An atmosphere consists of matter in a gaseous state. Matter close to the sun is so hot that it is gaseous, but farther from the sun, where the temperature is lower, matter begins to solidify.

     Rocks and minerals can be solid even at (to us) high temperatures. It is not surprising, then, that our inner planets, Mercury, Venus, Earth and Mars, have solid rocks on their surfaces. Mercury, however, has very little atmosphere. This is so partly because Mercury is a small planet with low gravity and partly because the sun is emitting radiation that forms "the solar wind." This stream of fast moving waves and particles tends to tear the atmosphere of Mercury away. Venus, Earth and Mars all have atmospheres. Mars, however, is a cooler planet than the earth, so that carbon dioxide, which is seen only as a gas in natural surroundings on earth, may freeze at the martian poles. Mars is also smaller than earth, and so has less mass. It is likely that a good deal of the original atmosphere of Mars escaped into space. Mars has a very thin atmosphere now compared to earth's atmosphere.

    The planets further from the sun receive less heat, and so are made up at least partly of solidified matter that would be gaseous on earth. Under cold enough conditions, the gases that we are familiar with on earth become liquids, and then solidify as temperatures drop. Think of how water solidifies into ice, and how lava becomes solid stone as it cools. Gases such as oxygen and nitrogen solidify on the cold outer planets.

    A planet (or moon) must be large enough to be able to use its gravity to hold onto an atmosphere. Where there is an atmosphere, there will be atmospheric currents, wind, perhaps even rain, ice, and snow if the temperatures permit that.. If there are oceans, there will be currents in the liquids, too. Weather is affected by the land forms of the planet, and in turn alters those land forms by erosion. Living things adapt to conditions where they live, and so weather patterns determine such things as where (and whether) there will be forests and grasslands on the planet. The vegetation, in turn, determines what kinds of animals can evolve in different areas.

     An atmosphere may also filter radiation.  On earth, the ozone layer protects all life from ultraviolet radiation, which can kill life forms.  The earliest life forms lived in water where water could filter out harmful radiation,  After photosynthesizing bacteria released oxygen into the air, it became possible for life forms to survive out of water.

     As you can see, the way you set up your planet will affect the life forms that will live there. Check these web sites to learn how weather works.

     Your may also go to Science Notes to see what material is available for your chapter. Then Return to Lesson 3 and begin work on the assignments.