Venus taken by the Galileo spacecraft in 1990.
Venus and Mercury are rather difficult to observe from the Earth. There are several reasons for this:
Venus taken by the Galileo spacecraft in 1990.
The Venus atmosphere is composed almost entirely of carbon dioxide (96%), which is very different from the Earth, where CO2 is a trace compound that is not very abundant. [Recall from our discussion of Earth's climate evolution that we think that at one time, the Earth DID have a lot more CO2 in its atmosphere.] The Venus surface pressure is 90 x that of the Earth, meaning that if you stood on the surface of Venus, the weight of the atmosphere above you would be 90 x greater than what you feel on Earth's surface. [This is about the pressure you would feel 1 km deep in the Earth's ocean.]
Venus images taken by the Mariner 10 spacecraft on its way to Mercury in 1973.
Venus' thick cloud layer is composed of sulfuric acid droplets, which is formed when sulfur dioxide combines with water. On Earth, we get sulfur dioxide from volcanic eruptions, but it gets washed away by rainfall ("acid rain"). On Venus, if there were volcanic eruptions in the past, the SO2 would have combined with whatever water was around to form sulfuric acid, and it stayed in the atmosphere.
The surface temperature of Venus is 730 K: extremely hot!!! This is much hotter than you would expect, even given that Venus is closer to the Sun than the Earth. The additional heating turns out to be due to the greenhouse effect. This effect is much more pronounced on Venus, since it has about a million times more CO2 (a greenhouse gas) than we do on Earth. The sunlight that eventually reaches the surface of Venus slowly heats it, and then cannot escape back to space. In the below image, the thermal infrared radiation from Venus' night side can be seen escaping through some spaces between the patchy clouds.
Infrared image of Venus taken by the Galileo spacecraft in 1990 on its way to Jupiter.
The question is: was Venus always this hot, or did it start out similar to the Earth and then evolve into something very different from the Earth? We think that Venus may have resembled Earth in its early history. Some small amount of heating (from an increase in energy output from the Sun, or from a small increase in atmospheric CO2) caused the oceans to evaporate and gas to be released from rocks on the surface. This led to more greenhouse gases in the atmosphere, which in turn increased the surface heating, which led to more evaporation, etc. This is known as the runaway greenhouse effect, which is an irreversible process and leads to drastic changes in its atmosphere and surface.