58 



Figure 1Y-4.— The ring system of Saturn is composed of ice. 



Earth? We are apprehensive about this question since our planet is so 

 different from its neighbors. 



The planets in our solar system that are most similar to Earth 

 are Mars and Venus. Historically, this apparent similarity in gross 

 properties led many scientists to anticipate that these planets might 

 also be populated with some form of life. Yet, as we learned more 



