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Venus, the second planet from the Sun. The
planet is named after Venus, the Roman goddess of love and beauty is Earth's closest planetary neighbor. Its similar size
and composition have caused it to be called a "sister" to Earth. But Venus is one of the most hostile environments
in the solar system. A noxious atmosphere, high temperatures, and crushing surface pressure make Mars and the Moon seem wonderful
by comparison. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of -4.6,
bright enough to cast shadows. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun:
its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after
sunset, for which reason it is often called the Morning Star or the Evening Star.
Venus is covered with an opaque
layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. Venus
has the densest atmosphere of all the terrestrial planets, consisting mostly of carbon dioxide, as it has no carbon cycle
to lock carbon back into rocks and surface features, nor organic life to absorb it in biomass. A younger Venus is believed
to have possessed Earth-like oceans, but these totally evaporated as the temperature rose, leaving a dusty dry desertscape
with many slab-like rocks. The water has most likely dissociated, and, because of the lack of a planetary magnetic field,
the hydrogen has been swept into interplanetary space by the solar wind. The atmospheric pressure at the planet's surface
is 92 times that of the Earth.
The Venusian surface was a subject of speculation until some of its secrets were
revealed by planetary science in the twentieth century. It was finally mapped in detail by Project Magellan in 1990–91.
The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate that there have been some
recent eruptions. However, the absence of evidence of lava flow accompanying any of the visible caldera remains an enigma.
The planet has few impact craters, demonstrating that the surface is relatively young, approximately half a billion years
old. There is no evidence for plate tectonics, possibly because its crust is too strong to subduct without water to make it
less viscous. Instead, Venus may lose its internal heat in periodic massive resurfacing events.
Venus is one of
the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is very similar
to the Earth, and is often described as Earth's "sister" or "twin". The diameter of Venus is only 650
km less than the Earth's, and its mass is 81.5% of the Earth's. However, conditions on the Venusian surface differ radically
from those on Earth, due to its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide,
with most of the remaining 3.5% being nitrogen.
Without seismic
data or knowledge of its moment of inertia, there is little direct information about the internal structure and geochemistry
of Venus. However, the similarity in size and density between Venus and Earth suggests that they share a similar internal
structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid because the two planets
have been cooling at about the same rate.The slightly smaller size of Venus suggests that pressures are significantly lower
in its deep interior than Earth. The principal difference between the two planets is the lack of plate tectonics on Venus,
likely due to the dry surface and mantle. This results in reduced heat loss from the planet, preventing it from cooling and
providing a likely explanation for its lack of an internally generated magnetic field.
About 80% of the Venusian
surface is covered by smooth volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains.
Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and
the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of
love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is
11 km above the Venusian average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess
of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults
covers much of this area.
Map of Venus, showing the elevated "continents" in yellow: Ishtar Terra at
the top and Aphrodite Terra just below the equator to the rightAs well as the impact craters, mountains, and valleys commonly
found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called
farra, which look somewhat like pancakes and range in size from 20–50 km across, and 100–1,000 m high; radial,
star-like fracture systems called novae; features with both radial and concentric fractures resembling spider webs, known
as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. These features are volcanic
in origin. Most Venusian surface features are named after historical and mythological women. Exceptions are Maxwell Montes,
named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio and Ovda Regio. The former three features were
named before the current system was adopted by the International Astronomical Union, the body that oversees planetary nomenclature.
Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes
as Earth, and it possesses some 167 massive volcanoes that are over 100 km across. The only volcanic complex of this size
on Earth is the Big Island of Hawaii. However, this is not because Venus is more volcanically active than Earth, but because
its crust is older. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and
has an average age of about 100 million years, while the Venusian surface is estimated to be about 500 million years old.
Several lines of evidence point to ongoing volcanic activity on Venus. During the Soviet Venera program, the Venera
11 and Venera 12 probes detected a constant stream of lightning, and Venera 12 recorded a powerful clap of thunder soon after
it landed. The European Space Agency's Venus Express recorded abundant lightning in the high atmosphere. While rainfall drives
thunderstorms on Earth, there is no rainfall on the surface of Venus (though it does rain sulfuric acid in the upper atmosphere
that evaporates around 25 km above the surface). One possibility is that ash from a volcanic eruption was generating the lightning.
Another piece of evidence comes from measurements of sulfur dioxide concentrations in the atmosphere, which were found to
drop by a factor of 10 between 1978 and 1986. This may imply that the levels had earlier been boosted by a large volcanic
eruption.
Impact craters on the surface of Venus (image reconstructed from radar data)There are almost a thousand
impact craters on Venus evenly distributed across its surface. On other cratered bodies, such as the Earth and the Moon, craters
show a range of states of degradation. On the Moon, degradation is caused by subsequent impacts, while on Earth, it is caused
by wind and rain erosion. However, on Venus, about 85% of craters are in pristine condition. The number of craters together
with their well-preserved condition indicates that the planet underwent a global resurfacing event about 500 million years
ago, followed by a decay in volcanism. Earth's crust is in continuous motion, but it is thought that Venus cannot sustain
such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which
mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million
years, subduction occurs on an enormous scale, completely recycling the crust.
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Venusian craters range from 3 km to 280
km in diameter. There are no craters smaller than 3 km, because of the effects of the dense atmosphere on incoming objects.
Objects with less than a certain kinetic energy are slowed down so much by the atmosphere that they do not create an impact
crater. Incoming projectiles less than 50 meters in diameter will fragment and burn up in the atmosphere before reaching the
ground.
Cloud structure in The Venusian atmosphere, revealed by ultraviolet observationsVenus has an extremely
dense atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The atmospheric mass is 93 times
that of Earth's atmosphere while the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure
equivalent to that at a depth of nearly 1 kilometer under Earth's oceans. The density at the surface is 65 kg/m³ (6.5%
that of water). The CO2-rich atmosphere, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect
in the Solar System, creating surface temperatures of over 460 °C (860 °F). This makes the Venusian surface hotter
than Mercury's which has a minimum surface temperature of -220 °C and maximum surface temperature of 420 °C, even
though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. The
surface of Venus is often said to resemble Hell. Studies have suggested that several billion years ago the Venusian atmosphere
was much more like Earth's than it is now, and that there were probably substantial quantities of liquid water on the surface,
but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of
greenhouse gases in its atmosphere. Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the
temperature of the Venusian surface does not vary significantly between the night and day sides, despite the planet's extremely
slow rotation. Winds at the surface are slow, moving at a few kilometers per hour, but because of the high density of the
atmosphere at the Venusian surface, they exert a significant amount of force against obstructions, and transport dust and
small stones across the surface. This alone would make it difficult for a human to walk through, even if the heat were not
a problem.
Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.
These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of the
Venusian surface in visible light. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the
Venusian surface is not as well lit. Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth
days. Venusian winds move at up to 60 times the speed of the planet's rotation, while Earth's fastest winds are only 10% to
20% rotation speed.
The surface of Venus is effectively isothermal; it retains a constant temperature not only
between day and night but between the equator and the poles. The planet's minute axial tilt (less than three degrees, compared
with 23 degrees for Earth), also minimizes seasonal temperature variation. The only appreciable variation in temperature occurs
with altitude. In 1995, the Magellan probe imaged a highly reflective substance at the tops of the highest mountain peaks
which bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit
at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where
it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from
elemental tellurium to lead sulfide (galena).
The clouds of Venus are capable of producing lightning much like
the clouds on Earth. The existence of lightning had been controversial since the first suspected bursts were detected by the
Soviet Venera probes. However, in 2006–07 Venus Express clearly detected whistler mode waves, the signatures of lightning.
Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of
that on Earth. In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of
the planet.
In 1980, the Pioneer Venus Orbiter found that the Venusian magnetic field is much weaker than that
of Earth's. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an
internal dynamo in the core like the one inside the Earth. Venus's small induced magnetosphere provides negligible protection
to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.
Venus rotates about its axis
in the opposite direction to most planets in the Solar SystemVenus orbits the Sun at an average distance of about 108 million
kilometers (about 0.7 AU), and completes an orbit every 224.65 days. Although all planetary orbits are elliptical, Venus is
the closest to circular, with an eccentricity of less than 0.01. When Venus lies between the Earth and the Sun, a position
known as "inferior conjunction", it makes the closest approach to Earth of any planet, lying at an average distance
of 41 million km during inferior conjunction. The planet reaches inferior conjunction every 584 days, on average. Due to the
decreasing eccentricity of both orbits of Earth and Venus, the minimum distances will become greater. From the year 1 to 5383,
there are 526 approaches less than 40 million km; then there are none for about 60,200 years. During periods of greater eccentricity
Venus can come as close as 38.2 million km.
If viewed from above the Sun's north pole, all the planets are orbiting
in a counter-clockwise direction; but while most planets also rotate counter-clockwise, Venus rotates clockwise in "retrograde"
rotation. The present rotation period of Venus represents an equilibrium state between gravitational tidal locking by the
Sun that tends to slow the rotation rate, and an atmospheric tide created by the solar heating of the thick Venusian atmosphere.
When it formed from the solar nebula, Venus may have begun with a different rotation period and obliquity, then migrated to
the current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere.
This change in the rotation period probably took place over the course of billions of years.
Venus rotates once
every 243 Earth days—by far the slowest rotation period of any of the major planets. At the equator, the Venusian surface
rotates at 6.5 km/h; on Earth, the rotation speed at the equator is about 1,670 km/h. A Venusian sidereal day thus lasts longer
than a Venusian year (243 versus 224.7 Earth days). However, because of the retrograde rotation, the length of a solar day
on Venus is significantly shorter than the sidereal day. To an observer on the surface of Venus the time from one sunrise
to the next would be 116.75 Earth days (making the Venusian solar day shorter than Mercury's 176 Earth days). Additionally,
the Sun would appear to rise in the west and set in the east. As a result of Venus's relatively long solar day, one Venus
year is about 1.92 Venus days long. Size: 6th largest planet - 12,104 kilometers - 0.9488 Earths across
Calendar: 1 Venus year = 0.615 Earth
years or 224.7 Earth days; 1 Venus day = 116.75 Earth days
Orbit: 108,210,000 kilometers - 0.723 times Earth's orbit
Axial tilt: 177.36 degrees (or, 2.64 degrees away from being upside down, 20.81 degrees less than Earth)
Number
of moons: none
Venus (Click on photo's
to enlarge)
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