The Bringer of Peace
Venus is the second planet from the Sun and the sixth largest. Venus' orbit
is the most nearly circular of that of any planet, with an eccentricity of less
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 (above).
Venus' 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 closest approach. Whether this is a resonance effect or merely a
coincidence is not known.
Venus is sometimes regarded as Earth's sister planet. In some ways they are
-- 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 of the
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 but winds at the surface
are very 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 dry. 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 (right). 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 hundred
There are no small craters on Venus. It seems that small meteoroids burn up
in Venus' dense atmosphere before reaching the surface. Craters on Venus seem to
come in bunches indicating that large meteoroids that do reach the surface
usually break up in the atmosphere.
The oldest terrains on Venus seem to be about 800 million years old.
Extensive volcanism at that time wiped out the earlier surface including any
large craters from early in Venus' history.
Magellan's images show a wide variety of interesting and unique features
including pancake volcanoes (left) which seem to be eruptions of very thick lava
and coronae (right) which seem to be collapsed domes over large magma chambers.
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 on Earth.
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 unaided eye. Sometimes (inaccurately)
referred to as the "morning star" or the "evening star", it is by far the
brightest "star" in the sky. There are several Web sites that show the current
position of Venus (and the other planets) in the sky. More detailed and
customized charts can be created with a planetarium program such as Starry
Bill Arnett; last updated: 1999 Apr 27