464
Guidepost
You have been to the moon and to Mercury, and now you are going to fi nd Venus and
Mars dramatically different from those small, inactive and airless worlds. Venus and
Mars have internal heat and atmospheres. The internal heat means they are geologically
active, and the atmospheres mean they have weather. As you explore, you will discover
answers to six essential questions:
Why is the atmosphere of Venus so thick?
What is the hidden surface of Venus really like?
How did Venus form and evolve?
Why is the atmosphere of Mars so thin?
What is the evidence that Mars once had water on its surface?
How did Mars form and evolve?
The comparative planetology questions that need to always be on your mind when you
explore another world are these: How and why is this world similar to Earth? How and
why is this world different from Earth? You will see that small initial differences can
have big effects.
You are a planet-walker, and you are becoming an expert on the kind of planets you
can imagine walking on. But there are other worlds beyond Mars in our solar system so
peculiar they have no surfaces to walk on, even in your imagination. You will explore
them in the next two chapters.Mars rover Spirit, about
the size of a riding
lawnmower, could not
take its own picture. It
has been digitally added
to this photo of the
Martian surface. Rovers
carry instruments to
analyze rocks and are
controlled from Earth.
(NASA/JPL-Caltech/Cornell)