Mean diameter: approximately 5,150 km · Orbits Saturn.
Saturn’s largest moon is the only moon known to have a substantial atmosphere. Titan’s surface has rivers, lakes and seas of liquid hydrocarbons, including methane and ethane, rather than Earth-like surface water.
The haze in the Cassini image hides much of that landscape. Titan is wider than Mercury, but the comparison concerns diameter alone: being larger than a planet does not make a satellite a planet.
Sources: NASA moon guide · JPL size data.
Image source: NASA/JPL-Caltech/Space Science Institute; reuse terms. Resized and JPEG-compressed.