Titan

Titan is Saturn's largest moon, with a diameter of 3200 miles (5150 km), and is the second-largest moon in the solar system after Jupiter's Ganymede. It is larger than the planet Mercury. Titan orbits Saturn at an average distance of 758,000 miles (1,222,000 km). Titan keeps the same face toward Saturn, so its rotation and orbital period around Saturn are the same (about 16 days).

Titan is unique in the solar system as the only moon known to have a dense atmosphere. The atmosphere is very thick and hazy, and completely obscures the surface at optical wavelengths. Titan's atmosphere extends about 600 km into space, about 10 times further than Earth's; because of this, Titan was once thought to be the largest moon in the solar system. The atmosphere is composed mostly of nitrogen, with some methane and traces of other hydrocarbon compounds.

The atmospheric pressure at Titan's surface is 50% higher than on Earth's. Titan's surface temperature is also very cold, at -290°F (-178°C). Though obscured by the atmosphere, radar observations of the surface revealed large, dark, flat areas which are now believed to be oceans of liquid methane.

Titan was visited in 2005 by the Cassini-Huygens spacecraft, which was the first mission to land on a moon of another planet. It revealed features similar to Earth's sand dunes, shorelines, lakes, and rivers. This confirmed that Titan is the second object in the solar system (after Earth) to have substantial portions of its surface covered by liquid.