What is the second-largest moon in the Solar System?

Titan is the second-largest moon in the Solar System, after Jupiter’s Ganymede. It is Saturn’s largest moon and has a diameter of about 5,149 kilometers. Titan is slightly larger than Mercury by diameter, although Mercury is far more massive because it contains much more dense iron and rock.

Titan’s exceptional size is only part of its importance. It is the only moon known to possess a dense atmosphere, composed mainly of nitrogen, and its surface pressure is higher than Earth’s. The atmosphere also contains methane, which forms clouds, rain, rivers, lakes, and seas in a weather cycle resembling Earth’s water cycle.

Before Voyager 1 flew past in 1980, astronomers thought Titan might be the Solar System’s largest moon. Its hazy atmosphere made the visible disk appear larger than the solid body really is. Voyager measurements established Ganymede’s lead. NASA’s Cassini mission and the Huygens probe later mapped Titan’s surface and confirmed its complex, organic-rich environment.