Original: Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.
Simplified: Saturn is a planet in the Solar System. Saturn is the second largest planet in the Solar System; the largest is Jupiter. The largest moon is Titan, which is larger than the planet Mercury. Titan is the largest moon, larger than the planet Mercury, and it is the only moon in the Solar System to have a thick, dense atmosphere.