Earth
The third planet out from the Sun is shown inFigure25.14. Does it look familiar? It’s
Earth! Because it is our home planet, we know a lot more about Earth than we do about
any other planet. But what are key features of Earth when viewed as a member of our solar
system?
Figure 25.14: This famous image of Earth was taken during the Apollo 17 mission to the
moon. ( 21 )
Oceans and Atmosphere
As you can see inFigure25.14, Earth has vast oceans of liquid water, large masses of land,
icecoveringthepoles, andadynamicatmospherewithcloudsofwatervapor. Earth’saverage
surface temperature is 14oC (57oF). As you know, water is a liquid at this temperature. The
oceans and the atmosphere help keep Earth’s surface temperatures fairly steady.
Earth is the only planet known to have life. The conditions on Earth, especially the presence
of liquid water, are ideal for life as we know it. The atmosphere filters out radiation that
would be harmful to life, such as ultraviolet radiation and X rays. The presence of life has