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Guidepost
In the preceding chapter, you learned how our solar system formed as a by-product
of the formation of the sun. You also saw how distance from the sun determined the
general composition of each planet. In this chapter, you begin your study of individual
planets with Earth. You will come to see Earth not only as your home, but as a planet
among other planets. On the way, you will answer four essential questions:
How does Earth fi t among the Terrestrial planets?
How has Earth changed since it formed?
What is the inside of Earth like?
How has Earth’s atmosphere formed and evolved?
Like a mountain climber establishing a base camp before attempting the summit, you
will, in this chapter, establish your basis of comparison on Earth. In the following chap-
ters you will visit worlds that are un-Earthly but, in some ways, familiar.
Earth: The Standard of
20 Comparative Planetology
The beauty of planet
Earth can be deceptive. It
was not always as it is
now, and it will not
survive unchanged
forever. It was born in
the solar nebula, and its
end will come when the
sun dies. (Jean-Bernard
Carillet/Getty Images)