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 . 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
Titan orbits Saturn once every 15 days and 22 hours. Like Earth's moon and many of the satellites of the giant planets, Titan is tidally locked in synchronous rotation with Saturn, and permanently shows one face to the planet. Consequently, its rotational period, or day, is identical to its orbital period. Longitudes on Titan are measured westward, starting from the prime meridian, which is defined as passing through the center of this face. Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.33 degrees relative to the Saturnian equator.
The small and irregularly shaped satellite Hyperion is locked in a 3:4 orbital resonance with Titan—that is, Hyperion orbits three times for every four times Titan orbits. Hyperion probably formed in a stable orbital island, whereas the massive Titan absorbed or ejected any other bodies that made close approaches.
Bulk characteristics
Titan is in diameter; it is 6% larger than the planet Mercury and 50% larger than Earth's Moon. Titan is the tenth-largest object known in the Solar system, including the Sun. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede, which has a diameter of , and thus the largest moon in the Solar System. This was an overestimation caused by Titan's dense, opaque atmosphere, with a haze layer 100–200 km above its surface. This increases its apparent diameter. Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto. Based on its bulk density of 1.881 g/cm3, Titan's composition is 40–60% rock, with the rest being water ice and other materials.
Titan is probably partially differentiated into distinct layers with a rocky center. This rocky center is believed to be surrounded by several layers composed of different crystalline forms of ice, and/or water. The exact structure depends heavily on the heat flux from within Titan itself, which is poorly constrained. The interior may still be hot enough for a liquid layer consisting of a "magma" composed of water and ammonia between the ice Ih (normal ice) crust and deeper ice layers made of high-pressure forms of ice. It is even possible that there is no high-pressure ice layer, the outer part of the moon instead consisting primarily of liquid water underneath a surface crust of ordinary ice. The presence of ammonia allows water to remain liquid even at a temperature as low as (for eutectic mixture with water).
The Cassini probe discovered evidence for the layered structure in the form of natural extremely low-frequency radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of extremely low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean. Surface features were observed by the Cassini spacecraft to systematically shift by up to between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer. Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn. Comparison of the gravity field with the RADAR-based topography observations also suggests that the ice shell may be substantially rigid.
Atmosphere
Titan is the only moon in the Solar System with an atmosphere denser than Earth's, with a surface pressure of , and it is one of only two moons whose atmospheres are able to support clouds, hazes, and weather—the other being Neptune's moon Triton. The presence of a significant atmosphere was first suspected by astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903. Due to the extensive, hazy atmosphere, Titan was once thought to be the largest moon in the Solar System until the Voyager missions revealed that Ganymede is slightly larger. The haze also shrouded Titan's surface from view, so direct images of its surface could not be taken until the Cassini–Huygens mission in 2004.
The primary constituents of Titan's atmosphere are nitrogen, methane, and hydrogen. The precise atmospheric composition varies depending on altitude and latitude due to methane cycling between a gas and a liquid in Titan's lower atmospherethe methane cycle. Nitrogen is the most abundant gas, with a concentration of around 98.6% in the stratosphere that decreases to 95.1% in the troposphere. Direct observations by the Huygens probe determined that methane concentrations are highest near the surface, with a concentration of 4.92% that remains relatively constant up to above the surface. Methane concentrations then gradually decrease with increasing altitude, down to a concentration of 1.41% in the stratosphere. Methane also increases in concentration near Titan's winter pole, probably due to evaporation from the surface in high-latitude regions. Hydrogen is the third-most abundant gas, with a concentration of around 0.1%. There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, and propane, and other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon, and helium. The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog.
Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself. The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.