Titan is the Saturnian moon larger in diameter than Mercury.
Titan measures about 5,149 kilometres across, approximately 6% wider than Mercury. It is the Solar System's second-largest moon, behind Jupiter's Ganymede, and the only moon known to possess a dense atmosphere comparable in scale to a planet's. Its thick orange haze once made Titan appear even larger than it really is.
The size comparison can be misleading because Titan is much less massive than Mercury. Titan is made largely of water ice and rock, whereas Mercury has a disproportionately large iron-rich core. As a result, Mercury contains about 2.5 times Titan's mass despite having the smaller diameter.
Titan was discovered by Christiaan Huygens on March 25, 1655. The Voyager missions refined its size and showed that Ganymede—not Titan—is the largest moon. Cassini-Huygens later revealed Titan's methane clouds, rivers and lakes, making it one of the most Earth-like worlds in the outer Solar System.