Venus clouds	Venus gulamons	Venus
Venus: Hubble pic	Venus: Pioneer pic	Venus: Mariner pic

VENUS (PLANET)

Venus, the second planet from the Sun, is Earth's near twin in
the solar system.  The two planets have approximately the same
diameter, mass, density, and probably composition as well.
Venus differs greatly from the Earth in having a thick and
highly reflective cloud layer that perpetually hides the
planet's surface and causes extremely high surface temperatures 
and pressure.  The surface is known to be far too hot to harbor 
any life.  Venus lacks a satellite and may never have had one,
or else any satellites that existed early in the planet's
history crashed into the surface long ago.
 
Venus has been visited by numerous space probes, beginning with 
the flight past the planet by the U.S. MARINER 2 in 1962.
Mariner 5 flew past Venus in 1967 and Mariner 10 in 1974.  In
1978 two U.S. spacecraft were sent there.  PIONEER Venus 1
orbited the planet, collecting data on surface heights and
returning radar images, while Pioneer Venus 2 released three
probes to the surface to collect atmospheric data.  The most
recent U.S. mission, Magellan, launched from the Space Shuttle
Atlantis on May 4, 1989, entered Venus orbit on Aug. 10, 1990.
By 1992 it had mapped 95% of the planet's surface.  The former
Soviet Union has also conducted an active Venus-exploration
program, including orbiters, landers, and balloon studies of
the planet's atmosphere (see VENERA).
 
PHYSICAL CHARACTERISTICS
 
Venus lies an average distance of about 108.2 million km (67
million mi) from the Sun.  It has a diameter of about 12,100 km 
(7,520 mi), and its mass and density are, respectively, about
81% and 90% that of the Earth.  Venus completes one trip around 
the Sun in 224.7 Earth-days, but Venus-days are quite different 
because the two planets rotate on their axes in very different
ways.  To determine the rotation rate of Venus, radar waves
from Earth have been bounced off both limbs of the planet to
observe the DOPPLER EFFECT.  That is, the waves reflected
from the side of Venus moving toward the Earth decrease
slightly in wavelength, while those reflected from the other
side increase.  Venus was thereby found to rotate once on its
axis every 243 days.  It rotates in a retrograde, or
"backward," direction, compared to Earth and most of the other
objects in the solar system.
 
ATMOSPHERE

Composition
 
Earth-based telescopic studies of the reflected spectrum of
Venus have shown that its atmosphere is very different from
that of Earth.  Carbon dioxide makes up 98% of the planet's
atmosphere as compared to 0.03% for the Earth, where most
carbon dioxide is bound up in the oceans and in rocks such as
limestone.  In contrast, nitrogen is the most abundant gas in
the Earth's atmosphere but constitutes less than 2% of the
atmosphere of Venus.  The noble gases helium, neon, and argon
are also present in the Venus atmosphere at the level of a few
parts per million--which, for the latter two gases, are 2,700
and 500 times greater, respectively, than in Earth's
atmosphere.  These abundances indicate that the Sun may have
begun to heat up only after most of the planets formed,
according to present theories of the solar system.
 
The major components of the clouds of Venus are sulfuric acid
droplets and solid sulfur particles.  Probes from the Pioneer
Venus mission discovered that, below the clouds, the atmosphere 
contains about 0.1% to 0.4% water vapor and 60 parts per
million of free oxygen.  These components indicate that Venus
may have had abundant water at one point early in its history,
water that has since been lost.
 
Temperature and Pressure
 
Venus also differs from the Earth in the extremely high surface 
pressure that its atmosphere exerts because of its thickness.
The pressure comes to about 88 atmospheres--that is, 88 times
the surface pressure on Earth at sea level--or about 88 kg per
sq cm (1,300 lb per sq in).
 
The thick carbon dioxide atmosphere of Venus causes high
surface temperatures as well as high pressure, because of the
phenomenon known as the GREENHOUSE EFFECT.  The effect works in 
the following way.  Sunlight filters through the clouds and
strikes the surface, which heats up as it absorbs the visible
light.  As the surface heats up it radiates away the absorbed
heat as infrared radiation, which has a longer wavelength than
visible light and is easily absorbed by the lower atmosphere.
Thus, as in a greenhouse, heat is trapped near the surface.
When the carbon dioxide atoms radiate away the infrared
radiation, a large fraction goes back into the surface and
causes the temperature to rise even further.  This process
causes the surface temperature of Venus to be about 480 deg C
(900 deg F) hotter than the surface of the planet Mercury,
despite the fact that Mercury is much closer to the Sun.
 
Visibility and Cloud Layers
 
The Soviet probes that were sent to land on Venus were equipped 
with artificial lighting, in case the thick cloud cover of the
planet prevented much illumination from reaching the surface.
The light there, however, proved to be more than adequate for
returning images of the rocky terrain.  This indicated that the 
lower atmosphere is clear.  From about 31 to 48 km (19 to 30
mi) above the surface, however, the atmosphere is hazy because
of the sulfuric acid particles it contains.  The next 3 km (2
mi) above this lies the densest layer of the atmosphere of
Venus, dominated by large sulfur particles.  A relatively clear 
layer lies above this one.  It is overlain in turn, at a height 
of from 52 to 58 km (32 to 36 mi), by another cloud layer
consisting of sulfuric acid droplets and liquid and solid
sulfur particles.  A third layer of clouds, from 65 to 70 km
(40 to 43 mi) above the surface, is made up again of sulfuric
acid droplets.  The uppermost, hazy atmospheric layer,
extending another 10 km (6 mi) above the clouds, probably
consists of water vapor or ice crystals.
 
Winds
 
Winds of about 13 km/h (8 mph) have been measured at the
surface of Venus.  These are relatively gentle, but they are
still strong enough to move sand-size particles around.  Faster 
winds exist at higher altitudes in the atmosphere.  At
distances above about 50 km (30 mi) above the surface, the
atmosphere of Venus has a four-day rotation period.  This is
called the superrotation of the atmosphere.  Winds moving at
175 km/h (110 mph) have been recorded at a height of 45 km (28
mi) above the surface, and strong vertical winds have also been 
detected.  A number of the Venus probes have made measurements
that have been interpreted as indicating lightning activity in
the planet's atmosphere.

GEOLOGY
 
Scientists have assumed that the geologies of Venus and Earth
should be fairly similar.  The reasoning behind this is that
the heat that drives geological activity is generated mostly by 
the decay of radioactive elements, so that the amount of
geological activity that a planet exhibits is believed to
increase with increasing size.  Thus the similarities in mass,
size, and probable overall composition of Venus and Earth would 
indicate that the planets would have similar rock compositions
and surface geology as well.  In fact, the evolutionary path
that Venus has traveled not only produced a greenhouse
atmosphere but also resulted in some major geological
differences between the two planets.  The most significant is
the lack of evidence for plate tectonics on Venus.
 
Surface Composition
 
The composition of the surface has been measured at several
landing sites.  Thus Venera 8 detected a composition similar to 
granitic rocks of Earth's continents, along with the presence
of radioactive isotopes of uranium, thorium, and potassium.
Venera 9 and Venera 10 found basaltic-type rock compositions at 
two other sites.  Basalt is a volcanic rock that occurs on
Earth on the ocean basins and at Hawaiian-type volcanoes, as
well as in the smoother areas of the Moon that are known as
mares, or "seas." Venera 13 and Venera 14 also measured the
electrical conductivity of Venus rocks, drilled holes to
retrieve material from deeper surface layers, and scooped up
samples of the soil.  These probes again detected Earth-like
basaltic compositions.
 
Surface Features
 
The surface of Venus has many features similar to those on
Earth.  Most of the planet is dominated by relatively low-lying 
plains characterized by abundant volcanic structures, but there 
are also continent-size highland regions with mountain ranges,
volcanoes, and rift systems.  The largest highland region,
named Aphrodite Terra, straddles the equatorial region of
Venus.  It is about the size of Africa.  The highest point on
the planet, lying in the Maxwell mountains, rises about 11 km
(7 mi) above the average surface elevation.  It might be noted
that the Maxwell range is named after physicist James Clerk
Maxwell.  It is the only Venus feature named after a man.  All
other features on the planet are named after women, following
planetary nomenclature rules established by the International
Astronomical Union.
 
The density of impact craters on Venus is similar to their
density on Earth, indicating a surface age of about 800 million 
years and suggesting that the surface may still be undergoing
active geological processes such as quakes and erupting
volcanoes.  Many of the impact craters have long outflows
extending from the rim, probably representing large volumes of
very fluid material produced during the extremely energetic
events.  Because the dense atmosphere screens out smaller
meteors and other impactors, the smallest impact crater on
Venus is about 3 km (1.9 mi) wide.
 
Mountain ranges, long faults, and deep troughs on the planet
indicate that horizontal surface movement has occurred there.
Among the most complex types of terrain identified on Venus are 
raised regions of the surface that are characterized by
complexly intersecting sets of faults and ridges.  These
regions, called tesserae from the Latin word for "tile," may
result from long episodes of compression and extension of the
surface.  Larger circular features on the surface, more than
300 km (185 mi) across, are surrounded by concentric ridges and 
troughs.  These features, called coronae from the Latin word
for "crown," are hot spots formed over hot material rising from 
deep in the planet's interior, in a way similar to the
formation of the Hawaiian islands on Earth.

Volcanism and Wind Activity
 
Volcanic features on Venus range from large volcanoes similar
to Earth's Mauna Loa, to abundant small shield volcanoes like
those found on Earth's ocean floors.  Flat-topped domes
resembling pancakes were produced by eruptions of very thick,
viscous lava.  This possibly indicates more evolved or
silica-containing magmas or erupted materials.  Sinuous
channels have been carved on the planet by very fluid lava.
One of them extends for a distance of more than 6,000 km (3,728 
mi) and is the longest such channel yet identified in the solar 
system.
 
Wind-produced features on Venus include long streaks that in
many places extend behind obstacles such as low-lying ridges
and small volcanic shields on the planet's plains.  Sand dunes
occur near several of the large impact craters, most sand-size
material on Venus being produced when such craters are formed.
 
Lack of Tectonics
 
As has been noted, images thus far returned from Venus indicate 
that although the surface of the planet has been very
geologically active, it appears to lack plate tectonics.
Instead most of the heat generated within Venus drives the
volcanic activity abundant there.  Features found at plate
boundaries on Earth also occur on Venus, such as the deep
asymmetrical troughs typical of subduction zones and the rifts
typical of spreading centers.  They do not, however, appear to
link up in an integrated system as on Earth.  The reason for
this may be the lack of water and the higher surface
temperatures of Venus.  Continued study of the images that are
being returned by Magellan--more data than all previous U.S.
planetary missions combined--will enable scientists to continue 
to compare Venus and Earth and to understand more fully how
planets evolve.
 

Bibliography:  Beatty, J. K., "Venus in the Radar Spotlight,"
Sky & Telescope, July 1991;  Bunge, Robert, "Lifting the Veil," 
Ad Astra, April 1992;  Burgess, Eric, Venus: An Errant Twin
(1985);  Goldman, S. J., "Venus Unveiled," Sky & Telescope,
March 1992;  Hunten, D. M., et al., eds., Venus (1983);
Russell, C. T., ed., Venus Aeronomy (1991);  Time-Life Books
Editors, The Inner Planets (1989);  Vogt, Gregory, Magellan and 
the Radar Mapping of Venus (1992).

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