The second-largest moon in the Solar System and the only one with a dense nitrogen atmosphere. Titan features liquid methane and ethane rain, rivers, and vast lakes like Kraken Mare.

Encased in a smooth shell of water ice, Europa shelters a global liquid water ocean beneath its crust containing twice the volume of all Earth's oceans combined — a top target for astrobiology.

Torn by extreme gravitational tidal flexing between Jupiter and Europa, Io is the most volcanically active body in the Solar System, with over 400 active volcanoes spewing sulfur plumes 500 km high.

A dazzling white icy world that reflects nearly 100% of sunlight. Hydrothermal geysers at its south pole erupt jets of water ice, salt, and organic compounds into space, feeding Saturn's E ring.

The only large moon with a retrograde orbit (orbiting opposite to Neptune's rotation), suggesting it is a captured Kuiper Belt object. Triton features nitrogen ice geysers and cantaloupe-like terrain.

Mars's tiny, lumpy, cratered moons are thought to be captured C-type asteroids. Phobos orbits so close to Mars (6,000 km) that it orbits faster than Mars rotates and will eventually break apart or crash.