3) An ocean within Europa? As the two spacecraft flew by the fourth-largest jovian moon, its icy crust showed a dizzying series of intersecting cracks. Calculations indicated the possibility of a liquid ocean deep beneath the ice. Such a feature likely exists because of the tidal interaction between the moon and Jupiter. But Europa’s orbit is closer to circular than Io’s, so the internal heating isn’t enough to create volcanoes — just enough to melt vast quantities of ice.
4) The Io torus. Voyager 1 found a thick ring of ionized sulfur and oxygen shed by Io that inflates Jupiter’s giant magnetic field. The material originates within the moon’s volcanoes, some of which are so powerful that they erupt it directly into space.
5) Saturn’s ring structure. Before 1980, astronomers recognized fewer than six rings around Saturn. But Voyagers’ cameras showed that each ring had numerous subdivisions. In addition, Voyager 1 discovered that the enigmatic F ring has two small “shepherding” satellites, Pandora and Prometheus, whose gravity keeps the ring in place.
6) Titan’s atmosphere. Voyager 1 showed that Titan has a nitrogen atmosphere with a surface pressure 45 percent greater than on Earth. Voyager data hinted at the possibility (later confirmed) that this satellite experiences clouds of methane and other hydrocarbons, and that rain falling from those clouds creates lakes of liquid methane on the surface.
7) The Great Dark Spot. As Voyager 2 approached Neptune, scientists identified a gigantic dark feature. It was dubbed the Great Dark Spot, and researchers were at a loss to explain how such a storm could form given the small amount of energy Neptune receives from the Sun. Further study showed the Great Dark Spot, and similar features observed since Voyager 2 passed by, are cyclones that exist as holes in the planet’s upper atmosphere.
8) Neptune’s supersonic winds. The discovery of the fastest winds in the solar system in the atmosphere of the most distant planet was a stunner. Voyager 2 measured wind speeds of 1,100 mph (1,600 km/h) above Neptune. Because the planet radiates 2.6 times as much energy as it receives from the Sun, researchers think the decay of radioactive elements deep within Neptune powers the currents.