Learn about Venus

Venus in astronomy, second major planet from the Sun. Named for the Roman goddess of love and beauty, it is, after the Moon, the most brilliant natural object in the nighttime sky. Venus comes closer to the Earth than any other planet, approaching to within about 42,000,000 km (26,100,000 miles) at inferior conjunction--i.e., when Venus comes between the Earth and the Sun.

Venus revolves around the Sun at a mean distance of 107,500,000 km (66,650,000 miles) in a nearly circular orbit. As viewed from the Earth, and as it travels around the Sun, Venus undergoes phase changes similar to those of the Moon. It completes one orbital revolution in 225 Earth days but goes through one cycle of phases in 584 days.

Venus is a near twin of the Earth in size and mass. Its diameter is about 12,103 km (7,516 miles) as compared to 12,756 km (7,921 miles) for the Earth; and its mass is approximately 0.81 of the latter.

The two planets, however, bear little resemblance to one another in other respects. Venus is completely enveloped by a thick layer of clouds consisting chiefly of droplets of concentrated sulfuric acid; this main cloud deck extends from an altitude of about 45 km (28 miles) up to nearly 70 km (43 miles). Thin hazes extend a few kilometres below the deck's lowest layers and about 20 km (12 miles) above its highest ones. Some cloud-top regions appear dark in ultraviolet light, possibly owing to the presence of sulfur dioxide, chlorine, or solid sulfur.

The composition of Venus' atmosphere is quite different from that of the terrestrial atmosphere. Spacecraft measurements indicate that carbon dioxide comprises more than 96 percent of the constituent matter, which accounts for the extreme density of the Venusian atmosphere. Nitrogen makes up another 3.5 percent; trace amounts of argon, water vapour, carbon monoxide, helium, and sulfur dioxide are also present. The dense atmosphere, together with the thick cloud cover, traps incoming solar energy so efficiently that Venus has a surface temperature of about 460C (860F)--the highest of any planet in the solar system. This high temperature is accompanied by an equally high surface pressure of about 90 bars, or 90 times the atmospheric pressure at the Earth's surface.

Detailed information on the appearance and composition of Venus' surface was obtained by the Soviet Venera spacecraft in the 1970s and '80s. Photographs revealed plains strewn with flat, slabby rocks as well as a darker, fine-grained soil. The surface composition measured by Venera 13 and 14 suggested a composition similar to basalts found on Earth.

Radar mapping of Venus by the Venera mission as well as by the U.S. Pioneer and Magellan missions has shown a geologically complex and diverse surface topography. Most of the planet consists of gently rolling plains, although there are several lowland areas and two continent-sized highlands: Ishtar Terra and Aphrodite Terra. The Maxwell Montes are a huge mountain chain on eastern Ishtar that rise more than 10 km (6 miles) above the average Venusian surface elevation. Other surface features include impact craters, rift valleys, novae (radiating patterns of faults and fractures atop a topographic rise), coronae (oval patterns of faults, fractures, and ridges, with a raised outer rim and a depressed interior), tesserae (large, elevated regions composed of rugged, complex terrain), shield volcanoes, and long, sinuous lava-flow channels. Many of these features are associated with volcanic activity. Overall, the Venusian topography does not suggest plate-tectonic activity of the kind thought to have shaped much of the Earth's surface. However, there is strong evidence that Venus, like the Earth, is presently a geologically active body.

Unlike most of the other planets, Venus rotates in retrograde (from east to west), slowly turning on its axis once every 243 days. The axis itself is tilted only 3 degrees from the plane of the planet's orbit around the Sun. These facts indicate that seasonal changes must be very slight. Solar heating and the slow rotation of Venus result in an atmospheric circulation in which air rises owing to heating at the equator, migrates sluggishly at high altitudes to the poles, descends there as it cools, and then returns toward the equator along the surface. Such a simple pattern would be completely unstable on the rapidly rotating Earth. Even on Venus, remarkable instabilities appear in the form of intense wave patterns and modify the simple picture. Also, the rotation rate of the atmosphere increases with height from the surface to the upper atmosphere. Thus, features in the clouds have been observed to travel completely around Venus' equator in about four Earth days.