Venus

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 m, or 3000 ft.). 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
	Venus	Earth

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 3% nitrogen	77% nitrogen 21% oxygen
mean surface temperature	464°C	15°C
moons	0	1
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 m/s 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.

Surface features

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 km, since meteors that would create smaller impacts burn up in the dense atmosphere.

Photo by Venera 13 from the surface of Venus

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 planet’s surface compared to 25% for Earth’s. Venus also has an extreme lowland basin, Atalanta Planitia, about the size of Earth’s 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 km below the planet’s mean surface level and is deeper than any comparable feature on Earth. Another channel, known as Baltis Vallis, is 6,800 km long (slightly longer than the Nile), averages about 1.8 km 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 km (1,900 mi.) 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
density	5.25 g/cm³
axial period	243.0 days
axial inclination	177.3°
orbital period	224.7 days
orbital inclination	3.4°
orbital eccentricity	0.007
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)
albedo	0.76

Related entries

atmosphere of Venus
transits of Venus
ashen light
life on Venus
Magellan
Mariner
Pioneer Venus
Venera
Venus Express
Venus probes
mythology of Venus (Encyclopedia of History)

Related category

• PLANETS AND MOONS

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