earl G. 1975. "Venus in the Medieval Mythographic Tradition." Journal of English and Germanic Philology 74: 519-535.

Tinkle, Theresa Lynn. 1996. Medieval Venuses and Cupids: Sexuality, Hermeneutics, and English Poetry. Stanford, CA: Stanford University Press.

                                               Celia E. Schultz

VENUS

VENUS is perhaps the most singular example from among the divinized abstractions that make up the Roman pantheon. The word venus, in its origin, is a neuter noun of the same kind as genus or opus. It is discernible in the derived verb venerari (*venes-ari ), which is confined to religious usage by all the authors of the republican period, especially Plautus. The Plautinian construction (not maintained by classic use) is of particular interest: veneror … ut, which can be translated, "I work a charm [upon such-and-such a divinity] in order to [obtain a result]." This notion of charm or seduction that defines the word venus is represented in Hittite (wenzi ) and in the language of the Veneti (wontar ). Yet the root ven- did not produce a divinity anywhere except in Latin. It is significant that, in the Oscan region (where is recorded a form that is probably borrowed from Latin), the homologue of the Latin Venus is Herentas, formed from another root: her-, "to will."

The neuter venus is part of a remarkable semantic series of the same kind as genus/Genius/generare, except that here the first term and not the second was divinized, passing from the neuter to the feminine: Venus/venia/venerari (sometimes venerare in Plautus). To the persuasive charm that the goddess embodies and that the venerans ("he who venerates") practices upon the gods, there corresponds the symmetric notion of venia in the sense of "grace" or "favor"—a notion that belongs to the technical vocabulary of the pontiffs (Servius, Ad Aeneidem 1.519).

This metamorphosis of a neuter noun into a goddess (in contrast, it is the shift from feminine to masculine that marks the divinization of Cupido) was very likely furthered by the encounter of this divinity with the Trojan legend. This legend must have facilitated the relation drawn between a Venus embodying charm in its religious meaning and an Aphrodite personifying seduction in the profane sense. The notion of Aphrodite as mother of the Trojan hero Aeneas, the legendary founder of the Roman race, allowed for the application of a Greek legend to Roman benefit. The myth illustrated the rite. It made explicit in plain language the ritual employed by a Roman venerans when soliciting the venia deum, the favor of the gods. Set forth as their ancestor, the "pious" Aeneas conferred upon the Romans a privileged status in the eyes of the gods. Was it not therefore their lot as his descendants, the Aeneads, to be assured of obtaining the pax veniaque deum (the peace and grace of God), as frequently expressed by Livy, thanks to the mediation of Venus, the preferred daughter of Jupiter? This, to be sure, was on the condition that they fulfill the duties of pietas ("piety"). This explains the famous declarations whereby the Romans claimed the title of "the most religious people in the world" (Cicero, De natura deorum 2.3.8, De haruspicum responsis 9.19).

The divinization of the notion of venus had to take place in a syncretic environment, Lavinium, which lent to Venus the smile of Aphrodite. According to tradition, Aeneas established at Lavinium, in Latium, a cult of Venus Frutis (the appellation Frutis is very likely connected etymologically to Aphrodite ), and in the same place a federal temple of Venus, common to all Latins, was set up. Archaeology has uncovered at that site a hērōiov, the shrine of a hero, which the discoverer identifies as the mausoleum of Aeneas mentioned by Dionysius of Halicarnassus (1.64.1–5).

The Trojan interpretation of Venus explains the development of her cult. Thanks to the enlightenment afforded by the association with the Trojan legend, the Romans were able to recognize their national Venus in the Aphrodite of Mount Eryx in Sicily at the time of the First Punic War and so erected a temple to her later on the Capitoline. On the basis of this same enlightenment, the goddess was associated with Jupiter in the cult of the Vinalia, the wine festival thought to have been instituted by Aeneas. The first temple erected in the goddess's honor had been dedicated to Venus Obsequens ("propitious Venus"). It had been vowed in 295 bce by Q. Fabius Gurges while battle raged against the Samnites. Its dedication day, August 19, coincided with the Vinalia Rustica. The Trojan interpretation was imposed in definitive and official fashion in the first century bce: Julius Caesar offered a temple in the middle of the forum to Venus Genetrix as the grandmother of the Julian gens and the mother of the Aeneades. Lucretius's literary expression Aeneadum genetrix thus was awarded liturgical consecration.

Bibliography

Dumézil, Georges. Idées romaines. Paris, 1969. See pages 245–252.

Dumézil, Georges. La religion romaine archaïque. 2d ed. Paris, 1974. Translated from the first edition by Philip Krapp as Archaic Roman Religion, 2 vols. (Chicago, 1970).

Schilling, Robert. "Le Culte de l'Indiges' à Lavinium." Revue des études latines 57 (1979): 49–68.

Schilling, Robert. Rites, cultes, dieux de Rome. Paris, 1979. See pages 290–333.

Schilling, Robert. La religion romaine de Venus depuis les origines jusqu'au temps d'Auguste. 2d ed. Paris, 1982.

New Sources

Freyburger, Gérard. "Vénus et Fides." In Hommages à Robert Schilling, pp. 101–108. Paris, 1983.

Johnson, Patricia J. "Construction of Venus in Ovid's Metamorphoses V." Arethusa 29 (1996): 125–149.

Lloyd-Morgan, Glenys. "Roman Venus: Public Worship and Private Rites." In Pagan Gods and Shrines of the Roman Empire, edited by Martin Henig and Anthony King, pp. 179–188. Oxford, 1986.

Magini, Leonardo. Le feste di Venere. Fertilità femminile e configurazioni astrali nel calendario di Roma antica. Rome, 1996.

Speidel, Michael. "Venus Victrix. Roman and Oriental." In Auf-stieg und Niedergang der Römischen Welt 2.17.4, pp. 2225–2238. Berlin and New York, 1984.

Wlosok, Antonie. Die Göttin Venus in Vergils Aeneis. Heidelberg, 1967.

Robert Schilling (1987)

Translated from French by Paul C. Duggan
Revised Bibliography