Venus is the second
planet from the
Sun, orbiting it every 224.7
Earth days.
[11] The planet is named after the
Roman goddess of love and beauty. 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.
[13] 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 has
been referred to by ancient cultures as the Morning Star or Evening
Star.
Venus is classified as a
terrestrial planet
and is sometimes called Earth's "sister planet" owing to their similar
size, gravity, and bulk composition (Venus is both the closest planet to
Earth and the planet closest in size to Earth). However, it has been
shown to be very different from Earth in other respects. Venus is
shrouded by an opaque layer of highly reflective clouds of
sulfuric acid, preventing its surface from being seen from space in
visible light. It has the densest
atmosphere of the four terrestrial planets, consisting mostly of
carbon dioxide. The
atmospheric pressure at the planet's surface is 92 times that of Earth's. With a mean surface temperature of
735 K (462 °C; 863 °F), Venus is by far the hottest planet in the
Solar System. It has no
carbon cycle
to lock carbon back into rocks and surface features, nor does it seem
to have any organic life to absorb it in biomass. Venus is believed to
have previously possessed oceans,
but these vaporized as the temperature rose due to the
runaway greenhouse effect.
The water has most probably
photodissociated, and, because of the lack of a
planetary magnetic field, the free hydrogen has been
swept into interplanetary space by the
solar wind.
Venus's surface is a dry desertscape interspersed with slab-like rocks and periodically refreshed by
volcanism.
Physical characteristics
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 similar to the Earth, and is often described as Earth's "sister" or
"twin".
[17]
The diameter of Venus is 12,092 km (only 650 km less than the Earth's)
and its mass is 81.5% of the Earth's. Conditions on the Venusian surface
differ radically from those on Earth, owing 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.
[18]
Geography
The Venusian surface was a subject of speculation until some of its secrets were revealed by
planetary science in the 20th 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 there have been some recent eruptions.
[19][20]
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.
[21] 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.
[22]
The absence of evidence of
lava flow accompanying any of the visible
caldera remains an enigma. The planet has few
impact craters, demonstrating the surface is relatively young, approximately 300–600 million years old.
[23][24] In addition to 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.
[25]
Most Venusian surface features are named after historical and mythological women.
[26] 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.
[27]
The longitudes of physical features on Venus are expressed relative to its
prime meridian.
The original prime meridian passed through the radar-bright spot at the
center of the oval feature Eve, located south of Alpha Regio.
[28]
After the Venera missions were completed, the prime meridian was
redefined to pass through the central peak in the crater Ariadne.
[29][30]
Surface geology
Global radar view of the surface from Magellan radar imaging between 1990–1994
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 large volcanoes that are over 100 km across. The only
volcanic complex of this size on Earth is the
Big Island of Hawaii.
[25] 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,
[31] while the Venusian surface is estimated to be 300–600 million years old.
[23][25]
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.
[32] 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, which evaporates around 25 km above the
surface). One possibility is 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 the levels had earlier been
boosted by a large volcanic eruption.
[33]
Impact craters on the surface of Venus (image reconstructed from radar data)
Almost a thousand impact craters on Venus are 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. On Venus, about 85% of the craters are
in pristine condition. The number of craters, together with their
well-preserved condition, indicates the planet underwent a global
resurfacing event about 300–600 million years ago,
[23][24] followed by a decay in volcanism.
[34]
Whereas Earth's crust is in continuous motion, Venus is thought to be
unable to 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.
[25]
Venusian craters range from 3 km to 280 km in diameter. No craters
are 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, they do not create an impact crater.
[35] Incoming projectiles less than 50 meters in diameter will fragment and burn up in the atmosphere before reaching the ground.
[36]
Internal structure
Without seismic data or knowledge of its
moment of inertia, little direct information is available about the internal structure and
geochemistry of Venus.
[37] The similarity in size and density between Venus and Earth suggests 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.
[38]
The slightly smaller size of Venus suggests pressures are significantly
lower in its deep interior than Earth. The principal difference between
the two planets is the lack of evidence for
plate tectonics on Venus, possibly because its crust is too strong to
subduct without water to make it less
viscous.
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.
[39] Instead, Venus may lose its internal heat in periodic major resurfacing events.
Magnetic field and core
Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars in true-color.
In 1967,
Venera-4 found the Venusian
magnetic field is much weaker than that of Earth. This magnetic field is induced by an interaction between the
ionosphere and the
solar wind,
[60][61] 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.
[62]
The lack of an intrinsic magnetic field at Venus was surprising given
it is similar to Earth in size, and was expected also to contain a
dynamo at its core. A dynamo requires three things: A
conducting liquid, rotation, and
convection.
The core is thought to be electrically conductive and, while its
rotation is often thought to be too slow, simulations show it is
adequate to produce a dynamo.
[63][64]
This implies the dynamo is missing because of a lack of convection in
the Venusian core. On Earth, convection occurs in the liquid outer layer
of the core because the bottom of the liquid layer is much hotter than
the top. On Venus, a global resurfacing event may have shut down plate
tectonics and led to a reduced heat flux through the crust. This caused
the mantle temperature to increase, thereby reducing the heat flux out
of the core. As a result, no internal geodynamo is available to drive a
magnetic field. Instead, the heat energy from the core is being used to
reheat the crust.
[65]
One possibility is Venus has no solid inner core,
[66]
or its core is not currently cooling, so the entire liquid part of the
core is at approximately the same temperature. Another possibility is
its core has already completely solidified. The state of the core is
highly dependent on the concentration of
sulfur, which is unknown at present.
[65]
The weak magnetosphere around Venus means the
solar wind
is interacting directly with the outer atmosphere of the planet. Here,
ions of hydrogen and oxygen are being created by the dissociation of
neutral molecules from ultraviolet radiation. The solar wind then
supplies energy that gives some of these ions sufficient velocity to
escape the planet's gravity field. This erosion process results in a
steady loss of low-mass hydrogen, helium, and oxygen ions, while
higher-mass molecules, such as carbon dioxide, are more likely to be
retained. Atmospheric erosion by the solar wind most probably led to the
loss of most of the planet's water during the first billion years after
it formed. The erosion has increased the ratio of higher-mass
deuterium to lower-mass hydrogen in the upper atmosphere by a multiple of 150 times the ratio in the lower atmosphere.
[67]
Orbit and rotation
Venus orbits the Sun at an average distance of about 108 million kilometers (about 0.7 AU)
and completes an orbit every 224.65 days. Venus is the second planet
from the Sun and it revolves round the Sun approximately 1.6 times
(yellow trail) in Earth's 365 days (blue trail)
Venus orbits the Sun at an average distance of about 0.72
AU (108,000,000
km; 67,000,000
mi), and completes an orbit every 224.65 days. Although all
planetary orbits are
elliptical, Venus's orbit is the closest to
circular, with an
eccentricity of less than 0.01.
[2] 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 at an average distance of 41 million km.
[2] The planet reaches inferior conjunction every 584 days, on average.
[2] Owing to the
decreasing eccentricity of Earth's orbit,
the minimum distances will become greater over tens of thousands of
years. From the year 1 to 5383, there are 526 approaches less than
40 million km; then there are none for about 60,158 years.
[68] During periods of greater eccentricity, Venus can come as close as 38.2 million km.
[2]
All the planets of the Solar System orbit the Sun in a
counter-clockwise direction as viewed from above the Sun's north pole.
Most planets also rotate on their axis in a counter-clockwise direction,
but Venus rotates clockwise (called
"retrograde"
rotation) once every 243 Earth days—the slowest rotation period of any
planet. The equator of the Venusian surface rotates at 6.5 km/h, while
on Earth rotation speed at the equator is about 1,670 km/h.
[69] Venus's rotation has slowed down by 6.5 minutes per day since the Magellan spacecraft visited it 16 years ago.
[70] A Venusian
sidereal day thus lasts longer than a Venusian year (243 versus 224.7 Earth days). Because of the retrograde rotation, the length of a
solar day on Venus is significantly shorter than the sidereal day, at 116.75 Earth days (making the Venusian solar day shorter than
Mercury's 176 Earth days); one Venusian year is about 1.92 Venusian (solar) days long.
[12] To an observer on the surface of Venus, the Sun would rise in the west and set in the east.
[12]
Venus may have formed from the
solar nebula
with a different rotation period and obliquity, reaching to its current
state because of chaotic spin changes caused by planetary perturbations
and
tidal
effects on its dense atmosphere, a change that would have occurred over
the course of billions of years. The rotation period of Venus may
represent an equilibrium state between tidal locking to the Sun's
gravitation, which tends to slow rotation, and an atmospheric tide
created by solar heating of the thick Venusian atmosphere.
[71][72]
A curious aspect of the Venusian orbit and rotation periods is the
584-day average interval between successive close approaches to the
Earth is almost exactly equal to five Venusian solar days.
[73] However, the hypothesis of a spin–orbit resonance with Earth has been discounted.
[74]
Venus has no natural satellite,
[75] though the
asteroid 2002 VE68 presently maintains a
quasi-orbital relationship with it.
[76] In the 17th century,
Giovanni Cassini reported a moon orbiting Venus, which was named
Neith
and numerous sightings were reported over the following 200 years, but
most were determined to be stars in the vicinity. Alex Alemi's and
David Stevenson's 2006 study of models of the early Solar System at the
California Institute of Technology shows Venus likely had at least one moon created by a huge
impact event billions of years ago.
[77][78]
About 10 million years later, according to the study, another impact
reversed the planet's spin direction and caused the Venusian moon
gradually to
spiral inward[79]
until it collided and merged with Venus. If later impacts created
moons, these also were absorbed in the same way. An alternative
explanation for the lack of satellites is the effect of strong solar
tides, which can destabilize large satellites orbiting the inner
terrestrial planets.
[75]
Observation
Venus is always brighter than the brightest stars outside our solar system, as can be seen here over the Pacific Ocean
Phases of Venus and evolution of its apparent diameter
Venus is always brighter than any star (apart from the Sun). The greatest luminosity,
apparent magnitude −4.9,
[9] occurs during crescent phase when it is near the Earth. Venus fades to about magnitude −3 when it is backlit by the Sun.
[8] The planet is bright enough to be seen in a mid-day clear sky,
[80] and the planet can be easy to see when the Sun is low on the horizon. As an
inferior planet, it always lies within about 47° of the
Sun.
[10]
Venus "overtakes" the Earth every 584 days as it orbits the Sun.
[2] As it does so, it changes from the "Evening Star", visible after sunset, to the "Morning Star", visible before sunrise. While
Mercury, the other inferior planet, reaches a maximum
elongation
of only 28° and is often difficult to discern in twilight, Venus is
hard to miss when it is at its brightest. Its greater maximum elongation
means it is visible in dark skies long after sunset. As the brightest
point-like object in the sky, Venus is a commonly misreported "
unidentified flying object". U.S. President
Jimmy Carter reported having seen a UFO
in 1969, which later analysis suggested was probably the planet.
Countless other people have mistaken Venus for something more exotic.
[81]
As it moves around its orbit, Venus displays
phases in a
telescopic view like those of the
Moon: In the
phases of Venus,
the planet presents a small "full" image when it is on the opposite
side of the Sun. It shows a larger "quarter phase" when it is at its
maximum elongations from the Sun, and is at its brightest in the night
sky, and presents a much larger "thin crescent" in telescopic views as
it comes around to the near side between the Earth and the Sun. Venus is
at its largest and presents its "new phase" when it is between the
Earth and the Sun. Its atmosphere can be seen in a telescope by the halo
of light refracted around the planet.