The planet Venus is the third brightest
celestial object in our sky, making it a focal point of observations and
studies both now and throughout history. For centuries cultures have
recorded Venus's path across the sky and interpreted its purpose. The Mayans
formed a calendar using its movements. Romans and Greeks viewed
the planet as a god and a symbol of creation, love, and fertility.
People have produced many legends, myths, and truths about the "Evening
star" that survive today.
Scientists have long assumed that because
of their relative structures that Venus and the Earth were identical planets.
Venus is similar to Earth in size, mass, and density. Each planet
is terrestrial, meaning a non-gaseous object orbiting close to the sun,
and composed primarily of the elements silicon, oxygen, iron, magnesium,
aluminum, nickel, and sulfur. They appear to have formed similarly.
Each planet was heated early in history, perhaps during the formation from
the original solar nebula. This heating led to differentiation where
denser materials sink towards the core, remaining molten, and lighter materials
rise to the surface and cool forming a solid layer or crust (Morrison 88).
Science fiction writers even began to describe Venus, as a planet covered
in vast vegetation, where dinosaurs roamand elaborate fantasy civilizations
exist (Cooper 5). However the clouds that encompass Venus have hidden,
for some time, the evidence that would support these claims. Finally
with the discovery of radio astronomy and the many satellites sent to observe
Venus we have gained a clearer understanding of Earth's sister planet.
The clouds that surround Venus have an intense
concentration of sulfuric acid droplets. These droplets reflect seventy-six
percent of the sun's rays, that is ten percent less than an average mirror,
making Venus one of the most brilliant objects in the sky
(Kaufmann 260). Venus is four times brighter than Mars, fifty times
brighter than Jupiter, and six hundred times brighter than the North Star
(Berman 96). Venus's brilliance allows viewing early in the morning
and evening up to three hours before sunset and sunrise. Because
its orbit is so large, one hundred and eight million kilometers on
average, we can view it from Earth at great distances from the Sun.
When Venus reaches its greatest eastern elongation, the angular distance
between a planet and the sun as viewed from Earth, at forty-seven degrees
east of the Sun, we call it the "Evening Star". Alternatively at
its extreme western elongation of forty-seven degrees we call it the "Morning
Star" (Kaufmann 261). Every nineteen months Venus, Earth, and the
Sun repeat the same alignment to produce a similar Morning and Evening
star. The tilt of the Earth's axis can have a dramatic effect
when viewing Venus especially as a Morning or Evening star. In fall
it appears lower in the horizon, whereas in Spring the situation reverses.
Venus sits high in the sky and will stay above the horizon for several
hours. Because Venus circles the sun thirteen times to every eight
Earth orbits, the position of Venus in the sky repeats almost exactly every
eight years (Berman 96). Another interesting relationship is the
synodic period of time for Venus, 583 Earth days. It is almost exactly
five times the Venus solar day, approx. one hundred and seventeen earth
days, and nearly the same amount of time it takes Venus to cycle back to
either the Morning or Evening Star. The rotation of Venus itself is unique.
All our planets and other objects in our solar system orbit the sun in
a counter-clockwise motion. Similarly most planets and objects rotate
on their axis in a counter-clockwise motion, called a prograde motion.
Venus, however, rotates on its axis clockwise. When a planet rotates
in opposition of its orbital direction it has a retrograde rotation.
In addition, the rotation of Venus is quite a bit slower than that of other
planets. One Venus day, meaning the time it takes to complete one full
rotation on its axis, is equal to approximately 243 earth days (Kaufmann
263). Further explorations with satellites and radio Astronomy has
revealed new information about the atmosphere and what lies hidden underneath
it.
About forty-eight kilometers above the surface
of Venus exists a dense atmosphere and thick cloud cover. The atmosphere
is nearly ninety-six percent carbon dioxide and four percent nitrogen and
other substances. Sulfuric acid droplets compose the cloud cover.
These droplets help contribute to the Greenhouse effect that describes
the climate of Venus. As the sun's rays charge toward the atmosphere
of Venus the sulfuric acid reflects the majority of the rays, the
wavelength of these rays being much larger than the space in these dense
clouds. However, some of the rays manage to penetrate to the surface,
where they increase temperature, by absorbing into the surface. The
surface in turn radiates Infrared waves. The dense atmosphere reflects
most of these back toward the surface where they are again absorbed, and
heat the surface further. This process continues until the temperature
reaches about seven hundred and fifty Kelvin. At this point thermal
equilibrium establishes a balance between the amount of radiation escaping
the atmosphere and temperature of the surface. This has two effects
on Venus. First it creates a surface pressure near ninety atmospheres,
one atmosphere being approximately thirteen hundred pounds per square inch
of pressure, and second the constant heating creates two huge convection
currents in the cloud layers. One convection current exists in the
each the northern and southern hemispheres. These cause the clouds
to move rapidly in a retrograde motion, opposite the spin of the
planet, with a period of about four earth day (Kaufmann 265, Morrison
103).
Modern scientists have always viewed Venus
as a twin sister to earth due to their same relative size, shape, and distance
to 
the
sun. Recent information gathered by Magellan and other such
projects have brought to light new revelations on the true physical characteristics
of Venus. The relatively young age of the surface on Venus creates
a grand argument about the nature of the surface's formation. Scientists
currently are using data from the Magellan satellite to prove their theories
revolving around ideas of volcanism and plate tectonics on Venus. As satellites
began to penetrate farther into the atmosphere and began mapping the surface
of Venus, we can see the similarities and differences to our topography
and gain insight into what kinds of tectonics exist on Venus. Magellan
is perhaps the most successful of these projects mapping some ninety-seven
percent of the Venusian surface with a resolution less than one hundred
meters. Beneath the planet's veil lies a host of geologic structures
sculpted by interior forces. Five-sixths of Venus consists of gently rolling
Volcanic plains with a total vertical scale of about two kilometers.
The remaining sixth comprises two large continents called the Ishtar and
Aphrodite Terras (Morrison 101). The Ishtar terra is roughly
the size of Australia and closely resembles the Himalayas in altitude.
It contains numerous mountains some reaching heights nearly eleven kilometers
from the surface. The Aphrodite terra is about the size of Africa.
It stretches nearly one-third the way around the Venusian equator,
is close to sixteen thousand kilometers in length and two thousand kilometers
in width (Eberhart 91).
Volcanoes are common on Venus. The largest are Shield Volcanoes;
one hundred and fifty of them are over one hundred kilometers in breadth.
The most plentiful are basaltic eruptions that immerse large areas in lava.
Explosive types of eruptions are rare, perhaps due to the high pressure
of the atmosphere. There are several types of volcanism exclusive
to Venus. There are dozens of near perfect circular flat domes with
step sides called Pancake domes. They are typically twenty-five kilometers
wide and two kilometers high and are apparently the product of a highly
liquid lava erupting suddenly from a single vent. Coronae are circular
structures with diameters up to two thousand kilometers with slightly raised
interiors surrounded by a low circular ridge and a moat. There are
several hundred known and they are the possible result of a mantle plume
becoming inactive before it could form a shield volcano. Finally
about forty or more lava flows reach distances more than one hundred kilometers.
One lava flow, the longest known channel in the solar system, reaches a
length of almost seven thousand kilometers. These rivers of are evidence
of lava flowing with extremely low viscosity, however on earth it is rare
they travel such distances. Magellan has also
revealed over nine hundred impact craters, most exceed twenty
kilometers in diameter, the largest nearly one hundred and fifty kilometers
wide. Apparently smaller projectiles can not penetrate the harsh
atmosphere. From the number of impact craters scientists can determine
the age of the surface. They have determined the surface to be relatively
young at five hundred million years old. This is a source of great
controversy. Fewer than five percent of the craters show evidence
of erosion or sedimentation, preserving a pristine appearance. In
addition the craters are all about the same age. Evidently the process
that removes crater marks on Venus is not gradual but a rather sudden process.
Here is the foundation of the arguments over the tectonics of Venus (Morrison
91).
Tectonics is the name geologist give to stresses
acting on the crust of a planet. Tectonic forces squeeze together or pull
apart the crustal rocks, often accompanied by earthquakes and volcanic
eruptions (Morrison 101). Before the Magellan project, researchers
speculated plate tectonics, the primary reconstructing force on Earth,
transformed Venus's surface. Plate tectonics occurs when you have
a crust broken up into pieces which "float" on a molten material called
a Mantle. Heat rising from the interior of a planet creates convection
currents, which provide the power to move these massive pieces or plates.
Two actions occur when these plates move. First, as the plates separate,
giant "rifts" open up allowing molten rock to rise to the surface, which
attaches itself to the receding plates as it cools. Nearly all the
earth's oceanic lithosphere, some sixty percent of the total surface of
the Earth, forms this way. The second event occurs when a thin oceanic
plate collides into a continental plate. It is often subducted, meaning
one plate edge is pushes below the other, creating an oceanic trench where
the two plates meet. The oceanic plate heats up at great depths releasing
carbon dioxide and forcing volcanic eruptions on the surface of the continental
plate. Finally, these volcanoes form offshore islands (Eberhart 91).
Virtually the entire crust of Venus is subject to tectonic forces. Ridges
and cracks, to rolling lowlands and shield volcanoes are all evidence of
pushing and pulling of interior forces. Venus like Earth has continental
highlands, volcanoes, a few high mountains, and a scattering of impact
craters. However, Venus lacks ocean basins-an important feature in
terrestrial geology-, its continents are smaller, and it displays distinct
geology like Coronae and Pancake domes (Morrison 99). Images
from Magellan failed to demonstrate the types of faults prevalent on spreading
centers of Earth, exist in the equatorial Highlands of Venus. These
"transform" faults connect segments of spreading plates with a characteristic
staircase like ridges. The Aphrodite terra, in addition, lacks signs
of vast horizontal movement that typically indicates the journey of a new
lithosphere from its birthplace. The absence of evidence of Plate
tectonics and the numerous plumes like Coronae, Pancake domes, and Shield
Volcanoes, lead to the idea that Venus operates on a diverse small scale
tectonic system. Yet, due to atmospheric pressure and temperature,
a system of tectonics that is in use on Earth may appear very different
on Venus (Eberhart 91).
The brilliance of Venus is no longer limited
to its position in the sky as it was for the Maya, Greeks, and Romans.
Over the centuries we have gained more and more knowledge of our sister
planet and now can began to use our understanding of Venus to help direct
our ideas of the order of the universe. Learning the phases of Venus
helps progress our universe model. Understanding Venus's topography
and atmospheric content help us realize where our planet could potentially
be in the future. As we study ancient cultures and how they viewed
the universe we can understand our origins better. Although their are many
unanswered questions concerning Venus, there have been a great many gains,
perhaps eventually all the answers will be there.
Image
Information and Sources
This global view of the surface of Venus is centered
at 0 degrees east longitude. Magellan synthetic aperture radar mosaics
from the first cycle of Magellan mapping are mapped onto a computer- simulated
globe to create this image. Data gaps are filled with Pioneer-Venus Orbiter
data, or a constant mid-range value. Simulated color is used to enhance
small-scale structure. The
simulated hues are based on color images recorded by the Soviet Venera
13 and 14 spacecraft. The image was produced by the
Solar System Visualization project and the Magellan Science team at
the JPL Multimission Image Processing Laboratory, and is a single frame
from a video released at the October 29, 1991, JPL news conference.
Source: http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/mgn_p39224.html
Link: http://nssdc.gsfc.nasa.gov/imgcat
Three impact craters are displayed
in this three-dimensional perspective view of the surface of Venus. The
center of the image is located at approximately 27 degrees south latitude,
339 degrees east longitude in the northwestern portion of Lavinia Planitia.
The viewpoint is located southwest of Howe Crater, which appears
centered in the lower portion of the image. Howe is a crater with a diameter
of 37.3 kilometers (23.1 miles) located at 28.6 degrees south latitude,
337.1 degrees east longitude. Danilova, a crater with a diameter
of 47.6 kilometers (29.5 miles), located at 26.35 degrees south latitude,
337.25 degrees east longitude, appears above and to the left of Howe in
the image. Aglaonice, a crater with a diameter of 62.7 kilometers (38.9
miles), located at 26.5 degrees south latitude, 340 degrees east
longitude, is shown to the right of Danilova. Magellan synthetic aperture
radar data is combined with radar altimetry to develop a three-dimensional
map of the surface. Rays cast in a computer intersect the surface to create
a three- dimensional perspective view. Simulated color and a digital elevation
map developed by the U.S. Geological Survey are used to enhance small-scale
structure. The simulated hues are based on color images recorded by the
Soviet Venera 13 and 14 spacecraft. The image was produced at the JPL Multimission
Image Processing Laboratory and is a single frame from a video released
at the May 29, 1991, JPL news conference. Source:
http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/mgn_p39146.html
Link: http://nssdc.gsfc.nasa.gov/imgcat
Sif Mons is displayed in this
computer-simulated view of the surface of Venus. The viewpoint is located
360 kilometers (223
miles) north of Sif Mons at a height of 7.5 kilometers (4.7 miles)
above the lava flows. Lava flows extend for hundreds of
kilometers across the fractured plains shown in the foreground to the
base of Sif Mons. The view is to the south. Sif Mons, a
volcano with a diameter of 300 kilometers (186 miles) and a height
of 2 kilometers (1.2 miles), appears in the upper half of the
image. Magellan synthetic aperture radar data is combined with radar
altimetry to produce a three-dimensional map of the
surface. Rays, cast in a computer, intersect the surface to create
a three-dimensional perspective view. Simulated color and a
digital elevation map developed by the U.S. Geological Survey are used
to enhance small-scale structure. The simulated hues are
based on color images recorded by the Soviet Venera 13 and 14 spacecraft.
The image was produced at the JPL Multimission
Image Processing Laboratory and is a single frame from a video released
at the March 5, 1991, JPL news conference.
Link: http://nssdc.gsfc.nasa.gov/imgcat/
Galileo image of Venus taken from 2.7 million km.
In order to emphasize cloud features a high-pass spatial filter was applied
and the image was colorized blue. The original image was taken through
the violet filter. The sub solar point, indicating local noon, is the bright
spot at right, and the evening terminator is at left. The clouds are moving
to the west (left) at about 100 m/s
(230 miles/hr) at the equator. Also visible are the polar hoods at
top and bottom. Venus is 12,100 km in diameter and north is up. (Galileo,
P-37222)
Link: http://nssdc.gsfc.nasa.gov/imgcat/
Magellan Space Satellite
Link: http://nssdc.gsfc.nasa.gov/imgcat/
Return to Evergreen Home Page
Return to Individuals'
Home Page
Madeby: Jason Mateljak and Shira Stefanik
E-mail: 
jmateljak@hotmail.com
Last modified: 10/9/1995
