2. What is the atmosphere?  The thin layer of gases that surrounds our planet.  Weather is the condition of the atmosphere at a certain place and time.  Our atmosphere makes life possible on our planet.  Provides us with oxygen.  Keeps surface warm so water can exist as a liquid.  Protects the surface from dangerous radiation from the sun.  Protects us from most meteoroids or rocks from space.

3. The Composition of the Atmosphere  Nitrogen makes up 78% of our atmosphere  Oxygen makes up 21%  Argon makes up almost 1%  All other gases have only trace amounts

4. Trace elements/compounds  Carbon dioxide: plants use to produce food. Plants take in CO 2 and give off oxygen as a waste product.  Animals take in oxygen to make energy and give off CO 2 as a waste product.  Fossil fuels (coal, oil, natural gas) give off CO 2 when they are burned. Rising CO 2 levels may cause global warming.  The fizz in soda is really CO 2 bubbles.  Dry ice is CO 2 in the solid state.

5. Ozone  Ozone is a molecule of oxygen with 3 oxygen atoms instead of 2.  Often formed when lightning interacts with oxygen in the air.  Forms a layer in the atmosphere that absorbs ultraviolet radiation from the sun. Without this layer life might not be possible on the surface of the planet.

6. Earth’s Early Atmosphere  The Precambrian atmosphere was composed mainly of nitrogen and carbon dioxide, along with some methane and ammonia.  Volcanoes spewed water vapor, carbon monoxide, and even more nitrogen and carbon dioxide into the air. Comets also contributed water vapor, but no free oxygen.  So where did the abundant oxygen that has made life as we know it come from?

8. Precambrian Life  The first life on Earth was cyanobacteria (like single celled blue-green algae).  They used photosynthesis to turn CO 2 into the food and energy they needed to survive.  Photosynthesis gives off oxygen as a waste product.  So over the millions of years, oxygen given off by algae built up in the atmosphere, while the amount of CO 2 declined.