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Geology 130F

 Lecture Fourteen


General reading: parts of Chapters 9, 10, and 11 in Moons & Planets (Hartmann);
Chapter 7 in Christiansen and Hamblin

Venus and Earth Compared

Characteristic Venus Earth
semimajor axis (AU)  0.723 1.0
Radius 6,051km 6,378km
Mass (Earth=1) 0.815  1.0 
Density (g/cm3 5.2 5.52 
Composition  Fe, Mg, Ca, Al, Si, O  Fe, Mg, Ca, Al, Si, O 
Atmosphere  CO2 96% N2 76.1% 
N2 3.5%% O2 21% 
Surface Temperature (K)  750 235-350
Rotation Period  -243 days  23h 45m 
Orbital peroid  224.7 days  365.3 days
Magnetic Field  ~0 Gauss  1 Gauss 
Obliquity  2.7 degrees  23.45 degrees 

Why are the atmospheric conditions so different?

 What caused the dramatic differences in rotational characteristics?

Geologic Characterstics

The surface of Venus consists of three major types of topography (revealed by by radar mapping):
  1. Lowlands including rolling plains ((70% of the surface) and circular depressions (20%) like Atalanta Planitia.
    1. The plains appear to be basaltic.
    2. Large belts of compressional ridges transect the lowlands.
    3. Little cratering
  2. Uplands or volcanic rises (15%). Beta Regio is an example of a Upland.
    1. Uplands are centers of intense volcanism.
    2. They may mark the location of mantle plumes
    3. They are similar to Martian bulges
  3. Highlands (15%), which are similar to continents on Earth. Ishtar Terra is the largest.
    1. Highlands are intensly deformed by compression and extension
    2. Mountains up to 11 km high
    3. Volcanoes are rare.
Radar imaging reveals a number of interesting features seen only on Venus. Coronae consist of concentric ridges. The image of Onatah and Bahet coronae is from JPL, and is accompanied by this linked caption. In the lower left is a "pancake" dome, another volcanic feature.

Impact craters are rare, but they are found. Dickinson crater is an example.

The Atmosphere of Venus

The surface temperature of Venus is about T=700K, more than twice that of Earth, while the surface gravity g is similar. The atmosphere is primarily carbon dioxide, CO2 (see table above), which has a molecular weight of about m=44, roughly twice the mean molecular weight of the Earth's atmosphere. The scale height of the atmosphere H=kT/(mg) is similar to that of Earth, about 10km. On the other hand the density and pressure are much higher than that of Earth's atmosphere, by a factor of about 90. As a result the total amount of gas between the surface and space is much larger than on Earth (by about the same factor of 90). The cloud cover (at about 45-75km) is enough to make it difficult to see the surface, but photographs from the Venera space craft show that some sunlight does penetrate to the ground. The amount of sunlight reaching the ground is less that that reaching the surface of the Earth. In spite of this reduced heat input, the surface temperature of Venus is higher than that of the Earth. Why?

The answer, worked out by Carl Sagan, is that Venus is an extreme example of the "green house effect". Both the composition and the total amount of gas add to the green house effect. As already noted, sunlight reaches the surface of the planet, heating it. The surface reprocesses this light, and emitts it as infrared photons, which have longer wavelengths than visible light. CO2 is very opaque to infrared photons, so the thick atmosphere traps the radiant energy from the sun. Rather than being radiated away, the heat must be carried out by convection. However, convection occurs only when the temperature gradient is large, that is, when the difference in temperature between the top of the atmosphere and the surface is large. The air at the surface is heated by radiation from the ground until it reaches 700K, when it has a low enough density to rise upward. When it moves up about a scale height, it mixes with the air around it and cools, which causes it to increase in density and then fall back to the surface. It is this convective motion that drives the strong winds seen in the patterns of clouds high in the atmosphere.

Venus formed out of roughly the same mix of materials that Earth did. That being the case, why do the atmospheres of the two planets differ so strongly? A related question is why is there no water on Venus, when there is so much on Earth?

The first question is probably easier to understand. Much of the surface of Earth is covered by deposits of limestone (calcite and dolomite). Both organic and inorganic processes cause limestone to form in Earth's oceans, removing CO2 from the atmosphere. If all the limestone on the surface of the Earth were released into the atmosphere, the composition would be similar to that of Venus (although only about 1/2 as dense). The early atmosphere of Earth may have been similar to that of Venus.

The reason for the lack of water on Venus is not so clear. Most likely the early atmosphere contained large quantities of water, but the high surface temperatures prohibitied the formation of liquid water oceans. Over geologic time, the water vaopor in the atmosphere could have been removed by disassociation---the breakup of H2O by ultraviolet radiation, allowing the hydrogen to escape from the planet. The oxygen was presumably combined with rocks on the surface of Venus; a similar process is known to have occured early in the Earth's history.

When the Oroville dam in northern California was filled a number of years ago, a series of moderate earthquakes followed. Geologists eventually realized that the increased amount of water in the rock in the immediate vicinity of the dam had acted as a lubricant, allowing the rocks to slide past each other to relieve stress. The result was felt as earthquakes. A similar process is believed to occur in the Earth's mantle; a small amount of water greatly enhances the fluidity of the rock. This may play a role in the largescale convective motions that are seen in the mantle. Venus, which lacks any surface or atmospheric water, may well be deficient in subsurface water as well. If so, large scale convective motions may be inhibited, possibly contributing to the lack of crustal motion on Venus. The dense, hot atmosphere of Venus may have affected the geologic history of the planet.

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