various theories explaining why Mars cannot sustain life

 various causes of geological changes on Mars

 the development of views about the nature of Mars

why it has been difficult to obtain information about Mars 

 it had a few geological features of Interest

 it was similar to Earth but colder

 it had at one time supported life

 it had water under its surface 

 Mars was probably even drier than it is today

the atmospheric pressure and the temperature may have been higher than they are today

Mars was inhabited by organisms that have since become fossilized

large floods were shaping the planet's surface 

 it may indicate current volcanic activity on Mars

 they may indicate that the surfaces Mars is becoming increasingly drier

 they may indicate  the current existence of water on Mars

 they may hold fossils of organisms that once existed on Mars 

 to stress that Mars is no longer interesting to explore

 to describe items that the spacecraft brought back from Mars

 to share his interest in the study of fossils

 to show how much the view of Mars has changed based on new evidence 

 to ask for clarification of a previous statement

 to convey his opinion

 to rephrase an earlier question

 to express his approval