Titan is the largest moon of Saturn and the second largest in the Solar System. It is the only moon that is known to have a dense atmosphere (denser than that of Earth), and is the only known object in the Solar System besides Earth with clear evidence of stable bodies of surface liquid. Titan is one of seven gravitationally rounded moons of Saturn, and is the second-most distant among them. Frequently described as a planet-like moon, Titan is 48.16% larger in diameter than Earth's moon, and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's Ganymede, and is larger than Mercury, and yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock, whereas Titan is mostly ice, which is less dense. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii or 1.2 million km above Saturn's apparent surface. From Titan's surface, Saturn, disregarding its rings, subtends an arc of 5.09 degrees, rendering it more than ten times as large in Titan's sky as the Moon is in Earth's sky. Titan is primarily composed of ice and rocky material, with a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions and the discovery of its atmospheric super-rotation. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found. The atmosphere of Titan is mainly nitrogen and methane; minor components lead to the formation of hydrocarbon clouds and heavy organonitrogen haze. Its climate, including wind and rain, creates surface features similar to those of Earth, such as dunes, rivers, lakes, 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 nearly resembles Earth's water cycle, albeit at a much lower temperature of about 94 K (−179 °C; −290 °F). Due to these factors, Titan is sometimes called the most Earth-like celestial object in the Solar System.
Discovery and naming
The Dutch astronomer Christiaan Huygens discovered Titan on March 25, 1655. Fascinated by Galileo's 1610 discovery of Jupiter's four largest moons and his advancements in telescope technology, Huygens, with the help of his elder brother Constantijn Huygens Jr., began building telescopes around 1650, and discovered the first observed moon orbiting Saturn with one of the telescopes they built. Huygens named his discovery Saturni Luna (or Luna Saturni, Latin for "moon of Saturn"), publishing in the 1655 tract De Saturni Luna Observatio Nova (A New Observation of Saturn's Moon). After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers began 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". The International Astronomical Union officially numbers Titan as "Saturn VI". The name Titan, and the names of all seven satellites of Saturn which were known at the time, came from John Herschel (son of William Herschel, who had discovered two other moons of Saturn, namely Mimas and Enceladus), in his 1847 publication Results of Astronomical Observations Made during the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope. Numerous small moons have been discovered around Saturn since then. Saturnian moons are named after mythological giants. The name Titan comes from the Titans, a race of immortals in Greek mythology. Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek tau (the initial of Titan) combined with the crook of the Saturn symbol as the symbol of Titan (). This symbol is not widely used.
Formation The regular moons of Jupiter and Saturn likely formed via co-accretion, similar to the process believed to have formed the planets in the Solar System. As the young gas giants formed, they were surrounded by discs of material that gradually coalesced into moons. While the four Galilean moons of Jupiter exist in highly regular, planet-like orbits, Titan overwhelmingly dominates Saturn's system, and has a high orbital eccentricity which is not immediately explained by co-accretion alone. A proposed model for the formation of Titan is that Saturn's system began with a group of moons which were similar to Jupiter's Galilean moons, but that they were disrupted by a series of giant impacts, which would go on to form Titan. Saturn's mid-sized moons, such as Iapetus and Rhea, were formed from the debris of these collisions. Such a violent beginning would also explain Titan's orbital eccentricity. A 2014 analysis of Titan's atmospheric nitrogen suggested that it was possibly sourced from material which is similar to that found in the Oort cloud, and not from sources which were present during the co-accretion of materials around Saturn. Titan may have played a critical role in the formation of Hyperion. Numerical simulations suggest that a mid-sized satellite of Saturn collided with Titan a few hundred million years ago, and the resulting debris accreted to form Hyperion.
Orbit and rotation
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