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Venus Moons - Orbital and Rotational Detail - Physical Properties - Atmosphere - Surface and Geology - Interior - Magnetic Field - Phases of Venus Surface and Geology False color image of Venusian craters from the orbiting Magellan spacecraft. Venus is covered by thick clouds that are opaque to visible light. In order to "see" the surface and study the geology, we need to penetrate its clouds with radar. Pioneer Venus showed that Venus does not have large continents or ocean basins, and only 15% of its surface is highlands. The best resolution imaging was done by the spacecraft Magellan in the first part of the 1990s; it found about 1000 impact craters ranging in size from 2 to 280 km. Smaller craters are absent because the meteors burn as they enter the thick atmosphere, and larger craters are absent because the surface must be younger than the time of heavy bombardment that ended 3.9 billion years ago. Astronomers can estimate the age of the surface from the number of craters and find that it is about 500 million years old. Venus does not have well defined tectonic plates. Tectonic activity is spread over the whole surface as is evident by the presence of bands, ridges and cracks. The surface doesn't show motions of the crust or mountain ranges formed by plate collisions, like the Himalayas on Earth. The five-mile high volcano Maat Mons, as recreated from Magellan radar data taken in October, 1991 About 80% of the surface are lava plains, indicating recent—in geological terms—volcanic activity (less than 500 million years old). Compared to Earth (including oceanic eruptions) Venus's volcanoes are less active. The appearance of the meteor craters today suggest that in the past volcanoes were more active than at present. Some volcanoes on Venus are similar to those on Earth, and lava flows are seen on their slopes. Others are circular domes, tens of kilometers in diameter and about 2-3 km high. Because of their shape they are called pancake domes and were produced by the eruption of thick, viscous lava. Long lava channels (longer than 100 km) are also observed; these were formed by the eruption and flow of low viscosity, fluid lava. The longest one is 7000 km long. Circular or oval features are seen in Venus, but not on Earth. These are called coronae and are hundreds to thousands of kilometers across, often filled with lava. They have concentric and radial patterns that result from a mantle hot spot (a plume of rising magma) and subsequent collapse. No one knows what the surface was like before 500 million years ago, or how the planet got "resurfaced." One theory is that if Venus has a crust thicker than Earth's (~300 km deep), it could insulate the interior. The mantle heats up by radioactive elements, and when it gets too hot, it can melt the crust. Every several million years the entire surface of Venus liquefies, the heat escapes from the planet and the surface solidifies again. Interior Venus is thought to have a metallic core (an inner solid core surrounded by an outer liquid core), probably made up of nickel and iron, and a large rocky mantle and a rocky crust thicker than Earth's. The lower density of Venus, compared to Earth's, indicates that it might have a smaller core than Earth's. Astronomers think that Venus has less sulfur than Earth, and from this fact they conclude that Venus' core might have less iron sulfide and might have formed later. Magnetic Field No probe sent to Venus has yet detected a magnetic field; if one exists, it is at least 10,000 times weaker than Earth's. Since the core of Venus is thought to be metallic, this poses a problem for the theories of formation of planetary magnetic fields. According to these theories, we expect a magnetic for Venus, although weaker than Earth's because of Venus's slower rotation rate and because its core might have less iron sulfide. Phases of Venus Because Venus is located between the Earth and the Sun, it shows phases when viewed through a telescope or powerful binoculars. When Galileo first saw the phases of Venus with a telescope, he immediately noticed that the cycle of phases that he saw supported the heliocentric universe of Copernicus, where the planets revolve around the Sun, as opposed to the geocentric universe of Ptolemy, where everything revolves around the Earth. The phases of Venus are correlated with the planet's angular distance from the Sun and its angular size. The angular size changes between 63 and 10 arc seconds, depending on the relative position of Earth and Venus. Venus is brightest when its elongation is 39º, shining at –4.6 magnitude. Bibliography Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co. Astrophysical Data: Planets and Stars, K.R. Lang, 1992, Springer-Verlag The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc. <<< Previous \| Next >>> Home

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Back to the MIRA Website | Field Trips to the Stars Home > Introduction to the Solar System > Venus

Venus

Moons - Orbital and Rotational Detail - Physical Properties - Atmosphere - Surface and Geology - Interior - Magnetic Field - Phases of Venus

Surface and Geology

False color image of Venusian craters from the orbiting Magellan spacecraft.

Venus is covered by thick clouds that are opaque to visible light. In order to "see" the surface and study the geology, we need to penetrate its clouds with radar. Pioneer Venus showed that Venus does not have large continents or ocean basins, and only 15% of its surface is highlands. The best resolution imaging was done by the spacecraft Magellan in the first part of the 1990s; it found about 1000 impact craters ranging in size from 2 to 280 km. Smaller craters are absent because the meteors burn as they enter the thick atmosphere, and larger craters are absent because the surface must be younger than the time of heavy bombardment that ended 3.9 billion years ago. Astronomers can estimate the age of the surface from the number of craters and find that it is about 500 million years old.

Venus does not have well defined tectonic plates. Tectonic activity is spread over the whole surface as is evident by the presence of bands, ridges and cracks. The surface doesn't show motions of the crust or mountain ranges formed by plate collisions, like the Himalayas on Earth.

The five-mile high volcano Maat Mons, as recreated from Magellan radar data taken in October, 1991

About 80% of the surface are lava plains, indicating recent—in geological terms—volcanic activity (less than 500 million years old). Compared to Earth (including oceanic eruptions) Venus's volcanoes are less active. The appearance of the meteor craters today suggest that in the past volcanoes were more active than at present. Some volcanoes on Venus are similar to those on Earth, and lava flows are seen on their slopes. Others are circular domes, tens of kilometers in diameter and about 2-3 km high. Because of their shape they are called pancake domes and were produced by the eruption of thick, viscous lava. Long lava channels (longer than 100 km) are also observed; these were formed by the eruption and flow of low viscosity, fluid lava. The longest one is 7000 km long.

Circular or oval features are seen in Venus, but not on Earth. These are called coronae and are hundreds to thousands of kilometers across, often filled with lava. They have concentric and radial patterns that result from a mantle hot spot (a plume of rising magma) and subsequent collapse.

No one knows what the surface was like before 500 million years ago, or how the planet got "resurfaced." One theory is that if Venus has a crust thicker than Earth's (~300 km deep), it could insulate the interior. The mantle heats up by radioactive elements, and when it gets too hot, it can melt the crust. Every several million years the entire surface of Venus liquefies, the heat escapes from the planet and the surface solidifies again.

Interior

Venus is thought to have a metallic core (an inner solid core surrounded by an outer liquid core), probably made up of nickel and iron, and a large rocky mantle and a rocky crust thicker than Earth's. The lower density of Venus, compared to Earth's, indicates that it might have a smaller core than Earth's. Astronomers think that Venus has less sulfur than Earth, and from this fact they conclude that Venus' core might have less iron sulfide and might have formed later.

Magnetic Field

No probe sent to Venus has yet detected a magnetic field; if one exists, it is at least 10,000 times weaker than Earth's. Since the core of Venus is thought to be metallic, this poses a problem for the theories of formation of planetary magnetic fields. According to these theories, we expect a magnetic for Venus, although weaker than Earth's because of Venus's slower rotation rate and because its core might have less iron sulfide.

Phases of Venus

Because Venus is located between the Earth and the Sun, it shows phases when viewed through a telescope or powerful binoculars. When Galileo first saw the phases of Venus with a telescope, he immediately noticed that the cycle of phases that he saw supported the heliocentric universe of Copernicus, where the planets revolve around the Sun, as opposed to the geocentric universe of Ptolemy, where everything revolves around the Earth. The phases of Venus are correlated with the planet's angular distance from the Sun and its angular size. The angular size changes between 63 and 10 arc seconds, depending on the relative position of Earth and Venus. Venus is brightest when its elongation is 39º, shining at –4.6 magnitude.

Bibliography

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
Astrophysical Data: Planets and Stars, K.R. Lang, 1992, Springer-Verlag
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc.

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