Venus is the second planet from the Sun and almost a twin of Earth in size. It is the brightest object in the sky after the Sun and the Moon, and is popularly known as the Evening Star or the Morning Star depending on when it is on view.
Venus has a slow retrograde (east-to-west) spin, opposite in direction to that of every other planet in the Solar System and presumably the result of a massive ancient collision. Venus is also the hottest planet – a result of of its dense atmosphere, predominantly of carbon dioxide, which traps the heat radiating from the surface (see Venus, atmosphere). The runaway greenhouse on Venus gives rise to temperatures high enough to melt lead, while the weight of the atmosphere results in a surface pressure 90 times greater than that of Earth (equivalent to standing on the ocean floor at a depth of about 900 meters, or 3000 feet). Due to the thermal inertia and convection of its atmosphere, Venus' surface temperature, averaging about 464 °C (867°F) alters very little between the night and day sides of planet despite the extremely slow rotation.
|Venus and Earth compared|
|distance from Sun||108,200,000 km||149,600,000 km|
|diameter||12,103.6 km||12,756.6 km|
|mass||0.82 Earth masses||1.0 earth mass|
|axial rotation period||243 Earth days (retrograde)||23 hour 53 minutes|
|lenth of year||224.7 Earth days||365 days|
|atmosphere||96% carbon dioxide
|mean surface temperature||464°C||15°C|
|global magnetic field||no||yes|
Wind and acid
Measurements of Venus' atmosphere and its cloud patterns show nearly constant high-speed zonal winds, about 100 meters oer second at the equator. The winds decrease toward the poles so that the atmosphere at cloud-top level rotates almost like a solid body. The wind speeds at the equator correspond to Venus's rotation period of four to five days at most latitudes. Layers of haze in the atmosphere contain small aerosol particles, possibly droplets of sulfuric acid. A concentration of sulfur dioxide above the cloud tops has been seen to decrease since 1978. The source of sulfur dioxide at this altitude is unknown, though one possibility is injection by volcanic explosions.
The volcanoes of Venus
It's an open question whether there are still any active volcanoes on Venus. Radar images from the Magellan probe provided compelling evidence that the planet has been dominated by volcanism on a global scale in the past. The photos also showed that the second highest mountain, Maat Mons, appears to be covered with fresh lava. Venus's surface seems to consist largely of recently solidified basalt, with very few meteor craters, which suggests some recent global resurfacing event. According to one theory, although Venus lacks mobile plate tectonics of the kind found on Earth, it undergoes massive volcanic upwellings at regular intervals that inundate its surface with fresh lava.
Earth and Venus are similar in density and chemical composition, and both have relatively young surfaces. That on Venus is no more than 300 million to 500 million years old and has been shaped by volcanism, impacts, and deformation of the crust. Approximately 70% of the surface is covered by rolling hills, 20% by lowland plains, and 10% by highlands. Some weathering takes place due to the corrosive effects of the acid rain and to gentle winds of a few km per hour, which are just strong enough to carry small dust grains. No craters exist on Venus with diameters less than about 1.5 to 2 kilometers, since meteors that would create smaller impacts burn up in the dense atmosphere.
|Photo by Venera 13 from the surface
Venus has two major continent-like plateaus: the northern plateau of Ishtar Terra, an elevated, lava-filled basin bigger than the continental United States which has the planet's highest mountains, the Maxwell Montes, and the larger, Africa-sized southern plateau of Aphrodite Terra. These "continents" make up only 8% of the planets surface compared to 25% for Earths. Venus also has an extreme lowland basin, Atalanta Planitia, about the size of Earths North Atlantic Ocean basin, with a smooth surface that resembles the mare basins of the Moon. The lowest point on the planet is in the rift valley Diana Chasma in Aphrodite Terra, which has much in common with Valles Marineris on Mars; it lies up to 2.9 kilometers below the planets mean surface level and is deeper than any comparable feature on Earth. Another channel, known as Baltis Vallis, is 6,800 kilometers long (slightly longer than the Nile), averages about 1.8 kilometers wide, and presents a riddle. High-temperature lava is unlikely to have caused such a long-distance flow, and there are no known substances that could remain liquid long enough under the planet's atmospheric pressure and temperature to have carved out this snakelike feature.
Internally, Venus is probably much like the Earth with an iron core about 3,000 kilometers (1,900 miles) in radius and a molten rocky mantle comprising the planet's bulk. Although Venus lacks an intrinsic magnetic field, a pseudo-field is set up around the planet by its interaction with the solar wind.
|mean distance from Sun||108.2 million km (67.3 million mi., 0.723 AU)|
|equatorial diameter||12,103.6 km (7,522.4 miles), Earth × 0.949|
|mass (Earth = 1)||0.815|
|axial period||243.0 days|
|orbital period||224.7 days|
|number of moons||0|
|atmospheric composition||96% carbon dioxide, 3.5% nitrogen|
|surface temperature (mean)||464°C (867°F)|
|surface gravity (Earth=1)||0.903|
|escape velocity||10.36 km/s (37,296 km/hr, 23,180 mph)|