Titan

Titan (or Saturn VI) is the largest moon of Saturn. It is the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth ellipsoidal moon from Saturn. Frequently described as a planet-like moon, Titan has a diameter roughly 50% larger than the Moon and is 80% more massive. It is the second-largest moon in the Solar System, after Jupiter's moon Ganymede, and is larger by volume than the smallest planet, Mercury, although only about 41% as massive. Titan was the first known moon of Saturn, discovered in 1655 by the Dutch astronomer Christiaan Huygens, and was the fifth moon (excluding Earth's Moon) to be discovered.

Titan is primarily composed of water ice and rocky material. Much as with Venus prior to the Space Age, the dense, opaque atmosphere prevented understanding of Titan's surface until new information accumulated with the arrival of the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan's polar regions. The surface is geologically young; although mountains and several possible cryovolcanoes have been discovered, it is smooth and few impact craters have been found.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes and seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle is viewed as an analog to Earth's water cycle, although at a much lower temperature.

DISCOVERY AND NAMING
Titan was discovered on March 25, 1655, by the Dutch astronomer/physicist Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr., began building telescopes around 1650. Christiaan Huygens discovered this first observed moon orbiting Saturn with the first telescope they built.

He named it simply Saturni Luna (or Luna Saturni, Latin for "Saturn's moon"), publishing in the 1655 tract De Saturni Luna Observatio Nova. After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V (with Titan then in fourth position). Other early epithets for Titan include "Saturn's ordinary satellite". Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion (Titan having borne the numbers II and IV as well as VI). Numerous small moons have been discovered closer to Saturn since then.

The name Titan, and the names of all seven satellites of Saturn then known, came from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations Made at the Cape of Good Hope. He suggested the names of the mythological Titans (Greek: Τῑτάν), sisters and brothers of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age.