Imagine you are entering our solar system in your travels from a distant star system, looking for habitable worlds. Your spacecraft approaches the inner solar system on the opposite side of the Sun from Earth. You encounter Mars, with its thin, chilled, unbreathable atmosphere and barren desert landscapes. Not very promising. Moving much closer to the Sun, the next planet you happen to encounter, Venus, is enshrouded in corrosive clouds hiding a menacingly hot surface. Finally, moving around the Sun and heading back outward, you spy Earth, with its liquid-water oceans orbiting our star between these two relatively forbidding worlds—one too cold, the other too hot. As noted near the end in Chapter 5, Earth is in the goldilocks zone around our Sun: not too hot, not too cold, just right (for life). Ever-changing white clouds pirouette above the browns and blues of Earth’s continents and oceans. Its close companion, the Moon, pockmarked by countless craters, helps cause those oceans to move up and down in a ceaseless rhythm. Earth is the planet that you seek.

In this chapter, we will explore Earth and the Moon, including what makes Earth suitable for life. Our detailed and rapidly developing knowledge of Earth allows us to present material on topics that range from the magnetic fields surrounding it, to its atmosphere and climate, to the depth and composition of its crust, and to the properties extending to its metal-rich core. While also increasing, our knowledge about the Moon is much more limited. We will explore in the next chapter the other terrestrial worlds in our solar system, all with a variety of different properties from Earth, leading to vastly different environments from our own.