Jupiter and
Saturn are the best‐studied of the jovian planets. It is from these huge, gassy planets that we have learned most about the primordial gases from which all of the planets are believed to have condensed. Jupiter's atmosphere consists mostly of hydrogen, ammonia, helium, and methane. Jupiter and Saturn give off twice as much energy as they receive from the Sun, which suggests they are still undergoing the process of gravitational contraction and condensation. One of Jupiter's moons, Io, is volcanically active, and its bright orange colors may indicate sulfur‐rich rocks. Photography has revealed long lava flows radiating out from the volcanic cones. Abundant andesitic volcanism has created lava plains and huge shield volcanoes. Few impact craters are seen because they are quickly covered by volcanic flows. Another kind of volcanic activity throws explosive sprays of sulfurous material up to 300 kilometers into the atmosphere. Jupiter's outer moons—Europa, Ganymede, and Callisto—are covered in ice. Intriguing patterns of grooves, striations, and cracks may be the result of some kind of plate tectonic activity that is affecting their frozen, rigid surfaces. Saturn's moons are also ice covered and pitted with craters. The moon Titan has an atmosphere that consists mostly of nitrogen, acetylene, ethane, ethylene, and hydrogen cyanide. It is conceivable that Titan's surface is covered by continents of ice and by liquid ethane and methane.
Much of what we know about our solar system is through the application of uniformitarianism—that the “present is the key to the past” and thus that the geologic processes we see today were active in the geologic past. The integration of our ongoing exploration of Earth and the planets around us will lead us to a better understanding of how our solar system formed. Our heightened awareness of geology, its active processes, the resources we consume, and the end products we generate can lead us to a better coexistence with the planet Earth.