orbit: 108,200,000 km (0.72 AU) from Sun
diameter: 12,103.6 km
mass: 4.869e24 kg
Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and
beauty. The planet is so named
probably because it is the brightest of the planets known to the ancients. (With a few exceptions, the surface
features on Venus are named for female figures.)
Venus has been known since prehistoric times. It is the brightest object
in the sky except for the Sun and the
Moon. Like Mercury, it was popularly thought to be two separate bodies: Eosphorus as the morning star and
Hesperus as the evening star, but the Greek astronomers knew better.
Since Venus is an inferior planet, it shows phases when viewed with a telescope
from the perspective of
Earth. Galileo's observation of this phenomenon was important evidence in favor of Copernicus's
heliocentric theory of the solar system.
The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface (left). Most recently, the orbiting US spacecraft Magellan produced detailed maps of Venus' surface using radar (right).
rotation is somewhat unusual in that it is both very slow (243 Earth days
per Venus day, slightly
longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are
synchronized such that it always presents the same face toward Earth when the two planets are at their
is sometimes regarded as Earth's sister planet. In some ways they are very
-- Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).
-- Both have few craters indicating relatively young surfaces.
-- Their densities and chemical compositions are similar.
Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and
might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it
is radically different from Earth.
The pressure of Venus' atmosphere at the surface is 90 atmospheres (about
the same as the pressure
at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers
of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view
surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus' surface
temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus' surface is actually
hotter than Mercury's despite being nearly twice as far from the Sun.
There are strong (350 kph) winds at the cloud tops winds at the surface
slow, no more than a few
kilometers per hour.
Venus probably once had large amounts of water like Earth but it all boiled
away. Venus is now quite
Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about
Earth by learning why the basically similar Venus turned out so differently.
Most of Venus' surface consists of gently rolling plains with little relief.
There are also several broad
depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar
Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about
the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is
surrounded by the highest mountains on Venus including the enormous Maxwell Montes.
Data from Magellan's imaging radar shows that much of the surface of Venus
is covered by lava
flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons.
Recently announced findings indicate that Venus is still volcanically active, but only in a few hot
spots; for the most part it has been geologically rather quiet for the past few years.
There are no small craters on Venus. It seems that small meteoroidss burn
up in Venus' dense atmosphere
before reaching the surface. Craters on Venus seem to come in bunches indicating that large meteoriods
that do reach the surface usually break up in the atmosphere.
Extensive volcanism during Venus' hisory has wiped out the earlier
surface including any
from early in Venus' history.
Magellan's images show a wide variety of interesting and unique features
which seem to be eruptions of very thick lava and coronae which seem to be collapsed domes over large
The interior of Venus is probably very similar to that of Earth: an iron
core about 3000
km in radius, a molten
rocky mantle comprising the majority of the planet. Recent results from the
Magellan gravity data indicate that Venus' crust is stronger and thicker
than had previously been assumed. Like Earth, convection in the mantle
produces stress on the surface which is relieved in many relatively small
regions instead of being concentrated at plate boundaries as is the case
Venus has no magnetic field, perhaps because of its slow rotation.
Venus has no satellites, and thereby hangs a tale.
Venus is usually visible with the naked eye. Sometimes (inaccurately) refered
to as the "morning star" or the
"evening star", it is by far the brightest "star" in the sky.
Obliquity (tilt of
Orbit inclination (degrees) 3.39
Orbit eccentricity (deviation from circular) 0.007
Mean surface temperature (K) 726
GM (x 106 km3/s2)
Bond albedo 0.72
Visual geometric albedo 0.65
Visual magnitude V(1,0) -4.40
Solar irradiance (W/m^2) 2660
Black-body temperature (K) 238.9
Topographic range (km) 15
Moment of inertia (I/MR^2) 0.33
J2 (x 10^-6) 4.458
Highest point on
-Maxwell Montes (17 km above mean planetary radius)
Surface materials basaltic rock and altered materials
Semimajor axis (106
Sidereal orbit period (days) 224.701
Tropical orbit period (days) 224.695
Perihelion (106 km) 107.5
Aphelion (106 km) 108.9
Synodic period (days) 583.92
Mean orbital velocity (km/s) 35.02
Orbit inclination (deg) 3.4
Orbit eccentricity 0.0068
Sidereal rotation period (hrs) 5832.5
Obliquity to orbit (deg) 177.3
Surface Density: ~65. kg/m3
Scale height: 15.9 km
Average temperature: 737 K
Diurnal temperature range: ~0
Wind speeds: 0.3 to 1.0 m/s (surface)
Mean molecular weight: 43.45 g/mole
Atmospheric composition (near surface, by volume):
Major: 96.5% Carbon Dioxide (CO2), 3.5% Nitrogen (N2)
Minor (ppm): Sulfur Dioxide (SO2) - 150, Argon (Ar) - 70, Water (H2O) -
Carbon Monoxide (CO) - 17, Helium (He) - 12, Neon (Ne) - 7