The largest and the sixth moon of Saturn discovered by Christiaan Huygens in 1655. Called also Saturn VI. Titan has a diameter of 5,150 km, about half the size of Earth and almost as large as Mars. It orbits Saturn at a mean distance of 1,221,830 km every 15.945 days. It is the only moon known to have an atmosphere. Its surface temperature is -179 °C, which makes water as hard as rocks and allows methane to be found in its liquid form. Its surface pressure is slightly higher than Earth's pressure (1.6 bars against 1 bar at sea level). The Huygens probe released from Cassini-Huygens landed on Titan on December 25, 2004. From the data obtained by Cassini-Huygens, we know that Titan is a world with lakes and seas composed of liquid methane and ethane near its poles, with vast, arid regions not made of silicates as on Earth, but of solid water ice coated with hydrocarbons that fall from the atmosphere. Titan's icy dunes are gigantic, reaching, on average, 1 to 2 km wide, hundreds kilometers long and around 100 m high. Titan is the only other place in the solar system known to have an Earth-like cycle of liquids flowing across its surface as the planet cycles through its seasons. Each Titan season lasts about 7.5 Earth years. The Huygens probe made the first direct measurements of Titan's lower atmosphere. Huygens also directly sampled aerosols in the atmosphere and confirmed that carbon and nitrogen are their major constituents. Cassini followed up Huygens' measurements from orbit, detecting other chemicals that include propylene and poisonous hydrogen cyanide, in Titan's atmosphere. Cassini's gravity measurements of Titan revealed that this moon is hiding an internal, liquid water and ammonia ocean beneath its surface. Huygens also measured radio signals during its descent that strongly suggested the presence of an ocean 55 to 80 km below the moon's surface.