Because of its slow rotation, Mercury's solar day is twice as long as its year. Although daytime temperatures are very high, temperature plummets during the long nights. The range of surface temperatures is greater for Mercury than for any other planet or satellite.
The dominant features of Mercury's surface are impact craters. Because of Mercury's relatively large gravity, debris from impacts was not thrown as far as for lunar craters. Thus, young craters have not been effective at erasing old craters.
A pattern of great scarps shows that Mercury's crust has been altered since it formed. This probably happened when Mercury shrank, fracturing the crust. All internal activity in Mercury probably ended about 3.8 billion years ago.
Mercury's atmosphere is extremely tenuous. It consists of gases trapped from the solar wind or released when meteoroids struck Mercury.
The average density of Mercury is almost as large as that of the Earth. This shows that Mercury contains a large proportion of heavy elements, the most abundant of which is probably iron.
Mercury's high iron content may have resulted from the loss of most of its rocky exterior. This may have happened when Mercury collided with another body almost as large as Mercury.
Venus is an especially important planet to understand, because it is similar to the Earth in many ways. By comparing the Earth and Venus, we can see how small initial differences in the two planets have led to significant differences today.
Unlike the other terrestrial planets, Venus has a retrograde rotation. The rotation of Venus is very slow, resulting in a solar day that is 117 Earth days long.
The lower atmosphere of Venus is as hot as an oven. The large amount of carbon dioxide in Venus's atmosphere results in a surface pressure 92 times as large as that of the Earth.
The thick cloud layer of Venus is made of concentrated sulfuric acid droplets. The main cloud layer is 25 km thick. Nevertheless, enough sunlight penetrates the clouds to make Venus's surface as bright as the Earth's on a cloudy day.
The thick atmosphere of Venus produces a strong greenhouse effect that is responsible for Venus's high atmospheric temperature.
Venus's surface has many impact craters. However, there are no small impact craters because small meteoroids are destroyed in the dense atmosphere before they reach the ground. Venus appears to have undergone planetwide lava flooding about a half billion years ago.
The surface of Venus shows abundant evidence of volcanic activity and crustal distortion. However, surface features seem to have been formed through vertical motion in Venus's crust rather than plate tectonics. A possible reason that plate tectonics is inhibited on Venus is that Venus's high crustal temperature may make the crust too buoyant to be easily forced into the interior.
Similar amounts of nitrogen and carbon dioxide have been released from the interiors of the Earth and Venus. In the case of Venus, almost all of the outgassed nitrogen and carbon dioxide remains in the atmosphere. Whatever water that was released from Venus's interior, however, has been removed from the atmosphere.
The climates of Venus and the Earth were probably quite similar at first. However, the warmer temperature of Venus's atmosphere caused it to be more humid than that of the Earth. The additional water vapor partially blocked the escape of infrared radiation and produced a greenhouse effect, further warming Venus. The atmosphere quickly became much hotter, evaporating surface water. Without oceans, carbon dioxide could not be deposited as carbonaceous rocks and therefore entered the atmosphere, resulting in the thick, hot atmosphere of Venus today.
