VENUS

Key Concepts

Venus, whose orbit has a semimajor axis of about 0.7 A.U., comes closer to the Earth than any other planet. At closest approach, it is a mere 38 million kilometers away (100 times the Earth-Moon distance). The rotation of Venus is retrograde, opposite to the direction of rotation of all the other planets. An observer located on the surface of Venus would discover that the time between one noon and the next is 117 days. Thanks to the retrograde rotation of Venus, the sun rises in the west and sets in the east.

Venus is sometimes called ``Earth's twin''; not only is it the closest planet to the Earth, it also is similar in some of its physical properties:

However, in some ways Venus is very different from the Earth. In particular, the venusian atmosphere is much denser and hotter than Earth's. (Vocabulary note: the correct adjectives to apply to the planet Venus are ``venerian'' or ``venusian''; the adjective ``venereal'' is reserved for other purposes.)

(1) The atmosphere of Venus is very hot because of a runaway greenhouse effect.

The atmosphere of Venus, down at ground level, is hot and dense; consequently, it has a very high pressure. Air pressure at the surface of Venus is about 90 times air pressure on Earth. (This works out to a pressure of two-thirds of a ton per square inch, the same as the pressure a kilometer deep in the Earth's oceans.) The composition of the venusian atmosphere: There is very little water vapor, and no oxygen, in the atmosphere of Venus.

Carbon dioxide is a greenhouse gas, so Venus, with its thick carbon dioxide atmosphere, has a gargantuan greenhouse effect. The surface temperature of Venus is 730 Kelvin (around 860 Fahrenheit), even hotter than noontime on Mercury. Moreover, the thick atmosphere of Venus acts as an insulating blanket, so the temperature drops very little at night. As the Sun slowly sinks below the eastern horizon, the night on Venus remains hot - above 700 Kelvin.

Once Venus was cooler, with a lower pressure atmosphere. The temperatures were probably cool enough for small seas of liquid water to exist on the surface. However, volcanos on Venus steadily belched out carbon dioxide and water vapor into the atmosphere. The addition of these greenhouse gases made the temperature rise. The rise in temperature caused water to evaporate from the seas and carbon dioxide to be released from the seas and the rocks. The further addition of greenhouse gases caused a further rise in temperature, which caused a further addition of greenhouse gases, which caused a further rise in temperature, which caused a further addition of greenhouse gases, which ... Well, you get the idea. This feedback cycle, which ended only when the oceans of Venus were totally evaporated, is called a runaway greenhouse effect. There is some concern that the burning of fossil fuels on Earth might trigger a similar runaway greenhouse effect. If all the carbon dioxide locked up in the Earth's limestone were liberated, the Earth's atmosphere would be similar to that of Venus; hot carbon dioxide at very high pressure.

Venus is completely covered with clouds that consist of droplets of sulfuric acid (H2SO4). These clouds are nearly opaque at visible wavelengths. At ultraviolet wavelengths (as shown in the image below), the clouds are not completely opaque:

The layer of sulfuric acid clouds lies about 50 to 60 kilometers above the surface of Venus. The temperature (300 Kelvin) and pressure within the clouds are moderate, but the winds within the cloud deck are very rapid (up to 350 kilometers/hour, enough to carry a cloud completely around Venus in four days). At the surface of Venus, the very dense air moves much more sluggishly, at only 3 kilometers/hour. The surface is only dimly illuminated by the light which seeps through the clouds; at the surface of Venus, even at high noon, it is as dim as a heavily overcast day on Earth.

(2) The surface of Venus shows volcanic activity, but no plate tectonics.

Since the clouds of Venus are nearly opaque, the surface has only been mapped by radar. An orbiting spacecraft, such as the Magellan spacecraft, which mapped Venus during the early 1990s, bounces radio signals off the surface of the planet. A radio signal which bounces off a high mountain has a shorter round trip journey, and hence returns more quickly than a signal which bounces off a deep valley. A map of Venus, found by radar, is shown below. The map is color-coded so that the lowest valleys are blue and the highest mountains are red. The larger version of the map (which you can summon up by clicking on the small image) is labeled with the names of prominent geological (venerological??) features.

click on image for a larger map

Most of Venus consists of low, rolling plains, but there are two prominent highland regions:

Note that Ishtar and Aphrodite are not exactly analogous to continents on Earth. Terrestrial continents are made of low-density granite, which rides at a higher level than the dense basalts of the ocean floor. On Venus, Ishtar and Aphrodite are made of the same basalt as the low, rolling plains.

Venus has many volcanos (about 100,000, according to one estimate). One of the larger volcanos is shown below:

Venus has experienced extensive lava flows in the relatively recent past. The average age of the crust is about half a billion years, indicating that Venus has been extensively resurfaced by lava during the last billion years or so. Venus has relatively few craters (the Magellan spacecraft saw only a thousand of them larger than 100 meters across). The scarcity of craters is partly due to the thickness of the venusian atmosphere (small meteoroids break up and are vaporized), but mainly due to the relatively recent resurfacing of Venus (most craters older than 500 million years have been paved over with fresh lava).

There is no evidence for plate tectonics on Venus; no mid-ocean rifts, no subduction trenches. The volcanos of Venus are spread evenly across the surface instead of being concentrated at plate boundaries, as they are on Earth. The lithosphere of Venus has not been broken into plates; probably because the heat at the surface of Venus is enough to slightly soften the lithosphere.

There is evidence, however, that the mantle of Venus is undergoing convection (or that it has done so in the past). Some surface features on Venus are the direct result of convective motions in the mantle. Coronae are large dome-shaped bulges in the crust of Venus, created when a rising plume of hot lava in the mantle shoves upward on the crust above it. Wrinkled mountains are created when convection in the mantle compresses a region of the crust. The crust is shoved together into folds. Fractures are created when convection in the mantle stretches a region of the crust apart. The soft and stretchy crust doesn't pull apart in a single deep wide rift, as the Earth's brittle crust would do, but it does show a large number of ``stretch marks'', as displayed in the image below.

 

(3) The interior of Venus is similar to that of Earth.

The average density of Venus is slightly smaller than that of the Earth, but that is simply because the Earth is somewhat larger, and hence creates a somewhat larger gravitational compression of the material in its core. Ignoring the effects of gravitational compression, we find that the uncompressed densities of Venus and the Earth are both 4200 kg/m3. This argues (although it is not conclusive proof) that the internal structure of Venus and the Earth are similar; a metal core surrounded by a thick rocky mantle.

One puzzling fact is that Venus, unlike the Earth, has no magnetic field. (When exploring Venus, you can throw your compass away. It won't work). It's not certain why Venus has no magnetic field. The magnetic field of the Earth is caused by convection currents in the liquid outer core. Perhaps the metallic core of Venus is entirely solid; or perhaps the slow rotation of Venus is responsible for the lack of currents in its core.

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