Venus ... Earth's not really sister planet
Introduction to Venus
Venus was long considered to be Earth's twin, a planet whose
thick clouds must have harbored lush vegetation and exotic animal life. When
the true nature of the clouds was discovered and the term "greenhouse
effect" applied to the planet by Dr. Carl Sagan, Venus became more like
a hellish place than an Eden. Venus holds the distinction of being the hottest
of the planets, with surface tempertures exceeding 740K, and it is the hot
everywhere. Students who have trouble imagining this heat can do an experiment.
Go home, turn on the oven to "bake." After the oven bell chimes
that it is up to the desired temperature, open the oven door and stick your
arm in the oven for a while and say, "Wow!, Venus is twice as hot as
this oven!" In appearance, Venus looks like a yellow, cloud-covered world,
and the only way to see the surface is either by landing there or with radar
mapping since the entire planet is always covered by clouds. Venus is tipped
upside down, or more accurately, Venus is right-side up but spinning clockwise
so that the Sun would rise in the west and set in the east. Additionally,
Venus is the only planet whose rotation length exceeds its revolution, meaning
that a Venus day is longer than its year. Finally, Venus has so much atmosphere
that the pressure at the surface would be 90 times that of Earth, flattening
anyone who dared to be there, and besides, the clouds rain battery acid. There
is not a worse place to be in the entire solar system with the possible exception
of Jupiter's moon Io.
Mass (kg), and mass relative to Earth
4.869x1024 kg = .8150 earths
Equatorial diameter (km)
Mean density (gm/cm3)
Acceleration of gravity (m/s2)
Velocity of escape (km/s)
| Period of rotation
Period of revolution
Mean orbital distance from the sun (AU)
Mean orbital distance from the sun (km)
Orbital velocity (km/s)
Inclination to the ecliptic
Inclination of the equator to the orbit
Number of natural satellites
Names of natural satellites
More Information on the Planet Venus from the Nine Planets Website
Much of the information below is direct from the Nine
Planets Website. Some material has been altered by me for this course,
while other items and comments are directly copied. I hope to maintain a
continuous update of this material to keep up with the findings from space
satellites and telescopes.
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 than 1%.
orbit: 108,200,000 km (0.72 AU) from Sun
diameter: 12,103.6 km
mass: 4.869x1024 kg
Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and
beauty. The planet is so named probably because it was the brightest of
the planets known to the ancients. (With a few exceptions, the surface features
on Venus are named for female figures.) It is the brightest object to appear
in the night sky, with only the Sun and Moon being brighter, unless a star
nearby happens to go supernova.
Venus has been known since prehistoric times, and due to its brightness
in the sky, it has been an object of much study and speculation. 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, meaning that it orbits inside the orbit
of Earth, andtherefore 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 images above and below depict the phases of Venus that astronomers
can view through their telescopes, and which Galileo observed such a long
time ago. The phases of Venus are due to its interior position relative
to Earth's. Of interest is that the crescent phases is significantly larger
because during that phase, Venus is closer to the Earth. While the crescent
exposes the least amount of sunlit Venus to Earth, it is still the time
when Venus will appear most bright to the naked eye in the evening or morning
Spacecraft Visits to Venus
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,
Venera 7 that was the first spacecraft to land on another planet, and
Venera 9 that was the first to return photographs of the surface. The Venera
13 images are seen below. During the height of the Cold War, the American
and Soviet scientists worked out an informal gentlemen's agreement that
was actually followed in spite of the paranoia and spying between the two
world superpowers. It was decided that the Soviets would explore Venus and
share their findings while the Americans would explore Mars and share those
findings. While the Soviets experienced many failed attempts to land craft
on Venus, they were successful several times and the images are a real surprise
recently, the orbiting US spacecraft Magellan
produced detailed maps of Venus' surface using radar (left). From the image,
it is obvious that Venus has the same type of surface as does the Earth,
without the oceans. The blue denotes basaltic lowlands, and the green and
yellow areas are continental highlands, sort of like Asia or North America.
While Venus lacks any plate tectonic movement, there is still geologic activity
with volcanism, but all of the volcanoes are shield domes like Hawaii and
Below is a topographical map of the Venusian surface taken from radar
imagry of Magellan. You can click on the map to see a larger version of
the image, or click on topo map to get a map
with labels. Venus surface.
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
(meaning that Venus spins in a clockwise direction such that the Sun would
rise in the west and set in the east). 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.
I find two interesting points here:
1) Astronomers like order in their study. To them, Venus offers some
challenge to this order. All planets spin counter-clockwise except Venus.
Instead of simply stating that Venus has almost no tilt and spins clockwise,
astronomers have instead chosen to designate Venus as a planet that spins
in the "proper" counterclockwise direction, but happens to do
2) Imagine going to school on Venus where a day is 243 days long. You
would be in school for 80 straight days, with four classes of 20 days each.
Can you sense the joy of math or English for 20 consecutive days? However,
you would get a nice lunch break and a long time to sleep :)
Venus is sometimes regarded as Earth's sister planet. In some ways they
are very similar:
-- 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). I have found
this concept of excessive surface pressure hard for my students to grasp,
so here is an analogy. On earth, 14.2 pounds of air pressure squeezes down
on each square inch of your body. You are totally unaware of this since
you were born into this world and adapt to its conditions. On Venus, the
pressure is 90 times greater. It is as if you were to go to the weight room
of a gym and pick up a few of those large 45 pound circular plates that
are put on bars for bench pressing. In fact, you would grab 27 of those
plates and pile them on your finger, or your chest, or even your head. This
is 1200 pounds of pressure per square inch. You would be quickly flattened.
This intensely thick atmosphere is composed mostly of carbon dioxide. There
are several layers of clouds many kilometers thick composed of sulfuric
acid. This is the same stuff that you might find in your car battery. I
recall a time when my battery died, and I needed to trade it in to get a
new one. During the drive to the NAPA store, I held the battery on my lap.
When I got to the store, the leaking battery juices had eaten large holes
in my pants, and I had to ask my friend to take the battery into the store
because my pants were ruined. Now imagine having that same kind of acid
rain down from the clouds!
Furthermore, 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. What is unusual about the clouds
is that their bottoms are about 30 km up from the surface. The greenhouse
trapping of solar heat has inflated the atmosphere such that if any rain
were to fall, it would evaporate before hitting the ground.
Well, isn't this great? You go outside of your little house on Venus and
are immediately flattened by the pressure, spontaneously combust into flames
from the excessive surface heat, and hope that acid rain does not scour
away whatever is left of you. Actually, the rain would never strike your
burnt body because it evaporates and rises back into the clouds well before
reaching the surface.
There are strong (350 km/hr) winds at the cloud tops but winds at the
surface are very slow, no more than a few kilometers per hour. The Soviets
deployed a collection of metal spheres into the upper clouds of Venus. The
transmitters in these metal mini-satellites returned information to the
Earth and allowed the scientists to measure wind velocity as these metal
spheres circled the planet is only 4 days. It took Phineas Fogg 88 days
to go around the Earth in his balloon. Imagine his experience in the clouds
Venus probably once had large amounts of water like Earth but it all boiled
away. You will learn why later on this page. 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.
Why is the atmosphere of Venus (Earth's sister) so different from Earth?
On Earth, CO2 is absorbed in the oceans and rocks. If the absorbed CO2 on
Earth were released into the atmosphere, 98% would be CO2 and the atmospheric
pressure would be 70x what it is now. So, except for the O2 and water, Earth's
atmosphere would be similar to Venus' if the CO2 has not gotten absorbed.
The oxygen on Earth is a product of life, the result of photosynthetic splitting
of water into O2 and H+ ions that are used to drive the future synthesis
of ATP and Glucose.
The water on Venus has disappeared due to the extreme temperatures brought
on by the runaway greenhouse effect. The trapped radiant heat from the solar-heated
ground caused water vapor to rise to high elevations where is was then broken
down into oxygen and hydrogen by ultraviolet radiation (photolysis). The
hydrogen, being light, escaped from the gravitational control of Venus.
The remaining heavier oxygen radicles combined with other atmospheric gases,
and thus water on Venus was lost forever!
Summary of Atmospheric Components
(percentages by volume)
absorbed in rock
free in atmosphere (96.5%)
free in atmophere (78%)
free in atmosphere (3.5%)
mostly condensed on surface
decomponsed long ago, and hydrogen escaped
free in atmosphere (21%)
no life to produce it
Evolution of Venus' Atmosphere
During secondary atmosphere formation on Venus, the temperature was so
high that no oceans formed. (Water is broken down by a process called photolysis,
and H2 escapes) and gases are not absorbed back into the rock as they are
on Earth. The remaining carbon dioxide gases in the atmosphere (pumped out
via volcanism) trapped heat (greenhouse effect). As the temperature rose
even more, the planet found itself in a runaway greenhouse effect. The atmospheric
heating was unstoppable. Given the closer proximity to the Sun and the extremely
slow rotational speed of the planet, Venus was doomed to be lifeless. What
early planetary astronomers believed would be a world teeming with life
turned out to be a dry, barren, and exceedingly harsh environment, and perhaps
the worst surface place in the Solar System to visit.
The variations in concentration from the Earth to Mars and Venus result
from the different processes that influenced the development of each atmosphere.
While Venus is too warm and Mars is too cold for liquid water the Earth
is at just such a distance from the Sun that water was able to form in all
three phases, gaseous, liquid and solid. Through condensation the water
vapor in our atmosphere was removed over time to form the oceans. Additionally,
because carbon dioxide is slightly soluble in water it too was removed slowly
from the atmosphere leaving the relatively scarce but unreactive nitrogen
to build up to the 78% is holds today.
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 million years.
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 image to the left is of several large craters
whose interior is now covered with lava. Following impact, lava welled up
from the interior of the planet and smoothed over the crater's interior.
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.
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 (below left) which seem to be collapsed domes over large magma chambers.
The volcanism of Venus produces mountains identical to the Hawaiian Islands
on Earth, and apparently by the same geological process of hotspot venting.
While Venus lacks the plate tectonic activity of Earth, there is difficulty
explaining the chain of volcanoes seen in the image to the left.
This image is interesting because it deomonstrates the unusual nature of
high surface heating on the rock. The 740K temperature does not melt the
surface rock, but is does soften it, and thus these mountains have collapsed
somewhat into the surface after formation. The result is a "spider-like"
image to the left depicts what astronomers believe the interior of Venus
to be like. It 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 that
is insufficient to generate the field from within a molten interior. It
has no mechanism to generate the magnetic field and this no mechanism to
protect itself from much of the harmful solar energy that Earth's magnetic
field protects us from!
The Venus Moon?
Venus has no satellites, and thereby hangs a tale
of a time when astronomers thought there might be a small moon orbiting
Even more fascinating is Percival
Lowell's sighting of canals (these pages may take a long time to load),
a report recently appearing in the October, 2002 Sky and Telescope.
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. It is so bright
that it is often confused with alien spacecraft. 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 Night.
The information shown above is from the Nine
Planets website, and courtesy of author Bill Arnett. Please go to that
site to find any updates I might not have caught.
Here are a few questions I would like you to be able to answer:
1) Give a brief description of the physical appearance of Venus as seen
from a telescope.
2) Why is Venus considered Earth's twin?
3) What is it like on the surface of the planet (describe pressure, heat,
4) Why should WE concern ourselves with Earth's greenhouse gasses, just
because it got bad on Venus?
5) What Russian satellites landed on Venus?
6) What kind of volcanoes are on the surface of Venus?
7) What are the clouds of Venus made of and what is the rain like?
8) What does the Sun do in the Venus sky? Why does it move that way?
9) How fast do the clouds blow high in Venus' atmosphere?
10) What US satellite did the primary mapping of the surface of Venus?
When you think you know the answers to these questions,
go to Venus
Quiz and send them in to me.
Some connecting links to Venus information are listed below:
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