KNOW THE PLANET VENUS

The Atmosphere of Venus

The atmosphere of Venus is mostly carbon dioxide, 96.5% by volume. Most of the remaining 3.5% is nitrogen. Early evidence pointed to the sulfuric acid contentin the atmosphere, but we now know that that is a rather minor constituent of the atmosphere.

The mass of the Venus atmosphere is about 90 times that of the Earth's atmosphere. 90% of the Earth's atmosphere is within 10 km of the surface, whereas you have to go to 50 km to capture 90% of the atmosphere of Venus. The clouds of Venus may extend from about 50 to 70 km and may be divided into three distinct layers. Below the clouds is a layer of haze down to about 30 km and below that it is clear.

Above the clouds there is a high-speed "jet stream" which blows from west to east at about 300-400 km/h. This wind is fastest at the equator and slows toward the poles, often giving a "V" type pattern in the visible cloud cover. At the surface there is almost no prevailing wind, with measured surface wind speeds typically less than 2 m/s.

High Temperature and Pressure on Venus

The temperature and pressure on the surface of Venus are so extreme that none of the armored Russian spacecraft of the Venera series lasted more than an hour on the surface. Within that short period, they and the multiprobe of the Magellan mission have provided us with all the direct data we have about the harsh Venusian surface. The pressure at the surface is about 90 Earth atmospheres! This is a pressure of about 900 Newtons per square centimeter or about 1300 pounds per square inch.

The surface temperature of about 750 K or about 480°C or nearly 900°F is hot enough to melt lead, hotter than any household oven temperature. This temperature destroyed the well-insulated and highly protected Russian electronics of the Venera surface craft within an hour.

Absence of Water on Venus

Since Venus is so similar to Earth in size and composition, one would expect a lot of water there, yet it is virtually absent. Because of the great similarities between the two planets, one would surmise that it must have had water in the past. Models for the loss of the water involve the runaway greenhouse effect on Venus. Being closer to the Sun and having the carbon dioxide in the atmosphere, the atmosphere continued to heat, perhaps with enhanced trapping of heat due to water vapor. With sufficiently high temperatures, the water vapor could rise high enough in the atmosphere for the water molecules to be broken up by ultraviolet radiation from the Sun. The freed hydrogen could then escape from the atmosphere, leaving the oxygen only in the form of carbon and sulfur oxides.

The upper atmosphere of Venus is more heavily bombarded by the solar windsince it has no magnetic field to redirect some of the solar wind particles. This would hasten the breakup of the water molecules if the above model is correct.

Sulfuric Acid Clouds on Venus

The benign images evoked by the name Venus are quickly dispelled by the notion of sulfuric acid clouds. Infrared observations starting in the 1970s indicated the presence of sulfur dioxide in the upper cloud layers and this was confirmed by later spacecraft observations. There may also be suspended sulfur particles in the upper atmosphere, contributing to the slightly yellow color of the visual images.

Presumably early volcanic activity contributed the sulfur to the atmosphere and the temperatures remained so high that it could not be trapped out into solid compounds on the surface as it did on the Earth. The melting point of sulfur is 386K, so the surface temperature of about 750K on Venus is well above the melting point. It is even above the boiling point of sulfur on the Earth (717K). Of course, in the 90 Earth-atmosphere pressure cooker of Venus, the sulfur would not be boiling, but it would be volatile enough to evaporate and form the sulfur dioxide compounds which would then remain airborne.

Besides the sulfuric acid clouds in the high atmosphere of Venus, there is evidence for sulfur dioxide in the haze which persists from 30-50km in the atmosphere. Despite the ominous-sounding presence of the sulfuric acid, it is a very minor constituent in the atmospheric composition of Venus.

Why So Much Carbon Dioxide on Venus and Not on Earth?

With a name like Venus, one expects the planet to be heavenly, but it is more like hell! With hotter-than-an-oven temperatures and 90 times the Earth's pressure, it's 96% carbon dioxide atmosphere gives us an example of the greenhouse effectgone wild! But a glance at the planet data table for Venus makes it clear that it is very much like the Earth. So why doesn't the Earth have a lot of carbon dioxide in the atmosphere?

One straightforward answer is that the carbon dioxide which may have been present in greater concentrations in the Earth's early atmosphere got dissolved into the oceans and trapped into solid compounds in the surface minerals, so that nearly all of it is gone from the present atmosphere. In the frightening scenario of a runaway greenhouse effect on the earth, which transformed all the presently trapped carbon into gaseous carbon dioxide, the Earth could become similar to the present Venus. So the vast differences in the current states of the two planets arise from rather small differences in initial temperatures and atmospheric concentrations. The heat-trapping by the Earth's atmosphere was not sufficient to lead to a progressively rising temperature. The profoundly different histories of the Earth and Venus should cause us to be attentive to the prospects of global warming.

VENUS

     Venus is the second closest planet to our Sun at an average distance 67 million miles.  It revolves around the Sun every 225 Earth days, but its rotation takes 243 Earth days.  One other interesting note is that Venus rotates clockwise, the only planet in our solar system to do so, and contrary to its own orbit around the Sun.  Research suggests that the "backward" rotation is caused by tides which are raised in the thick atmosphere by the Sun, and with friction interaction between the atmosphere and planet itself.  It is hypothesized that these interactions caused Venus's rotation to slow, stop, and then reverse.  This is somewhat similar to what is happening here on Earth, as our Moon's pull on our oceans causes tides whose subsequent friction is gradually slowing Earth's rotation.  Other recent research suggests that Venus' clockwise rotation was produced by an extreme impact from a large body early in Venus' development, 4 to 5 billion years ago (similar to how a major impact created our Moon from Earth).  This large impact on Venus may have resulted in the planet reversing its rotation if the blow was from the opposite direction of its rotation.  Venus is one of only three planets in our solar system which has a perfect circular sphere (the other two being Mercury and Pluto).

Atmosphere and Weather:  Venus has a thick atmosphere made up almost entirely of carbon dioxide.  It also exerts a pressure 92 times higher than the earth's, resulting in a pressure-cooker environment.  Venus is covered by dense clouds made up primarily of sulfuric acid.  The clouds are so thick it is impossible to see its surface without using sophisticated radar systems.  Imaging radar systems aboard the NASA space probes Pioneer (1978) and Magellan (1990-1994) produced a map of the Venusian surface.  These systems also detected lightning, hypothesizing that the lightning is generated in thick clouds around 35 miles above the surface.  The surface is desolate and continent like features exist complete with canyons, mountains, plains and waterless oceans.  The Venusian surface is pock-marked with a low number of large craters along with volcanic rock and lava flows.  It appears that over 85% of the surface is covered by volcanic rock with giant lava flows having flooded areas producing the plains.  Research suggests that some of these volcanoes have erupted within the past few centuries and there is evidence that the entire planet was "resurfaced" three to five hundred million years ago.      

 The incident visible and infrared spectrum from the Sun are major contributors to the extremely hot temperatures on Venus, with the dense, carbon dioxide atmosphere not letting the infrared rays escape back into space.  These temperatures resemble something more of a raging fire due to an uncontrolled greenhouse effect which more than doubles the temperature on the planet's surface.  It appears that the surface temperature ranges from about 820 degrees to nearly 900 degrees F.  The average surface temperature is 847 degrees F., hot enough to melt lead.  No wonder the ocean basins are "dry".  At these temperatures any water would evaporate nearly instantaneously.  However research from NASA's Ames Research Center suggests that liquid water in the oceans and seas were prevalent on Venus for hundreds of millions of years in earlier times.  Due to Venus' immense pressure, water could have existed with temperatures as warm as 200 to 300 degrees F.  But that is still likely too hot to have sustained any primitive life forms.  As Venus continued to warm, any liquid water was boiled off into the atmosphere.  

The Pioneer Spacecraft in December 1978 revealed very high winds aloft in its atmosphere, approaching speeds of 200 mph.  Along with the thick cloud cover, this could explain why the night side of the planet has temperatures nearly the same as the day side, as strong winds aloft transfer heat around the globe.  However it appears that the surface winds are very light. 

In short, Venus is by far the hottest planet in our solar system, with a very minimal diurnal temperature spread.

QUICK FACTS(Data is from NASA Goddard)

Average distance from Sun	67,000,000 miles
Perihelion	66,600,000 miles
Aphelion	67,500,000 miles
Sidereal Rotation	243 Earth days
Length of Day	116.75 Earth days
Sidereal Revolution	225 Earth days
Diameter at Equator	7,504 miles (only about 5% smaller than Earth's)
Tilt of axis	177.4 degrees
Moons	None
Atmosphere	Carbon Dioxide 96.5%, Nitrogen 3.5%
Discoverer	Unknown
Discovery Date	Prehistoric