Uranus and Neptune remain two of the most mysterious objects in the solar system, primarily because they have been visited only by NASA's Voyager 2 spacecraft in 1986 and 1989, respectively. Their "ice giant" moniker comes from longstanding hypotheses that their interiors are comprised of an icy mantle beneath their hydrogen-helium atmospheres. While Jupiter and Saturn are also composed primarily of hydrogen and helium, Uranus and Neptune are hypothesized to have a layered structure composed of icy elements in their interiors.