For the past four hundred years, astronomers have gazed at the planets through telescopes and wondered if those worlds are like our own planet Earth. Now, in just the past few decades, advances in spacecraft technology have allowed us to send orbiting probes and robotic rovers to carefully study the planets and their moons. In Chapter 5, we systematically looked at some of the geologic processes that shape Earth, resulting from an energetic molten interior. Using these ideas, we are now in a position to look at other solar system objects and to see how they are similar to or different from our own. This way of thinking leads us to a fascinating question: If planets and moons formed more than 4 billion years ago, how can modern-day astronomers determine the seemingly complex histories of planets and moons beyond Earth? Just as ancient fossils on Earth provide evidence of how life has evolved on our planet, ancient rocks and geological formations help us understand how planets’ surfaces and atmospheres have evolved. For example, the oldest rocks on Earth have a chemical structure showing that they formed in a time when there was little oxygen in the atmosphere and, hence, before the appearance of photosynthetic planet life. Planetary scientists who want to learn about the history of Mars are therefore very interested in looking carefully at ancient rocks on that world. Now that we understand the interior, surface, and atmosphere of Earth, we can compare it to the other planets and moons in our solar system, such as the landslide and crater on Iapetus, one of Saturn’s moons, in the chapter-opening image. In this chapter we will focus on the terrestrial planets Mercury, Venus, and Mars. We will look first at the forces that have molded them—impacts, volcanism, and tectonics—and then briefly consider their atmospheres. We can also begin to explore where else in the solar system water exists and speculate about whether or not sufficient water is present to sustain living organisms.