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Terrestrial Planets
Identify the terrestrial planets -- also called inner planets. They are the planets closest to the Sun, and include Mercury, Venus, Earth and Mars. Although the Moon is not a planet, scientists often include it on the list of terrestrial planets because its size, composition and history are closely related to the development of the inner planets. The terrestrial planets have dense rocky surfaces and relatively thin atmospheres. They also spin more slowly than the Jovian planets.
Terrestrial Climates
Recognize differences in the climates of terrestrial planets. These vary considerably. For example, Mercury is a dry, lifeless rock covered in craters, with a very thin atmosphere; its average daytime temperature is 750 degrees Fahrenheit, while its average nighttime temperature is 200 degrees below zero. So much heat is trapped in Venus's atmosphere -- which consists of 95 percent carbon dioxide -- that Venus's average temperature day or night is about 864 degrees. Instead of water, sulfuric acid clouds drench the planet's surface in sulfuric acid. Mars' atmosphere is also made of carbon dioxide. However, unlike Venus, where volcanic activity adds carbon dioxide to the atmosphere, Mars has no volcanoes to replace heat-trapping carbon dioxide. Mars is a frozen wasteland where daytime temperatures average 20 degrees and nighttime temperatures drop to 120 degrees below zero.
Jovian Planets
Note that the Jovian, or Jupiter-like, planets -- also called the outer planets and gas giants -- are those farthest from the Sun. They include Jupiter, Saturn, Uranus and Neptune. The giant Jovian planets are more massive than the Earth and have tiny rocky cores surrounded by thick layers of gas, particularly hydrogen and helium. None of the Jovian planets has a rocky surface. The Jovian planets spin faster than the terrestrial planets and are flatter at their poles than the terrestrial planets.
Jovian Climates
Recognize the varied and changeable Jovian climates. Jupiter and Saturn have relatively stable climates without seasonal changes. Scientists believe that Jupiter lacks seasons because it tilts very little on its axis, while Saturn lacks seasons because its internal heat keeps planet-wide temperatures the same all year. Jupiter, Saturn and Neptune are known for their powerful storms. Jupiter, in particular, is known for the Great Red Spot, a storm more than twice the size of Earth. Scientists believe this storm has been raging for centuries. Neptune is known for a similar storm called the Great Dark Spot. Although this storm disappeared in 1994, it was replaced by another massive storm soon after. When Voyager 2 visited Uranus in 1986, very little atmospheric activity was noted. However, new observations in 2004 showed rapid development of clouds as Uranus's southern atmosphere turned towards the sun for the first time in decades.