Rivers on Earth, Titan, and Mars

One of the more distinctive things about Earth among the planets is that we have plate tectonics. In other words, the hard, outer shell of the planet (called the lithosphere) is divided into several cool, rigid plates that float atop a hotter, more fluid layer of rock (the asthenosphere). These rigid surface plates do not float placidly: their grinding, colliding, shifting, and diving causes earthquakes, fuels volcanoes, builds mountains, tears open oceans, and constantly remodels and resurfaces the planet.