Introduction to Venus (planet)

Venus, in astronomy, the second planet from the sun. Except for the sun and the moon, Venus is generally the brightest astronomical object in the sky, displaying a brilliant white starlike appearance. It can be seen even during the day when it is at its brightest. At specific times during a year, Venus is either the first object seen in the western sky in the evening (when it moves toward Earth), or the last seen in the eastern sky in the morning (when it moves away from Earth). Ancient astronomers named the object they saw in the morning Phosphorus, and the one in the evening Hesperus. Later, when they realized it was the same object they were seeing twice, they called it Venus in honor of the Roman goddess of love and beauty.

Venus at a glance
Distance from the sun: Shortest--66,780,000 mi (107,480,000 km); Greatest--67,690,000 mi (108,940,000 km); Average--67,240,000 mi (108,210,000 km).
Distance from Earth: Shortest--23,700,000 mi (38,200,000 km); Greatest--162,200,000 mi (261,000,000 km).
Diameter: 7,521 mi (12,104 km).
Length of year: 225 Earth days.
Rotation period: 243 Earth days.
Temperature: 870 °F (465 °C).
Atmosphere: Carbon dioxide, nitrogen, water vapor, argon, carbon monoxide, neon, sulfur dioxide.
Number of satellites: None.

Orbit

Venus's orbit lies about midway between those of Mercury and Earth and is inclined some 3 12 to Earth's orbit. Its orbit is almost circular. When Venus is at inferior conjunction (between the sun and Earth), it comes closer to Earth than any other planet. Its farthest distance from the sun is around 67,700,000 miles (108,900,000 km); its nearest is around 66,800,000 miles (107,500,000 km); and its mean (average) distance from the sun is around 67,200,000 miles (108,200,000 km).

Observed with a telescope over a period of months, Venus is seen to pass through phases (apparent changes in shape and size) much like those of Earth's moon. These phases are the result of the changing relative positions of the sun, Venus, and Earth.

Venus may be seen close to the far side of the sun once every 584 days of its orbit. Its sunlit area can be seen then almost in its entirety, but appears to decrease during the rest of its orbit. Also, it seems to grow in size. Only half the planet can be seen around 221 days later, and only a thin area after another 71 days, when it nears the same side of the sun as Earth. Venus rotates very slowly about its axis (a hypothetical line down its center) and completes a full turn in slightly more than one Venusian year (243 Earth days). The direction is east-to-west or retrograde, i.e. opposite to that of all the other planets. The axis tilts at an angle of approximately 178 from the perpendicular position.

Density and Mass

The mass of Venus is about four-fifths that of Earth. The gravitational force on Venus is slightly less than on Earth. An object weighing 100 pounds on Earth would weigh about 91 pounds on Venus. Venus is also slightly less dense than Earth.

Temperature

The temperature of the topmost layer of Venus's clouds is around 55 F. (13 C.), and the entire surface gets as hot as 870 F. (465 C.)—hotter than that of any other planet. There is no water on Venus's surface—any water would evaporate because of the extremely high temperature. Earth's plant and animal life could not survive there. Scientists are not sure there is any life form on the planet. Most astronomers think that the high temperature is caused by Venus's atmosphere.

Atmosphere

Venus's atmosphere is the heaviest of those of all the planets. It is made up of carbon dioxide, with small quantities of nitrogen and water vapor, and traces of argon, carbon monoxide, neon, and sulfur dioxide. After the sun's heat reaches the planet's dense atmosphere, the carbon dioxide and sulfuric acid in its thick clouds trap it and prevent it from escaping into space. As this phenomenon is very similar to the operation of a greenhouse, it is known as the greenhouse effect.

The planet's atmospheric pressure (the force per unit area exerted against a surface by the weight of the air above that surface) is calculated to be 1,323 pounds per square inch (9,122 kilopascals)—about 90 times that on Earth. The surface of the planet is completely and permanently shrouded by thick clouds made up of droplets of sulfuric acid. The atmospheric pressure is 90 times that at sea level on Earth. In the upper atmosphere, winds of up to 225 miles per hour (362 kmh) blow continuously from east to west in a broad band above the equator.

Surface

Venus's surface features include level ground, mountains, canyons, and valleys. Plains make up 65 per cent of the surface, and six mountainous areas cover about 35 percent of the surface. Thousands of volcanoes, between and 150 miles (45 and 240 km) wide, dot the plains. The tallest feature of the planet is a mountain range called Maxwell, about 7 miles (11.3 km) high and about 540 miles (870 km) long. There is a canyon 0.6 mile (1.0 km) deep in a region called Beta Regio. An impact crater forms when a planet collides with an asteroid. Venus has impact craters, but far fewer than the moon, Mars, or Mercury. Scientists conclude from this that Venus's surface is less than a billion years old. Some of its features are not seen on Earth. One is coronae, or crowns, which form when hot material from the planet's interior reaches the surface. Another is tessarae (tiles), which are elevated regions full of furrows and depressions all around.

Scientists have used radio astronomy, space probes, and radar to study Venus's surface, which is permanently shrouded under thick clouds of sulfuric acid. Two Soviet spacecraft that touched down on Venus in 1982 performed chemical analyses of rock samples from their respective landing sites. The analyses indicated that the samples were similar to basalt, a type of volcanic rock found on Earth. The most detailed studies that have been made of the surface of Venus are those made in 1990 and onwards by the United States spacecraft Magellan. While the craft was in orbit around Venus, its radar mapped virtually the entire surface of the planet. These radar studies showed that most of the surface of Venus consists of relatively smooth plains. There are also extensive mountainous regions, roughly circular basins, and numerous large craters. Other studies, including measurements of changing sulfur dioxide concentrations in the atmosphere, suggested that Venus has active volcanoes.

Exploration

Venus was the first planet to be tracked by spacecraft. Mariner 2, an unmanned United States spacecraft that passed within 21,600 miles (34,760 km) of Venus on December 14, 1962, over 3 12 months after leaving Earth. It measured various conditions like temperatures on and near Venus. In 1966, Venera 2 reached a point 15,000 miles (24,000 km) near the planet on February 27, and Venera 3 crashed into it on March 1. Both were unmanned Soviet spacecrafts.

On October 18, 1967, Venera 4, a Soviet spacecraft, dropped a container of instruments into Venus's atmosphere by parachute. On October 19, the United States spacecraft Mariner 5 flew within 2,480 miles (3,990 km) of the planet. No magnetic field was mapped, but both probes charted much atmospheric carbon dioxide. On December 15, 1970, the Soviet spacecraft Venera 7 landed on the planet. The United States planetary probe Mariner 10 flew nearby on February 5, 1974, and took the first photographs from close up.

On October 22, 1975, the unmanned Soviet spacecraft Venera 9 landed and took the first photographs of Venus's surface from close up. Three days later, another Soviet space vehicle, Venera 10, reached Venus's surface and took photographs, measured the atmospheric pressure, and analyzed the structure of rocks.

In December, 1978, four unmanned spacecrafts reached the planet. The United States craft Pioneer Venus 1 went into orbit on December 4, during which it measured the temperature at the top of the planet's clouds, drew and produced a map of the surface, and sent back radar images. On December 9, the United States Pioneer Venus 2 entered and calculated the atmosphere's density and chemical composition. Data on the lower atmosphere were sent back by Venera 12, which landed on December 21, and also by Venera 11, which reached the planet's surface on December 25.

In 1982, two more Soviet spacecrafts landed on Venus, examined samples of the soil, and took photographs—Venera 13 on March 1 and Venera 14 on March 5. In October, 1983, Venera 15 and Venera 16 began tracking the area of Venus north of 30 north latitude by radar, and sent back detailed photographs of features as small as 0.9 mile (1.5 km) wide. Venera 15 finished its mapping in July, 1984; Venera 16, in April, 1984.

On August 10, 1990, the United States spacecraft Magellan began its orbit and imaged features as small as 330 feet (100 m) wide on its radar. Scientists in charge lowered the craft into Venus's atmosphere on October 11, 1994 so that it could chart conditions there before crashing.

The Venus Express probe, designed by the European Space Agency to scrutinize all aspects of the planet's atmosphere and volcanic activity on the surface was launched on November 9, 2005 and sent into orbit on April 11, 2006. The craft found evidence of lightning in the planet's clouds and also corroborated earlier data suggesting that there was once more water vapor in Venus's atmosphere. Venus Express has also detected infrared light coming from the night sky of the planet, called nightglow. Since 2007, the probe has identified three of the chemicals responsible for the glow: oxygen (O2), hydroxyl (OH), and nitric oxide (NO). However, the nature of the chemical reactions that cause nightglow is still unknown.