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.

Planetary data

Mass (kg), and mass relative to Earth

4.869x1024 kg = .8150 earths

Equatorial diameter (km)

12,100 km

Mean density (gm/cm3)


Acceleration of gravity (m/s2)


Velocity of escape (km/s)


Period of rotation
243.01 days

Period of revolution

224.701 days

Aphelion (AU)


Aphelion (km)


Perihelion (AU)


Perihelion (km)


Mean orbital distance from the sun (AU)


Mean orbital distance from the sun (km)


Orbital velocity (km/s)






Inclination to the ecliptic

3.394 degrees

Inclination of the equator to the orbit

177.3 degrees

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.

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 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 sky.

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, the Soviet 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 to everyone.

Most 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 Mars.






Venus Topography

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 so upside-down.

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 of Venus!

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.

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 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.

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 (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" appearance.






The 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 the planet.

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, and rain)?

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:

Venus from SSE

Venus from JPL

Venus from LANL

You may go back to look at Mercury, or to Earth or move forward toMars. If these choice are worriesome to you, then return to either Inner Rocky World Introduction or the Planet Introduction, or the Syllabus.

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