So Close and Yet So Far The Inner Planets

In This Chapter

V Where do the planets' names come from?

V Some vital statistics

V Orbit and rotation

V Atmospheres on the terrestrial planets

V Planetary surfaces

V The geology of the terrestrials

V How everything might have been different

Our two closest neighbors in the solar system, Mars and Venus, are a constant reminder of how easily things could have turned out differently here on Earth. Venus is so hot and forbidding that it might be a good place to film Dante's Inferno, and while pictures of Mars may resemble the American Southwest, it has an atmosphere so cold and thin that it's hardly there. Putting a planet a little closer to the sun or a little farther away can truly make all the difference in the world. Equally amazing is that all three planets fall within what is called the "habitable zone" of the sun (see Chapter 24, "Table for One?"). The habitable zone is the distance range from the sun within which water could be liquid on the planet's surface. But only one, Earth, has abundant liquid water. Recent imaging of the surface of Mars indicates that water may still briefly exist in liquid form on its surface.

In this chapter, we will take a closer look at the four rocky planets that are closest to the sun. Astronomers call these four planets terrestrial, meaning Earth-like. To be sure, Mercury, Venus, and Mars resemble Earth more than they do the jovian planets— Jupiter, Saturn, Uranus, and Neptune—but, as we'll see, these four worlds are as different from each other as one could imagine. And the earth is the only one of the four that has sustained life.

Why is that? What is it about the earth that allowed life to thrive here and not on the other three planets in the inner solar system? Let's start looking for some answers.

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