Venus
Introduction
Venus was named after the ancient Roman goddess of love and beauty, but its conditions are anything but hospitable and inviting to humans. Unlike Earth, Venus is extremely hot and dry. The planet is always shrouded by a thick layer of clouds. Venus has a massive atmosphere, or surrounding layers of gases, composed mainly of carbon dioxide. This thick atmosphere traps heat, making Venus the hottest planet in the solar system.
The permanent blanket of clouds also makes it difficult to study the planet. Little was known about the surface and atmosphere until the 1960s, when astronomers made the first radar observations of Venus and unmanned spacecraft began visiting the planet.
Basic Planetary Data
Size, Mass, and Density Venus is the third smallest planet in the solar system, after Mercury and Mars. It is a near twin of Earth in size, mass, and density. Venus' diameter is about 7,521 miles (12,104 kilometers), compared with some 7,926 miles (12,756 kilometers) for Earth. Its mass is approximately 80 percent of Earth's, and its density is about 95 percent of Earth's. The surface gravity of the two planets is also of similar strength.
Appearance from Earth Along with Mercury, Venus is an “inferior” planet, or one whose orbit is smaller than Earth's. For this reason, Venus always appears in Earth's sky in roughly the same direction as the Sun. At some times of the year the planet can be seen as a “morning star,” appearing in the hours before sunrise. At other times it can be seen as an “evening star” in the hours after sunset. Venus often can be seen in clear skies during daylight, if the observer knows exactly where to look.
Because Venus orbits closer to the Sun than Earth does, it exhibits phase changes as viewed from Earth. These phases are similar to those of the Moon and Mercury. Venus sometimes appears as a thin crescent and sometimes as a half or fuller disk. It passes through one cycle of phases about every 584 Earth days. The phases can be seen easily in high-power binoculars or a small telescope.
Venus rarely but regularly passes directly between Earth and the Sun. This event is a type of eclipse called a transit. The planet then appears to observers on Earth as a small black disk crossing the bright disk of the Sun. Two transits of Venus occur about every 125 years. The transits occur in pairs eight years apart. There were no transits of Venus during the 20th century; the dates for the pair of transits in the 21st century are June 8, 2004, and June 6, 2012. After that, another transit of the planet will not occur until 2117. One should never look at a transit directly without certain kinds of protective equipment. Magnification provides the best views. For safe viewing, binoculars or a telescope can be outfitted with special solar filters or used to project the image of the transit onto white cardboard. (See also amateur astronomy; planet, “Apparent Motions.”)
Orbit and Spin All eight planets revolve around the Sun in elliptical, or oval-shaped, orbits. Venus' orbit is the most nearly circular of all the planets. It orbits the Sun at a mean distance of about 67 million miles (108 million kilometers). This is about 30 percent closer to the Sun than Earth's orbit is. At its closest approach to Earth, Venus is about 26 million miles (42 million kilometers) away; at its farthest, Venus is some 160 million miles (257 million kilometers) away. Venus completes one orbital revolution about every 225 Earth days, which is the length of one year on Venus.
Venus' rotation is unusual in a couple of ways. It spins about its axis very slowly, completing one rotation about every 243 Earth days. It is the only planet in the solar system that takes longer to rotate once about its axis than to travel once around the Sun. These two motions combine so that a day on Venus—the time it takes for the Sun to appear straight overhead, to set, and then to rise straight overhead again—lasts about 117 Earth days.
Because of its slow rotation, Venus is more nearly spherical than Earth and most other planets. The force from a planet's rotation generally causes some bulging at the equator and flattening at the poles. These distortions are minimized on Venus.
Venus also rotates in retrograde motion, or the direction opposite that of most of the other planets and members of the solar system. Six of the eight planets rotate clockwise when viewed from above the northern pole, while only Venus and Uranus rotate counterclockwise. To an observer on Venus, the Sun would appear to rise in the west and set in the east (if one could see through the thick clouds). Venus' spin axis is tilted only about 3 degrees relative to the plane of its orbit. This means that seasonal variations on the planet are probably very slight.
Unlike Earth and most of the other planets, Venus does not have a global magnetic field. This might result from its extremely slow rotation rate. Scientists think that a planet's rotation helps drive the fluid motions in the planet's core that generate a magnetic field.
Atmosphere, Surface, and Interior Atmosphere and Greenhouse Effect
Even though Venus is closer to the Sun than Earth is, Venus absorbs less sunlight than Earth does. Venus' thick clouds allow only a little light through. About 85 percent of the sunlight that strikes the clouds gets reflected back into space.
The sunlight that does penetrate the clouds is absorbed by the lower atmosphere and surface. As the light heats the lower atmosphere and the ground, they radiate some of the energy back at longer, infrared wavelengths. On Earth most such energy escapes back into space. This keeps Earth's surface relatively cool. On Venus, however, the thick atmosphere traps much of the reradiated infrared energy. Energy at infrared wavelengths cannot pass through a carbon dioxide-rich atmosphere as easily as shorter-wavelength visible light can.
This phenomenon, called the greenhouse effect, makes Venus extremely hot. The planet's average surface temperature is about 867° F (464° C), which is hot enough to melt lead. Venus is even hotter than Mercury, the planet closest to the Sun. The rocks on Venus may glow faintly red from their own heat.
Studying the greenhouse effect on Venus has given scientists an improved understanding of the more subtle but very important influence of greenhouse gases in Earth's atmosphere. (Rising levels of carbon dioxide, methane, and other so-called greenhouse gases in Earth's atmosphere are thought to be causing global warming on Earth.)
Surface
Interior What little is known about Venus' interior is mostly inferred from its similarity to Earth in terms of density and size. Planetary scientists theorize that Venus probably developed an interior roughly like that of Earth, with a metallic core and a rocky mantle and crust.
Venus' core probably extends outward about 1,860 miles (3,000 kilometers) from the planet's center. It likely contains iron and nickel like Earth's core. Venus' core probably also includes a less dense substance such as sulfur. Unlike most of the other planets, however, Venus has no magnetic field, so there is no direct evidence for a metallic core.
The mantle makes up the bulk of the planet. Gravitational data suggest that the crust is typically about 12–30 miles (20–50 kilometers) thick. It likely contains much basalt. As mentioned above, movements within Venus' mantle are thought to deform the crust. These movements are mainly vertical. Venus does not now seem to experience plate tectonics as Earth does. Plate tectonics involves mainly horizontal movements of a planet's crust and upper mantle.
Observation and Exploration Venus was observed from Earth for centuries before the invention of modern astronomical instruments. The Babylonians recorded its appearances in about 3000 BC, and ancient civilizations in China, Central America, Egypt, and Greece also observed the planet.
Telescopic Observation In the 17th century Galileo made the first telescopic observations of Venus. In 1610 he discovered the planet's phases. If Earth lay at the center of the solar system, as was then widely believed, Venus would not display such phases. Galileo's discovery was the first direct observational evidence to support Nicolaus Copernicus' then-controversial theory that Earth and the other planets orbit the Sun.
When viewed through an optical (visible-light) telescope, Venus appears yellow-white and fairly featureless because of its permanent veil of clouds. Since the early 20th century astronomers have used wavelengths of light that lie outside the visible spectrum to uncover features of the planet that are otherwise hidden. Ultraviolet rays reveal swirls, v-shaped bands, and distinctive bright and dark markings in the clouds. Astronomers have used infrared radiation to study the composition of the atmosphere and clouds. Microwave studies have revealed the high temperatures at the planet's surface.
Radar studies have been particularly important in helping astronomers “see” through the planet's clouds. A large radio telescope outfitted with a transmitter can send out radio waves that pierce Venus' cloud screen and bounce off the planet's surface. The telescope then detects the returning radio waves.
Spacecraft Exploration Exploration of the planet with spacecraft began in the 1960s. More than 20 unmanned spacecraft have visited Venus, including craft that have flown by, orbited, and landed on the planet and that have sent probes parachuting through its atmosphere. Like Earth-based telescopes, spacecraft flying near Venus use radar to penetrate the deck of clouds and map the surface below. Missions to the surface have also had to contend with the planet's extremely high temperatures and pressures.
Another early NASA mission to the planet, called Pioneer Venus, included two craft. Pioneer Venus 1 and Pioneer Venus 2 arrived at the planet in December 1978. The first of these spacecraft orbited Venus for several years, collecting comprehensive data on the atmosphere. Its radar instrument produced the first high-quality map of Venus' surface topography. One of the longest-lived planetary spacecraft, Pioneer Venus 1 returned data for more than 14 years. Pioneer Venus 2, known as the Multiprobe, released four probes to collect data at different points in the planet's atmosphere.
In addition to missions to Venus, several spacecraft have flown past Venus while on their way to other main targets. These flybys were designed as “gravity assists,” which transfer momentum from the planet to the spacecraft in order to increase the craft's velocity and adjust its course. Such gravity assists also allow spacecraft to investigate Venus while flying by. The first craft to use a gravity assist was Mariner 10, which flew past Venus in 1974 on its way to Mercury. Others have included NASA's Galileo, which flew by Venus in 1990 on its way to Jupiter, and NASA's Cassini, which flew by Venus in 1998 and 1999 on its way to Saturn.
- Scientists use radar to pierce the thick clouds shrouding Venus and “see” the surface …
Venus was named after the ancient Roman goddess of love and beauty, but its conditions are anything but hospitable and inviting to humans. Unlike Earth, Venus is extremely hot and dry. The planet is always shrouded by a thick layer of clouds. Venus has a massive atmosphere, or surrounding layers of gases, composed mainly of carbon dioxide. This thick atmosphere traps heat, making Venus the hottest planet in the solar system.
The permanent blanket of clouds also makes it difficult to study the planet. Little was known about the surface and atmosphere until the 1960s, when astronomers made the first radar observations of Venus and unmanned spacecraft began visiting the planet.
Basic Planetary Data
- Simulations and other images provide an overview of Venus, the second planet from the Sun. Venus is …
Size, Mass, and Density Venus is the third smallest planet in the solar system, after Mercury and Mars. It is a near twin of Earth in size, mass, and density. Venus' diameter is about 7,521 miles (12,104 kilometers), compared with some 7,926 miles (12,756 kilometers) for Earth. Its mass is approximately 80 percent of Earth's, and its density is about 95 percent of Earth's. The surface gravity of the two planets is also of similar strength.
Appearance from Earth Along with Mercury, Venus is an “inferior” planet, or one whose orbit is smaller than Earth's. For this reason, Venus always appears in Earth's sky in roughly the same direction as the Sun. At some times of the year the planet can be seen as a “morning star,” appearing in the hours before sunrise. At other times it can be seen as an “evening star” in the hours after sunset. Venus often can be seen in clear skies during daylight, if the observer knows exactly where to look.
Because Venus orbits closer to the Sun than Earth does, it exhibits phase changes as viewed from Earth. These phases are similar to those of the Moon and Mercury. Venus sometimes appears as a thin crescent and sometimes as a half or fuller disk. It passes through one cycle of phases about every 584 Earth days. The phases can be seen easily in high-power binoculars or a small telescope.
Venus rarely but regularly passes directly between Earth and the Sun. This event is a type of eclipse called a transit. The planet then appears to observers on Earth as a small black disk crossing the bright disk of the Sun. Two transits of Venus occur about every 125 years. The transits occur in pairs eight years apart. There were no transits of Venus during the 20th century; the dates for the pair of transits in the 21st century are June 8, 2004, and June 6, 2012. After that, another transit of the planet will not occur until 2117. One should never look at a transit directly without certain kinds of protective equipment. Magnification provides the best views. For safe viewing, binoculars or a telescope can be outfitted with special solar filters or used to project the image of the transit onto white cardboard. (See also amateur astronomy; planet, “Apparent Motions.”)
Orbit and Spin All eight planets revolve around the Sun in elliptical, or oval-shaped, orbits. Venus' orbit is the most nearly circular of all the planets. It orbits the Sun at a mean distance of about 67 million miles (108 million kilometers). This is about 30 percent closer to the Sun than Earth's orbit is. At its closest approach to Earth, Venus is about 26 million miles (42 million kilometers) away; at its farthest, Venus is some 160 million miles (257 million kilometers) away. Venus completes one orbital revolution about every 225 Earth days, which is the length of one year on Venus.
Venus' rotation is unusual in a couple of ways. It spins about its axis very slowly, completing one rotation about every 243 Earth days. It is the only planet in the solar system that takes longer to rotate once about its axis than to travel once around the Sun. These two motions combine so that a day on Venus—the time it takes for the Sun to appear straight overhead, to set, and then to rise straight overhead again—lasts about 117 Earth days.
Because of its slow rotation, Venus is more nearly spherical than Earth and most other planets. The force from a planet's rotation generally causes some bulging at the equator and flattening at the poles. These distortions are minimized on Venus.
Venus also rotates in retrograde motion, or the direction opposite that of most of the other planets and members of the solar system. Six of the eight planets rotate clockwise when viewed from above the northern pole, while only Venus and Uranus rotate counterclockwise. To an observer on Venus, the Sun would appear to rise in the west and set in the east (if one could see through the thick clouds). Venus' spin axis is tilted only about 3 degrees relative to the plane of its orbit. This means that seasonal variations on the planet are probably very slight.
Unlike Earth and most of the other planets, Venus does not have a global magnetic field. This might result from its extremely slow rotation rate. Scientists think that a planet's rotation helps drive the fluid motions in the planet's core that generate a magnetic field.
Atmosphere, Surface, and Interior Atmosphere and Greenhouse Effect
- Large V-shaped bands in Venus' clouds are revealed in a photograph taken in ultraviolet light by …
- An infrared image taken by the Venus Express orbiter shows wind-blown clouds in the night sky above …
- A graph shows temperatures and pressures at different levels in Venus' lower and middle atmosphere, …
Even though Venus is closer to the Sun than Earth is, Venus absorbs less sunlight than Earth does. Venus' thick clouds allow only a little light through. About 85 percent of the sunlight that strikes the clouds gets reflected back into space.
The sunlight that does penetrate the clouds is absorbed by the lower atmosphere and surface. As the light heats the lower atmosphere and the ground, they radiate some of the energy back at longer, infrared wavelengths. On Earth most such energy escapes back into space. This keeps Earth's surface relatively cool. On Venus, however, the thick atmosphere traps much of the reradiated infrared energy. Energy at infrared wavelengths cannot pass through a carbon dioxide-rich atmosphere as easily as shorter-wavelength visible light can.
This phenomenon, called the greenhouse effect, makes Venus extremely hot. The planet's average surface temperature is about 867° F (464° C), which is hot enough to melt lead. Venus is even hotter than Mercury, the planet closest to the Sun. The rocks on Venus may glow faintly red from their own heat.
Studying the greenhouse effect on Venus has given scientists an improved understanding of the more subtle but very important influence of greenhouse gases in Earth's atmosphere. (Rising levels of carbon dioxide, methane, and other so-called greenhouse gases in Earth's atmosphere are thought to be causing global warming on Earth.)
Surface
- Flat rock slabs and soil on the surface of Venus appear in panoramic 170° images taken by the …
- A surface map of Venus shows the planet's global topography. The map is color coded according to …
- Venus' complex surface features include coronae, which are sets of faults, fractures, and ridges in …
- A shield volcano on Venus named Sif Mons appears in a computer-generated image based on radar data …
- A large, characteristic impact crater appears on the surface of Venus in an image made from radar …
Interior What little is known about Venus' interior is mostly inferred from its similarity to Earth in terms of density and size. Planetary scientists theorize that Venus probably developed an interior roughly like that of Earth, with a metallic core and a rocky mantle and crust.
Venus' core probably extends outward about 1,860 miles (3,000 kilometers) from the planet's center. It likely contains iron and nickel like Earth's core. Venus' core probably also includes a less dense substance such as sulfur. Unlike most of the other planets, however, Venus has no magnetic field, so there is no direct evidence for a metallic core.
The mantle makes up the bulk of the planet. Gravitational data suggest that the crust is typically about 12–30 miles (20–50 kilometers) thick. It likely contains much basalt. As mentioned above, movements within Venus' mantle are thought to deform the crust. These movements are mainly vertical. Venus does not now seem to experience plate tectonics as Earth does. Plate tectonics involves mainly horizontal movements of a planet's crust and upper mantle.
Observation and Exploration Venus was observed from Earth for centuries before the invention of modern astronomical instruments. The Babylonians recorded its appearances in about 3000 BC, and ancient civilizations in China, Central America, Egypt, and Greece also observed the planet.
Telescopic Observation In the 17th century Galileo made the first telescopic observations of Venus. In 1610 he discovered the planet's phases. If Earth lay at the center of the solar system, as was then widely believed, Venus would not display such phases. Galileo's discovery was the first direct observational evidence to support Nicolaus Copernicus' then-controversial theory that Earth and the other planets orbit the Sun.
- Venus crosses the face of the Sun in a telescopic image recorded on a photographic plate on Dec. 6, …
When viewed through an optical (visible-light) telescope, Venus appears yellow-white and fairly featureless because of its permanent veil of clouds. Since the early 20th century astronomers have used wavelengths of light that lie outside the visible spectrum to uncover features of the planet that are otherwise hidden. Ultraviolet rays reveal swirls, v-shaped bands, and distinctive bright and dark markings in the clouds. Astronomers have used infrared radiation to study the composition of the atmosphere and clouds. Microwave studies have revealed the high temperatures at the planet's surface.
Radar studies have been particularly important in helping astronomers “see” through the planet's clouds. A large radio telescope outfitted with a transmitter can send out radio waves that pierce Venus' cloud screen and bounce off the planet's surface. The telescope then detects the returning radio waves.
Spacecraft Exploration Exploration of the planet with spacecraft began in the 1960s. More than 20 unmanned spacecraft have visited Venus, including craft that have flown by, orbited, and landed on the planet and that have sent probes parachuting through its atmosphere. Like Earth-based telescopes, spacecraft flying near Venus use radar to penetrate the deck of clouds and map the surface below. Missions to the surface have also had to contend with the planet's extremely high temperatures and pressures.
- The U.S. Mariner 5 spacecraft, shown being prepared for its launch on June 14, 1967, flew by Venus …
Another early NASA mission to the planet, called Pioneer Venus, included two craft. Pioneer Venus 1 and Pioneer Venus 2 arrived at the planet in December 1978. The first of these spacecraft orbited Venus for several years, collecting comprehensive data on the atmosphere. Its radar instrument produced the first high-quality map of Venus' surface topography. One of the longest-lived planetary spacecraft, Pioneer Venus 1 returned data for more than 14 years. Pioneer Venus 2, known as the Multiprobe, released four probes to collect data at different points in the planet's atmosphere.
- The Soviet Venera 4 atmospheric probe, shown on Earth before its launch, took direct measurements …
- The Magellan spacecraft and an attached rocket are released into a temporary orbit around Earth …
- The European Space Agency's Venus Express spacecraft and its launch rocket are shown prior to …
In addition to missions to Venus, several spacecraft have flown past Venus while on their way to other main targets. These flybys were designed as “gravity assists,” which transfer momentum from the planet to the spacecraft in order to increase the craft's velocity and adjust its course. Such gravity assists also allow spacecraft to investigate Venus while flying by. The first craft to use a gravity assist was Mariner 10, which flew past Venus in 1974 on its way to Mercury. Others have included NASA's Galileo, which flew by Venus in 1990 on its way to Jupiter, and NASA's Cassini, which flew by Venus in 1998 and 1999 on its way to Saturn.