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Guidepost

Want to fl y to the moon? You will need to pack more than your lunch. There is no air

and no water, and the sunlight is strong enough to kill you. Mercury is the same kind of

world. Take shelter in the shade, and you will freeze to death in moments. Earth seems

normal to you, and other worlds are, well, unearthly, but they are related to Earth in

surprising ways. Exploring these two airless worlds will answer fi ve essential questions:

Why is the moon airless and cratered?

How did the moon form and evolve?

In what ways is Mercury similar to, and different from, the moon?

How did Mercury form and evolve?

How are the histories of the moon and Mercury connected to Earth?

You are beginning your detailed study of planets by exploring airless worlds; in the next

chapter you will move on to bigger planets with atmospheres. They are not necessarily

more interesting places, but they are just a bit less unearthly.

The Moon and Mercury:

Comparing Airless

Worlds

21

When astronauts stepped

onto the surface of the

moon, they found an

unearthly world with no

air, no water, weak

gravity, and a dusty,

cratered surface. Through

comparative planetology,

the moon reveals a great

deal about our own

beautiful Earth. (JSC/NASA)