Venus, greenhouse effect

Almost 80% of the sunlight that hits Venus is reflected back into space by the thick clouds surrounding the planet before it ever reaches the surface. Even so, temperatures at the surface of Venus are much hotter than those on Earth. However, this is not because Venus is closer to the Sun than the Earth. Scientists believe that the difference in the temperatures of the two planets is due to a runaway greenhouse effect caused by the large amount of sulfur dioxide in Venus atmosphere. 

AU from the Sun, just outside the orbit of Mars, whereas the inner boundary is determined by the runaway-greenhouse effect as observed on Venus. If the surface temperature were too hot, above 373 K, this would vaporise all water on the surface of the planet. The inner boundary is around 0.85 AU so the habitable zone spans 0.85-1.7 AU for our Sun (Figure 7.7) but the current habitable zone spans 0.85 - 1.3 AU (t — 0) in Figure 7.7. The habitable zone was much larger when the Sun s luminosity was greater, and narrower when the luminosity was smaller. 

Water vapour makes a sizeable contribution, and probably the largest, to radiation trapping and as the temperature increases the water vapour concentration increases. Temperature rises as a result of increased water vapour concentration and hence a mechanism for a positive feedback in the greenhouse effect that might lead to a runaway greenhouse effect. When the vapour pressure for water reaches saturation, condensation occurs and water rains out of the atmosphere this is what happens on Earth and Mars. On Venus, however, the water vapour pressure never saturates and no precipitation occurs and the global warming continues to increase. Thus Venus suffers from extreme temperatures produced by both its proximity to the Sun and the presence of water vapour and carbon dioxide in its atmosphere. 

The combination of forces that caused these changes is still in question but it involves the stability of the climate. The climate has fluctuated between limits of plus or minus 15°C (27°F) for hundreds of millions of years. These limits are large enough to have a major influence on species extinction and evolution. A runaway greenhouse effect is thought to have changed Venus where the oceans boiled. 

Two important applications of radiation to determine molecular structure—X-ray crystallography and magnetic resonance—were discussed in Chapters 3 and 5. In this chapter we will discuss a variety of other techniques. Microwave absorption usually forces molecules to rotate more rapidly, and the frequencies of these absorptions provide a direct measure of bond distances. Individual bonds in a molecule can vibrate, as discussed classically in Chapter 3. Here we will do the quantum description, which explains why the greenhouse effect, which overheats the atmosphere of Venus and may be starting to affect the Earth s climate, is a direct result of infrared radiation inducing vibrations in molecules such as carbon dioxide. 

Venus has about as much carbon dioxide near its surface as Earth, but the carbon dioxide is held as gas, not bound in carbonate, so there is a massive greenhouse effect (Lewis Prinn... 

There does not seem to be much chance that the Earth would be vulnerable to a runaway greenhouse effect such as Venus where the oceans would boil away. But, climate changes as great as an ice age could be disastrous if they occurred rapidly. 

In fact, the conditions on Venus today are sometimes described as the result of a "runaway greenhouse effect. The planet s surface temperature of about 730 K (430°C) cannot be explained simply on the basis of the heat it receives from the Sun. Indeed, it is warmer... 

As time evolved and the energy provided by the sun increased, the gradual removal of carbon dioxide from the atmosphere became critical to avoid a runaway greenhouse effect (with extremely high surface temperatures) as observed on Venus. This removal of CO2 was accomplished by weathering of calcium silicate (CaSiOs) minerals by acidic C02-rich rainwater, leading to the formation of limestone (CaC03). 

A dramatic illustration of the greenhouse effect is found on Venus, where the atmosphere is 97 percent CO2 and the atmospheric pressure is 9 X 10 Pa (equivalent to 89 atm). The surface temperature of Venus is about 730 K ... 

Fig. 12-1. Surface temperatures expected on the three planets Venus, Earth, and Mars as a function of the water vapor pressure. An increase in the vapor pressure increases the retention of infrared radiation in the atmosphere, raising the temperature via the greenhouse effect. Overlaid is the phase diagram of water. On Earth and Mars the starting (radiation equilibrium) temperatures are low enough for water to condense out when the temperature intersects the condensation curve. On Venus, the temperature rises more rapidly and runs away. [Adapted from Walker (1977), originally modeled by Rasool and DeBergh (1970).]...

Venus -- has a slow rotation when compared to Earth. Venus and Uranus rotate in opposite directions from the other planets. This opposite rotation is called retrograde rotation. The surface of Venus is not visible due to the extensive cloud cover. The atmosphere is composed mostly of carbon dioxide. Sulfuric acid droplet in the dense cloud cover gives Venus a yellow appearance. Venus has a greater greenhouse effect than observed on Earth. The dense clouds Combined with carbon dioxide traps heat. Venus was named after the Roman goddess of love. 

Runaway Greenhouse Effect Effect produced when a planet builds up an intense surface temperature like that on Venus because its atmosphere tends to trap heat rather than allow it to escape to space. 

The greenhouse effect is obviously demonstrated by our neighboring planets. Venus has an atmosphere of almost pure COj. In addition it is covered by white clouds. Its surface temperature is about 430°C. Thus Hquid water cannot be present on its surface. If present in earHer periods, also then with high contents of carbon dioxide, the radiation would have increased the surface temperature to levels at which water has vaporized. Water vapor also is an effective greenhouse gas, so the temperature increased by the combined action of water and carbon dioxide. Venus is thus a nightmare scenario of the danger with the greenhouse effect. 

Compare Earth to its nearest planets—Venus and Mars. The atmosphere of Venus is so thick that observers cannot see the surface of Venus from Earth. (Our only images of Venus s surface have come from probes that parachuted through Venus s atmosphere to the surface, and then promptly melted.) As a result, Venus experiences a runaway greenhouse effect that keeps Venus too hot to support liquid water or (probably) Ufe as we know it. Humans could not Uve on Venus. 

Water vapor is an important trace gas in the lower Venus atmosphere and it contributes to the global greenhouse effect that maintains the high surface temperature. Most of the H2O is found below the cloud base at about 47 km. Its signature can be observed in a spectral window between 0.9 and 2.5 pm where the atmospheric absorption is weak, therefore H2O absorption features can be seen. For H2O abundance determinations the following windows are most often used 1.18, 1.74 and 2.3 pm. 

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