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The Planets .... brought to you by QuasArt Web Designs.
V E N U S (2nd planet from the Sun)
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.)
CHAPTERS
I II III IV V VI
NO MOONS
INTRODUCTION
I
In One
of the planets in the solar system, the second in distance from the
sun. Except for the sun and the moon, Venus is the brightest object
in the sky. The planet is 
called
the morning star when it appears in the east at sunrise, and the evening
star when it is in the west at sunset. In ancient
times the evening star was called Hesperus and the morning star Phosphorus
or Lucifer. Because of the distances of the orbits of Venus and earth
from the sun, Venus is never visible more than three hours before
sunrise or three hours after sunset. When viewed through a telescope,
the planet exhibits phases like the moon. Full Venus appears the smallest
because it is on the far side of the sun from earth. Maximum brilliance
(a stellar magnitude of -4.4, or 15 times the brightest star) is seen
in the crescent phase. The phases and positions of Venus in the sky
repeat with the synodic period of 1.6 years (see Time; Year). Transits
across the face of the sun are rare, occurring in pairs at intervals
of a little more than a century. The next two will be in 2004 and
2012.
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Venus
(left) is the brightest object in our sky, after the sun and moon.
Swirling clouds of sulfur and sulfuric acid obscure Venus's surface
and inhibited study of the planet from Earth until technology
permitted space vehicles, outfitted with probes, to visit it.
These probes determined that Venus is the hottest of the planets,
with a surface temperature of about 460° C (about 860° F). Scientists
believe that a greenhouse effect causes the extreme temperature,
hypothesizing that the planet's thick clouds and dense atmosphere
trap energy from the sun. Photo Researchers, Inc. |
EXPLORATION
II
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The
Magellan probe to Venus mapped almost all of the planet's surface
and provided scientists with a vast amount of data. The probe
is shown here in the cargo bay of the space shuttle Atlantis before
the shuttle's 1989 launch. Magellan reached Venus in 1990. N A S A "Magellan Spacecraft," |
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ATMOSPHERE
III
The surface temperature on Venus is highly uniform and is about 462° C (736 K/864° F); the surface pressure is 96 bars (compared with 1 bar for earth); the atmosphere of the planet consists of nearly all carbon dioxide (CO2). The cloud base is at 50 km (31 mi), and the cloud particles are mostly concentrated sulfuric acid. The planet has no detectable magnetic field. That 97 percent of Venus's atmosphere is CO2 is not as strange as it might seem; in fact, the crust of earth contains almost as much in the form of limestone. About 3 percent of the Venusian atmosphere is nitrogen gas (N2). By
contrast,
78 percent of earth's atmosphere is nitrogen. Water and water vapor
are extremely rare on Venus. Many scientists argue that Venus, being
closer to the sun, was subjected to a so-called runaway greenhouse effect,
which caused any oceans to evaporate into the atmosphere. The hydrogen
atoms of the water molecules could have been lost to space and the oxygen
atoms to the crust. Another possibility is that Venus had very little
water to begin with. The sulfuric acid of the clouds also has its analogue
on earth in a very thin haze in the stratosphere. On earth, sulfuric
acid is carried down in the rain and reacts with surface materials;
indeed, this so-called acid rain is damaging parts of the environment.
On Venus the acid evaporates at the cloud base and can only remain in
the atmosphere. The upper parts of the clouds, visible from earth and
from Pioneer Venus 1, extend as haze 70 to 80 km (44 to 50 mi) above
the planet surface. The clouds contain a pale yellow impurity, better
detected at near-ultraviolet wavelengths. Variations in the sulfur dioxide
content of the atmosphere may indicate active volcanism on the planet.
Certain cloud patterns and weather features can be discerned in the
cloud tops that give some information about wind motion in the atmosphere.
The upper-level winds circle the planet at 360 km/h (225 mph). These
winds cover the planet completely, blowing at virtually every latitude
from equator to pole. Tracking the motions of descending probes has
shown that, despite the scale of these high-speed, upper-level winds,
well more than half of Venus's tremendously dense atmosphere, near the
planet's surface, is almost stagnant. From the surface up to 10 km (6
mi) altitude, wind speeds are only about 3 to 18 km/h (2 to 11 mph).
The high-speed winds probably result from the transfer of momentum from
Venus's slow-moving, massive lower atmosphere to higher altitudes where
the atmosphere is less massive, so that the same momentum results in
a much higher velocity. The upper atmosphere and ionosphere have been
studied in great detail by Pioneer Venus 1, which passes through them
once each day. On earth this region is very hot; on Venus it is not,
even though Venus is closer to the sun. Surprisingly, the night side
of Venus is extremely cold. (Day-side temperatures are 40° C/104° F,
compared to night-side temperatures of -170° C/-274° F.) Scientists
suspect that strong winds blow from the day side toward the near vacuum
that is caused by the low temperatures on the night side. Such winds
would carry along light gases, such as hydrogen and helium, which are
concentrated in a night-side "bulge." On earth the ionosphere is isolated
from the solar wind by the magnetosphere. Venus lacks a magnetic field
of its own, but the solar wind seems to generate an induced magnetosphere,
probably by a dynamo action involving its own magnetic field.TOP OF PAGE
THE SURFACE OF VENUS
IV
This radar map shows the topography of the surface of the planet Venus.
Red and yellow indicate high continental elevations while blue
represents lower basins. Radar measures the time radiowaves take to
bounce off the surface of Venus and return. Higher elevations return the
signal sooner. Photo Researchers, Inc.
Venus rotates very slowly on its axis, and the direction is retrograde (opposite to that of earth). Curiously, Venus's synodic year is almost exactly five solar days long, with the result that the same side always faces earth when the two planets are closest. At such times, the side facing earth can be viewed and mapped by earth-based radio telescopes. In contrast to the very large antenna needed for earth-based radar mapping of Venus, a modest instrument of Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes and from earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet's far side as viewed from earth at closest approach. The more powerful radar aboard the Magellan spacecraft has revealed huge active volcanoes, large solidified lava flows, and a vast array of meteorite craters. The largest impact crater yet observed is almost 160 km (100 mi) across-the smallest about 5 km (3mi). Although the probe's radar could resolve even smaller craters, if any were present, Venus's dense atmosphere apparently prevents smaller meteorites from impacting the surface of the planet. The global survey and other probes have also revealed evidence that a great deal of tectonic activity has taken place on Venus, at least in the past. Such evidence includes ridges, canyons, a troughlike depression that extends across 1400 km (870 mi) of the surface, and a gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet probes have sent back photographs of the areas in which they set down, and have also measured the natural radioactivity of the rocks. The radioactivity resembles that of granite and strongly suggests that the material of Venus is differentiated, or chemically separated, by volcanic activity. Angular rocks that are visible in the Soviet pictures also suggest the existence of geologic activity that would counteract the forces of erosion.
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RECOMMENED READING
V
Alexander, Arthur F. The Planet Saturn: A History of Observation,
Theory and Discovery. Dover, 1980. P. Smith,
n.d. Source book on the planet Saturn.
Allaby, Michael. The Greening of Mars. St. Martin's/Marek,
1984. Presents a habitable Mars.
Briggs, Geoffrey and Taylor, Frederick.
Cambridge Photographic Atlas of the Planets.
Cambridge, 1986. Close-up photos from space probes.
Burgess, Eric. Venus, an Errant Twin. Columbia, 1985.
Nontechnical discussion of the planet closest to earth.
Chapman, Clark R. Planets of Rock and Ice:
From Mercury to the Moons of Saturn. Scribner, 1982.
This revised edition of The Inner Planets (1977)
includes results of more recent space missions.
Elliot, James and Kerr, Richard. Rings:
Discoveries from Galileo to Voyager. MIT, 1985.
An account of how the outer planets' rings were discovered.
Hughes, David. The Distant Planets. Facts on File, 1989.
A beginner's book.
Jones, Brian. Exploring the Planets. Smithmark, 1991.
Easy enough for many children.
Murray, Bruce and others. Earthlike Planets. Freeman, 1981.
"The Surfaces of Mercury, Venus, Earth, Moon, and Mars" (subtitle).
Shurkin, Joel N. Jupiter: The Star That Failed. Westminster, 1979.
Young people's history of Jupiter from Egyptian astronomers to NASA.
Time-Life Editors. The Near Planets. Time-Life, 1989.
Lavishly illustrated.
Vogt, Gregory. Mars and the Inner Planets. Watts, 1982.
Illustrated description of Mars, Venus, Mercury; for young people.
Washburn, Mark. Distant Encounters: The Exploration of Jupiter and
Saturn. Harcourt, 1983. What Voyager has taught us.
WEB LINKS
VI
COMING SOON
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Planets | Sun | Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto