Titan is the only moon in the Solar System that has a thick atmosphere (the gases that surround a planet or moon). After the spacecraft Voyager I visited the moon on 12 November 1979, it showed that Titan's surface (the ground level) is hidden under an atmosphere that is 900 km thick. Before this, everyone thought that Titan was the biggest moon in the Solar System. Now we know that it is the second biggest, after Ganymede, one of Jupiter's moons.