Jupiter the giant. Even its most obvious feature, the red spot, is wider than Earth. What we see from the outside are the outermost layers of its deep atmosphere, where the colored bands of swirling gases, in places rising and elsewhere falling, hold frozen substances, including ammonia and water. If we could ourselves descend into that atmosphere it’s interesting to imagine what we might see—perhaps at first, there would be sunlight sparkling through the ice crystals, but as we go deeper, the light would surely fade. We’d find ourselves not just in darkness but in an environment of ever-increasing pressure—an unknown world indeed. The moons have a more familiar kind of structure. Unlike their planet they have solid surfaces that, in principle, we could stand on. On some of them, such as fiery Io and iceworld Europa, we would need to be wary—the first of those has its volcanoes, and the second has a seemingly dynamic ice layer hiding an ocean of familiar liquid water.