The largest moon of Saturn, which has a thick, nitrogen-rich atmosphere, is:

Titan is Saturn's largest moon. It is bigger than the planet Mercury and is the only moon in the solar system with a dense atmosphere.

Discovery:

Titan was discovered in 1655, the first moon of Saturn to be found.

Atmosphere:

• Titan's atmosphere is made mostly of nitrogen (about 95 percent), with some methane.
• The surface pressure is about 1.5 times that of the Earth.
• A thick orange haze of organic compounds hides its surface from view.

Lakes and rivers:

• Titan is the only body in the solar system, other than the Earth, known to have stable liquids on its surface.
• But its lakes, seas and rivers are made of liquid methane and ethane, not water, because the surface temperature is about minus 179 degrees Celsius.
• It has a methane cycle, similar to the Earth's water cycle, with methane clouds and rain.
• Its largest sea, Kraken Mare, is larger than the Caspian Sea.

Interior:

• Scientists believe Titan has a hidden ocean of liquid water beneath its icy crust.
• Because of its organic chemistry and possible underground ocean, Titan is a major target in the search for conditions related to life.

Exploration:

Voyager 1 flew past Titan in 1980.

A European probe landed on Titan's surface in January 2005, after being carried there by the Cassini spacecraft. It was the first landing on a moon in the outer solar system.

NASA's Dragonfly mission, a drone-like rotorcraft, is planned to explore Titan's surface in the 2030s.

Other moons of Saturn:

• Enceladus, which shoots jets of water vapour from its south pole, indicating an underground ocean.
• Mimas, with a large crater that makes it look like the "Death Star" from Star Wars.
• Rhea, Iapetus and Dione are other large moons.

Why not the other options?

• ✗Europa and Io are moons of Jupiter.
• ✗Triton is the largest moon of Neptune, notable for orbiting in the direction opposite to Neptune's rotation.