Saturn's biggest moon is called TITAN, which has a diameter of 5, 150 km and is the second-largest moon in the Solar System. It is the only moon known to have a dense atmosphere and is the only object in space other than Earth where clear evidence of stable bodies of surface liquid has been found. TITAN is the sixth gravitationally rounded moon from Saturn and was discovered in 1655 by the Dutch astronomer Christiaan Huygens.