Titan (/ˈtaɪ.tən/, from Ancient Greek Τῑτάν) or Saturn VI is the largest moon of Saturn and the second largest moon in the Solar System, Until 2004, this atmosphere prevented understanding of Titan's surface, but with the arrival of the Cassini–Huygens mission, new information about it has accumulated, such as the discovery of liquid hydrocarbon lakes near its north pole. These are the only large, stable bodies of surface liquid known to exist anywhere other than Earth.
Titan is never visible to the naked eye, but can be observed through small telescopes (diameter greater than 5 cm) or strong binoculars. It has a maximum magnitude of +7.9, outshone by six asteroids (Vesta, Pallas, Iris, Hebe, Juno, Melpomene) and the dwarf planet Ceres. Titan reaches an angular distance of about 20 Saturn radii from Saturn and subtends a disk 0.8 arcseconds in diameter.

Name
At 5,150 km across, Titan is larger than the planet Mercury and is the second largest natural satellite in the solar system after Ganymede.

Physical characteristics
Titan's diameter and mass (and thus its density) are similar to Jovian moons Ganymede and Callisto. Its interior may still be hot and there may be a liquid layer consisting of water and ammonia between the ice crust and the rocky core. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser due to gravitational compression.

Internal structure
Titan is the only known moon with a fully developed atmosphere that consists of more than just trace gases. The presence of a significant atmosphere was first discovered by Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of an atmospheric partial pressure of methane of the order of 100 millibars (10 kPa).

Atmosphere
At the surface, Titan's temperature is about 94 K (−179 °C, or −290.2 °F). At this temperature water ice does not sublimate, so the atmosphere is nearly free of water vapor. Scattered variable clouds punctuate an overall haze in Titan's atmosphere. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must produce the orange color as seen from space.
Ground-based observations show that Titan's climate changes with the seasons. Over the course of Saturn's 30-year orbit, Titan's cloud systems appear to manifest for 25 years, and then fade for four to five years, before reappearing again.

Climate
The Cassini mission has revealed that Titan's surface is relatively smooth. The few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. The area mapped so far appears to have no height variation greater than 50 meters (165 feet); however, radar altimetry has so far only covered part of the north polar region.
Titan's surface is marked by broad regions of bright and dark terrain. These include a large, highly reflective area about the size of Australia identified in infrared images from the Hubble Space Telescope and the Cassini spacecraft. This region is named Xanadu and appears to represent an area of relatively high ground. There are dark areas of similar size elsewhere on the moon, observed from the ground and by Cassini; it had been speculated that these are methane or ethane seas, but Cassini observations seem to indicate otherwise (see below). Cassini has also spotted some enigmatic linear markings, which some scientists have suggested may indicate tectonic activity, as well as regions of bright material cross cut by dark lineaments within the dark terrain.
In order to understand Titanian surface features better, the Cassini spacecraft is currently using radar altimetry and synthetic aperture radar imaging to map portions of Titan during its close fly-bys of the moon. The first images have revealed a complex, diverse geology with both rough and smooth areas. There are features that seem volcanic in origin, which probably disgorge water mixed with ammonia. There are also streaky features that appear to be caused by windblown particles.

Surface features
It has long been believed that lakes or even seas of methane might exist on Titan's surface, but until recently, conclusive evidence has proven elusive.

Liquids on Titan
Radar SAR and imaging data from Cassini have revealed a relative paucity of impact craters on Titan's surface, suggesting a youthful surface. To date, only three impact craters have been confirmed, which includes a 440 km wide multi-ring impact basin named Menrva (seen by Cassini's ISS as a bright-dark concentric pattern),

Impact craters

Main article: Cryovolcano Cryovolcanism
A mountain range measuring 150 km (93 miles) long, 30 km (19 miles) wide and 1.5 km (1 mile) high was discovered by Cassini in 2006. This range lies in the southern hemisphere and is thought to be composed of icy material and covered in methane snow. The movement of tectonic plates, perhaps influenced by a nearby impact basin, could have opened a gap through which the mountain's material upwelled.

Mountains
In the first images of Titan's surface taken by Earth-based telescopes in the early 2000s, large regions of dark terrain were revealed straddling Titan's equator.

Dark terrain

Main article: Huygens probe Huygens landing site
Titan was examined by both Voyager 1 and Voyager 2, with Voyager 1's course being diverted specifically to make a closer pass of Titan. Unfortunately Voyager 1 did not possess any instruments that could penetrate Titan's haze, an unforeseen factor. Many years later, intensive digital processing of images taken through Voyager 1's orange filter did reveal hints of the light and dark features now known as Xanadu and the Sickle,

Exploration of Titan
Scientists believe that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan. Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution. The Miller-Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane forming hydrocyan and ethyne. Further reactions have been studied extensively.

See also