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Venus

the second planet from the Sun, and the sixth largest

Venus as seen by the Pioneer probe
Venus as seen by the Pioneer probe

popular symbol for Venus
popular symbol for Venus

Venus was named for the Roman goddess of love and beauty, probably because it was the brightest of the planets known to the ancients. With a few exceptions, the surface features on Venus are named for female figures.

Facts and Figures
Mean distance from Sun 67,000,000 miles
Distance from Earth  
  Least 24,200,000 miles
  Greatest 162,000,000 miles
Equatorial diameter 7,688 miles
Period of rotation (day) 243 earth days (retrograde)
Period of revolution (year) 224.7 earth days
Orbital speed 21.8 miles per second
Mass (Earth=1) 0.82
Mean density (water=1) 5.3
Surface gravity (Earth=1) 0.91
Atmospheric pressure at surface (Earth=1 bar) 96 bars
Mean surface temperature 459 C (858 F)
Natural satellites 0

Venus' rotation is unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach.

Venus is similar enough to Earth that the two have often been called "sister planets." It is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass); both have few craters, indicating relatively young surfaces; and their densities and chemical compositions are similar.

View from Earth

Except for the Sun and Moon, Venus is the brightest object in the sky. The planet is called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus (or Lucifer). Because of the distances of the orbits of Venus and Earth from the Sun, Venus is never visible more than three hours before sunrise or three hours after sunset.

When viewed through a telescope on Earth, Venus exhibits phases like the Moon. Full Venus appears the smallest because it is on the far side of the Sun from Earth. Maximum brilliance is seen in the crescent phase. The phases and positions of Venus repeat every 1.6 years. Galileo's observation of the phases of Venus was important evidence in favor of Copernicus' heliocentric theory of the Solar System.

left: the phases of Venus as seen from Earth
the phases of Venus as seen from Earth

History of Exploration

Venus' complete cloud cover and deep atmosphere make it difficult to study from Earth. Spectroscopic examination of its atmosphere shows just a trace of water vapor. But, since only the top layers of Venus' atmosphere can be analyzed from Earth, it was long suspected that more water vapor existed in the lower levels. For centuries astronomers had several different theories about how much water, if any, the planet has.

Some believed that moisture has to exist because the Venusian clouds have the optical properties of earthly clouds, which are composed of water vapor. Some even suggested that the surface of Venus is one vast ocean from pole to pole, without any land at all except for a few isolated islands.

artist's conception of Venus as one vast ocean with only a few specks of land
one theory suggested that Venus was one vast ocean with only a few specks of land

Other astronomers believed that Venus is a bone-dry world with virtually no surface moisture at all. These astronomers said that the atmosphere looks cloudy because it is filled with dust whipped up by winds from the dry Venusian plains. Subsequent investigations of Venus by space probes have shown this second theory to be more correct than the first.

artist's conception of Venus as a bone-dry, desert world
another theory suggested that Venus is a bone-dry, desert world

The first spacecraft to visit Venus was Mariner 2, which was launched by the United States in 1962. It has subsequently been visited by more than twenty probes, including Mariner 5 (1967) and Mariner 10 (1974). The first spacecraft to land on another planet was the Soviet Union's Venera 7, which landed on Venus in 1970. The first photographs of the Venusian surface were sent back by Venera 9 and 10, both of which landed on Venus in 1975.

one of the photographs sent back by Venera 9
one of the photographs sent back by Venera 9

The United States sent two Pioneer Venus missions in 1978. Pioneer Venus 2 sent four probes to the surface, while the remaining craft explored the upper atmosphere. Pioneer Venus 1, an orbiter, continues to measure the upper atmosphere. The Magellan probe, launched toward Venus in 1989, began transmitting radar images of the planet in 1990.

Atmosphere

The pressure of Venus' atmosphere at the surface is 96 bars, about the same as the pressure at a depth of one kilometer in Earth's oceans. Venus' atmosphere is composed primarily of carbon dioxide (approximately 97%). There are also several layers of clouds composed of sulfuric acid which completely obscure our view of the surface. This dense atmosphere results in a surface temperature of about 460 C (860 F), hot enough to melt lead. Venus' surface is actually hotter than Mercury's, despite being nearly twice as far from the Sun. The planet has no detectable magnetic field.

Water and water vapor are extremely rare on Venus. Many scientists argue that Venus was subjected to a "greenhouse effect," which caused any oceans to evaporate into the atmosphere. The hydrogen atoms of the water molecules could have been lost to space and the oxygen atoms to the crust. Of course, it is also possible that Venus had very little water to begin with.

Upper-level winds circle Venus at 360 km/hr (225 mph). These winds cover the planet completely, blowing at virtually every latitude from equator to pole. Winds at the surface, however, are almost non-existent, blowing at no more than 3 to 18 km/hr (2 to 11 mph).

Surface Features

Most of Venus' surface consists of gently rolling plains with little relief. There are several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia, etc. There are two large highland areas: Ishtar Terra in the northern hemisphere (which is about the size of Australia) and Aphrodite Terra along the equator (which is about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surround by the highest mountains on Venus, including the enormous Maxwell Montes.

Radar images from the Magellan probe show that much of Venus' surface is covered by lava flows. There are several large shield volcanoes (similar to those found in Hawaii), and there is indication that Venus is still volcanically active, but only in a few hot spots. Probes have also revealed evidence that a great deal of tectonic activity has taken place on Venus, at least in the past. Results from the Magellan gravity data indicate that Venus' crust is stronger and thicker than had previously been assumed. Like Earth, convection in the mantle produces stress on the surface which is relieved in many relatively small regions instead of being concentrated at plate boundaries as is the case on Earth.

SEE ALSO
Galileo
Nicolaus Copernicus
Mercury

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This page was last updated on 02/26/2017.