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Titan (moon)

Titan (moon)

Overview
Titan or Saturn VI, is the largest moon of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, the only natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 known to have a dense atmosphere,
and the only object other than Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 for which clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth ellipsoidal
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

 moon from Saturn. Frequently described as a planet-like moon, Titan has a diameter roughly 50% larger than Earth's moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 and is 80% more mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

ive.
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Titan or Saturn VI, is the largest moon of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, the only natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 known to have a dense atmosphere,
and the only object other than Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 for which clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth ellipsoidal
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

 moon from Saturn. Frequently described as a planet-like moon, Titan has a diameter roughly 50% larger than Earth's moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 and is 80% more mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

ive. It is the second-largest moon in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, after Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's moon Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

, and it is larger by volume than the smallest planet, Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, although only half as massive. Titan was the first known moon of Saturn, discovered in 1655 by the Dutch astronomer Christiaan Huygens, and was the fifth moon of a planet apart from the Earth to be discovered.

The moon itself is primarily composed of water ice and rocky material. Much as with Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 prior to the Space Age
Space Age
The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...

, 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
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 lakes in the satellite's polar regions. These are the only large, stable bodies of surface liquid known to exist anywhere other than Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. The surface is geologically young; although mountains and several possible cryovolcano
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...

es have been discovered, it is smooth and few impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s have been discovered.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

 clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as sand dunes, rivers, lakes and seas (probably of liquid methane or ethane) and shorelines, and, like on Earth, is dominated by seasonal weather patterns. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan is viewed as analogous to the early Earth, although at a much lower temperature. The satellite has thus been cited as a possible host for microbial
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

 extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

 or, at least, as a prebiotic environment rich in complex organic chemistry. Researchers have suggested a possible underground liquid ocean might serve as a biotic environment. It has also been suggested that a form of life may exist on the surface, using liquid methane as a medium instead of water; and anomalies in atmospheric composition have been reported which are consistent with the presence of such a life-form, but which could also be due to an exotic non-living chemistry.

Discovery and naming


Titan was discovered on March 25, 1655, by the Dutch
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

 in 1610 and his improvements on telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 technology. Christiaan, with the help of his brother Constantijn Huygens, Jr.
Constantijn Huygens, Jr.
Constantijn Huygens Jr. was a Dutch statesman also known for his work on scientific instruments and as a chronicler of his times...

, 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
Giovanni Domenico Cassini
This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

 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, come from John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

 (son of William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

, discoverer of Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

 and Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

) in his 1847 publication Results of Astronomical Observations Made at the Cape of Good Hope. He suggested the names of the mythological Titans
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

, sisters and brothers of Kronos
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

, the Greek Saturn.

Orbit and rotation


Titan orbits Saturn once every 15 days and 22 hours. Like the Earth's moon and many of the other gas giant satellites, its orbital period is identical to its rotational period; Titan is thus tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

 in synchronous rotation
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...

 with Saturn, and thus always shows one face to the planet. Because of this, there is a sub-Saturnian point on its surface, from which the planet would appear to hang directly overhead. Longitudes on Titan are measured westward from the meridian passing through this point. Its orbital eccentricity is 0.0288, and it is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, the moon reaches an angular distance of about 20 Saturn radii (just over 1.2 million kilometers) from Saturn and subtends a disk 0.8 arcseconds in diameter.

Titan is locked in a 3:4 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 with the small, irregularly shaped satellite Hyperion
Hyperion (moon)
Hyperion , also known as Saturn VII, is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance...

. A "slow and smooth" evolution of the resonance—in which Hyperion would have migrated from a chaotic orbit—is considered unlikely, based on models. Hyperion probably formed in a stable orbital island, while massive Titan absorbed or ejected bodies that made close approaches.

Bulk characteristics


Titan is 5,150 km across, compared to 4,879 km for the planet Mercury, 3,474 km for Earth's moon, and 12,742 km for the Earth. Before the arrival of Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

 in 1980, Titan was thought to be slightly larger than Ganymede (diameter 5,262 km) and thus the largest moon in the Solar System; this was an overestimation caused by Titan's dense, opaque atmosphere, which extends many kilometres above its surface and increases its apparent diameter. Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

. Based on its bulk density of 1.88 g/cm3, Titan's bulk composition is half water ice and half rocky material. Though similar in composition to Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

 and Enceladus, it is denser due to gravitational compression
Gravitational compression
Gravitational compression is a phenomenon in which gravity, acting on the mass of an object, compresses it, reducing its size and increasing the object's density....

.

Titan is probably differentiated into several layers with a 3,400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot and there may be a liquid layer consisting of a "magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

" composed of water and ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 between the ice Ih
Ice Ih
thumb|Photograph showing details of an ice cube under magnification. Ice Ih is the form of ice commonly seen on earth.Ice Ih is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic which is...

 crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at temperatures as low as 176 kelvin (-97.2 °C) (for eutectic mixture with water). Evidence for such an ocean has recently been uncovered by the Cassini probe in the form of natural extremely low frequency
Extremely low frequency
Extremely low frequency is a term used to describe radiation frequencies from 3 to 300 Hz. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz...

 (ELF) radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of ELF 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 30 km 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.

An early 2000 study by DLR Institute of Planetary Research at Berlin-Adlershof placed Titan in a "large icy satellite" group along with the Galilean moons Callisto and Ganymede.

Size comparison: Titan in infrared (lower left) with the Moon and Earth (top and right) Titan's theorized internal structure Mass comparison: At 96% of their total mass, Titan dominates the Saturnian moons

Atmosphere



Titan is the only known moon with a fully developed atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 that consists of more than just trace gas
Trace gas
A trace gas is a gas which makes up less than 1% by volume of the Earth's atmosphere, and it includes all gases except nitrogen and oxygen . The most abundant trace gas at 0.934% is argon, which is being continually produced by radioactive decay of in the earth's rocks...

es. Observations from the Voyager
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

 space probes have shown that the Titanian atmosphere is denser than Earth's, with a surface pressure about 1.45 times that of Earth's. Titan's atmosphere is about 1.19 times as massive as Earth's overall, or about 7.3 times more massive on a per surface area basis. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan's surface features obscure. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms. Titan's lower gravity means that its atmosphere is far more extended than Earth's; even at a distance of 975 km, the Cassini spacecraft had to make adjustments to maintain a stable orbit against atmospheric drag. The atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside. It was not until the arrival of the Cassini–Huygens mission in 2004 that the first direct images of Titan's surface were obtained. The Huygens
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....

 probe was unable to detect the direction of the Sun during its descent, and although it was able to take images from the surface, the Huygens team likened the process to "taking pictures of an asphalt parking lot at dusk".

The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas Solà
Josep Comas Solá
Josep Comas i Solà was a Catalan Spanish astronomer.He observed planets including Mars and Saturn, measuring the period of rotation of the latter...

, who observed distinct limb darkening
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...

 on Titan in 1903, and confirmed by Gerard P. Kuiper in 1944 using a spectroscopic technique
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 that yielded an estimate of an atmospheric partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

 of methane of the order of 100 millibars (10 kPa). Subsequent observations in the 1970s showed that Kuiper's figures had been significant underestimates; methane abundances in Titan's atmosphere were ten times higher, and the surface pressure was at least double what he had predicted. The high surface pressure meant that methane could only form a small fraction of Titan's atmosphere. In 1981, Voyager 1 made the first detailed observations of Titan's atmosphere, revealing that its surface pressure was higher than Earth's, at 1.5 bars.
Titan's atmosphere is the only dense, nitrogen-rich atmosphere in the Solar System aside from the Earth's. The atmospheric composition in the stratosphere is 98.4% nitrogen with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%). Because methane condenses out of Titan's atmosphere at high altitudes, its abundance increases as one descends below the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...

 at an altitude of 32 km, leveling off at a value of 4.9% between 8 km and the surface. There are trace amounts of other hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s, such as ethane, diacetylene
Diacetylene
Diacetylene , with the formula C4H2, is a highly unsaturated hydrocarbon that contains three single bonds and two triple bonds. It is the first in the series of polyynes.-Occurrence:...

, methylacetylene
Methylacetylene
Methylacetylene is an alkyne with the chemical formula H3C≡CH. It is a component of MAPP gas along with its isomer 1,2-propadiene , which is commonly used in gas welding...

, acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

 and propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

, and of other gases, such as cyanoacetylene
Cyanoacetylene
Cyanoacetylene is an organic compound with formula or H-C≡C-C≡N. It is the simplest cyanopolyyne. Cyanoacetylene has been detected by spectroscopic methods in interstellar clouds, in the coma of comet Hale–Bopp and in the atmosphere of Saturn's moon Titan....

, hydrogen cyanide, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

, cyanogen
Cyanogen
Cyanogen is the chemical compound with the formula 2. It is a colorless, toxic gas with a pungent odor.The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups — analogous to diatomic halogen molecules, such as Cl2, but far less oxidizing...

, argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. The orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s, tar-like organic precipitates. The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light, producing a thick orange smog. Titan has no magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

, although studies in 2008 showed that Titan retains remnants of Saturn's magnetic field on the brief occasions when it passes outside Saturn's magnetosphere and is directly exposed to the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

. This may ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

ize and carry away some molecules from the top of the atmosphere. In November 2007, scientists uncovered evidence of negative ions with roughly 10 000 times the mass of hydrogen in Titan's ionosphere, which are believed to fall into the lower regions to form the orange haze which obscures Titan's surface. Their structure is not currently known, but they are believed to be tholins, and may form the basis for the formation of more complex molecules, such as polycyclic aromatic hydrocarbons.

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 somehow replenished by a reservoir on or within Titan itself. That Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, since comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...

. Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes. A possible biological origin for the methane has not been discounted (see below).

There is also a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east. Observations of the atmosphere, made in 2004 by Cassini, also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.

Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural ELF waves (see above) on Titan is unclear as there does not appear to be extensive lightning activity.

Climate


Titan's surface temperature is about 94 K (−179 °C, or −290 °F). At this temperature water ice does not sublimate or evaporate
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

, so the atmosphere is nearly free of water vapor. The haze
Haze
Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...

 in Titan's atmosphere contributes to the moon's anti-greenhouse effect
Anti-Greenhouse Effect
The anti-greenhouse effect is a neologism used to describe two different effects that describe a cooling effect an atmosphere has on the ambient temperature of the planet. Unlike the greenhouse effect, which is common, an anti-greenhouse effect is only known to exist in one situation in our Solar...

 by reflecting sunlight back into space, making its surface significantly colder than its upper atmosphere. The moon receives just about 1% of the amount of sunlight Earth gets. Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze. This atmospheric methane conversely creates a greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 on Titan's surface, without which Titan would be far colder. The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface. In October 2007, observers noted an increase in apparent opacity in the clouds above the equatorial Xanadu
Xanadu (Titan)
Xanadu is a highly reflective area on the leading hemisphere of Saturn's moon Titan...

 region, suggestive of "methane drizzle", though this was not direct evidence for rain. However, subsequent images of lakes in Titan's southern hemisphere taken over one year show that they are enlarged and filled by seasonal hydrocarbon rainfall. It is possible that areas of Titan's surface may be coated in a layer of tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s, but this has not been confirmed. The presence of rain indicates that Titan is the only celestial body other than Earth upon which rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...

s could form. However, given the extreme opacity of the atmosphere in visible light, the vast majority of any rainbows would be visible only in the infrared.

Simulations of global wind patterns based on wind speed data taken by Huygens during its descent have suggested that Titan's atmosphere circulates in a single enormous Hadley cell
Hadley cell
The Hadley cell, named after George Hadley, is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10–15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface...

. Warm air rises in Titan's southern hemisphere—which was experiencing summer during Huygens descent—and sinks in the northern hemisphere, resulting in high-altitude air flow from south to north and low-altitude airflow from north to south. Such a large Hadley cell is only possible on a slowly rotating world such as Titan. The pole-to-pole wind circulation cell appears to be centered on the stratosphere; simulations suggest it ought to change every twelve years, with a three-year transition period, over the course of Titan's year (30 terrestrial years). This cell creates a global band of low pressure—what is in effect a variation of Earth's Intertropical Convergence Zone
Intertropical Convergence Zone
The Intertropical Convergence Zone , known by sailors as The Doldrums, is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together....

 (ITCZ). Unlike on Earth, however, where the oceans confine the ITCZ to the tropics, on Titan, the zone wanders from one pole to the other, taking methane rainclouds with it. This means that Titan, despite its frigid temperatures, can be said to have a tropical climate.

The number of methane lakes visible near Titan's southern pole is decidedly smaller than the number observed near the north pole. As the south pole is currently in summer and the north in winter, an emerging hypothesis is that methane rains onto the poles in winter and evaporates in summer.

Clouds



In September 2006, Cassini imaged a large cloud at a height of 40 km over Titan's north pole. Although methane is known to condense in Titan's atmosphere, the cloud was more likely to be ethane, as the detected size of the particles was only 1–3 micrometers
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 and ethane can also freeze at these altitudes. In December, Cassini again observed cloud cover and detected methane, ethane and other organics. The cloud was over 2400 km in diameter and was still visible during a following flyby a month later. One hypothesis is that it is currently raining (or, if cool enough, snowing) on the north pole; the downdrafts at high northern latitudes are strong enough to drive organic particles towards the surface. These were the strongest evidence yet for the long-hypothesized "methanological" cycle (analogous to Earth's hydrological cycle) on Titan.

Clouds have also been found over the south pole. While typically covering 1% of Titan's disk, outburst events have been observed in which the cloud cover rapidly expands to as much as 8%. One hypothesis asserts that the southern clouds are formed when heightened levels of sunlight
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...

 during the Titanian summer generate uplift in the atmosphere, resulting in convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

. This explanation is complicated by the fact that cloud formation has been observed not only post–summer solstice but also at mid-spring. Increased methane humidity at the south pole possibly contributes to the rapid increases in cloud size. There had been summer in Titan's southern hemisphere until 2010, when Saturn's orbit, which governs the moon's motion, tilted the northern hemisphere towards the Sun. When the seasons switch, it is expected that ethane will begin to condense over the south pole.

Research models that match well with observations suggest that clouds on Titan cluster at preferred coordinates and that cloud cover varies by distance from the surface on different parts of the satellite. In the polar regions (above 60 degrees latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

), widespread and permanent ethane clouds appear in and above the troposphere; at lower latitudes, mainly methane clouds are found between 15 and 18 km, and are more sporadic and localized. In the summer hemisphere, frequent, thick but sporadic methane clouds seem to cluster around 40°.

Ground-based observations also reveal seasonal variations in cloud cover. 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.

Cassini has also detected high-altitude, white, cirrus
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

-type clouds in Titan's upper atmosphere, likely formed of methane.

Although no evidence of lightning activity has yet been observed on Titan, computer models suggest that clouds in the moon's lower troposphere can accumulate enough charge to generate lightning from an altitude of roughly 20 km.

Surface features




The surface of Titan has been described as "complex, fluid-processed, [and] geologically young." The Cassini spacecraft has used radar altimetry and synthetic aperture radar
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...

 (SAR) imaging to map portions of Titan during its close fly-bys of the moon. The first images revealed a 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, some of them hundreds of kilometers in length, that appear to be caused by windblown particles. Examination has also shown the surface to be relatively smooth; the few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons or volcanoes. Radar altimetry suggests height variation is low, typically no more than 150 meters. Occasional elevation changes of 500 meters have been discovered and Titan has mountains that sometimes reach several hundred meters to more than 1 kilometer in height.

Titan's surface is marked by broad regions of bright and dark terrain. These include Xanadu, a large, reflective
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 equatorial area about the size of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. It was first identified in infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 images from the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 in 1994, and later viewed by the Cassini spacecraft. The convoluted region is filled with hills and cut by valleys and chasms. It is criss-crossed in places by dark lineaments—sinuous topographical features resembling ridges or crevices. These may represent tectonic activity, which would indicate that Xanadu is geologically young. Alternatively, the lineaments may be liquid-formed channels, suggesting old terrain that has been cut through by stream systems. 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).
Mosaic of Titan from Cassini's February 2005 flyby. The large dark region is Shangri-la. Titan in false color showing surface details and atmosphere with Xanadu in the bright region at the center-right. Titan Globe, a mosaic of infrared and radar images with nomenclature

Liquids



The possibility of hydrocarbon seas on Titan was first suggested based on Voyager 1 and 2 data that showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them, but direct evidence wasn't obtained until 1995 when data from Hubble and other observations suggested the existence of liquid methane on Titan, either in disconnected pockets or on the scale of satellite-wide oceans, similar to water on Earth.

The Cassini mission confirmed the former hypothesis, although not immediately. When the probe arrived in the Saturnian system in 2004, it was hoped that hydrocarbon lakes or oceans might be detectable by reflected sunlight from the surface of any liquid bodies, but no specular reflection
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...

s were initially observed. At Titan's south pole, an enigmatic dark feature named Ontario Lacus
Ontario Lacus
Ontario Lacūs is a lake composed of methane, ethane and propane near the south pole of Saturn's moon Titan. Its character as a hydrocarbon lake was confirmed by observations from the Cassini spacecraft, published in the 31 July 2008 edition of Nature...

 was identified (and later confirmed to be a lake). A possible shoreline was also identified at the pole via radar imagery. Following a flyby on July 22, 2006, in which the Cassini spacecraft's radar imaged the northern latitudes (which were then in winter), a number of large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole. Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007. The Cassini–Huygens team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found outside of Earth. Some appear to have channels associated with liquid and lie in topographical depressions. Overall, the Cassini radar observations have shown that lakes cover only a few per cent of the surface and are concentrated near the poles, making Titan much drier than Earth.

In June 2008, the Visual and Infrared Mapping Spectrometer on Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus. On December 21, 2008, Cassini passed directly over Ontario Lacus and observed specular reflection in radar. The strength of the reflection saturated the probe's receiver, indicating that the lake level did not vary by more than 3 mm (implying either that surface winds were minimal, or the lake's hydrocarbon fluid is viscous). On 8 July 2009, Cassini's VIMS observed a specular reflection in 5 micrometre infrared light off Jingpo Lacus, a lake in the north polar region, shortly after the area emerged from 15 years of winter darkness.

Radar measurements made in July 2009 and January 2010 indicate that Ontario Lacus is extremely shallow, with an average depth of 0.4–3.2 m, and a maximum depth of 2.9–7.4 m. In contrast, the northern hemisphere's Ligeia Mare
Ligeia Mare
Ligeia Mare is a lake located on Titan, the planet Saturn's largest moon. It is bigger then Lake Superior on Earth. It is composed of liquid hydrocarbons located at 79.0° N, 248.0° W, measuring roughly 500km in diameter and with surface area of about ~100,000km2...

 has depths exceeding 8 m, the maximum measurable by the radar instrument.
Photo of infrared specular reflection off Jingpo Lacus, a lake in the north polar region Perspective radar view of Bolsena Lacus (lower right) and other northern hemisphere hydrocarbon lakes Contrasting images of the number of lakes in Titan's northern hemisphere (left) and southern hemisphere (right) Two images of Titan's southern hemisphere acquired one year apart, showing changes in south polar lakes

Impact craters


Radar, SAR and imaging data from Cassini have revealed few impact craters on Titan's surface, suggesting it is relatively young. The few impact craters discovered include a 440 km wide multi-ring impact basin named Menrva seen by Cassini's ISS as a bright-dark concentric pattern. A smaller, 60 km wide, flat-floored crater named Sinlap and a 30 km crater with a central peak and dark floor named Ksa have also been observed. Radar and Cassini imaging have also revealed a number of "crateriforms", circular features on the surface of Titan that may be impact related, but lack certain features that would make identification certain. For example, a 90 km wide ring of bright, rough material known as Guabonito
Guabonito (Titan)
Guabonito is ring of bright features on Saturn's moon Titan. Guabonito was first seen in Cassini images taken in October 2004 and has been observed several times since. This ringed features, 90 kilometers across, is located in Titan's Shangri-la dark region, near the boundary with Xanadu, and is...

 has been observed by Cassini. This feature is thought to be an impact crater filled in by dark, windblown sediment. Several other similar features have been observed in the dark Shangri-la and Aaru regions. Radar observed several circular features that may be craters in the bright region Xanadu during Cassini's April 30, 2006 flyby of Titan.

Pre-Cassini models of impact trajectories and angles suggest that where the impactor strikes the water ice crust, a small amount of ejecta remains as liquid water within the crater. It may persist as liquid for centuries or longer, sufficient for "the synthesis of simple precursor molecules to the origin of life." While infill from various geological processes is one reason for Titan's relative deficiency of craters, atmospheric shielding also plays a role; it is estimated that Titan's atmosphere reduces the number of craters on its surface by a factor of two.

Cryovolcanism and mountains




Scientists have long speculated that conditions on Titan resemble those of early Earth, though at a much lower temperature. The detection of Argon 40 in the atmosphere in 2004 indicated that volcanoes had spawned plumes of "lava" composed of water and ammonia. Global maps of the lake distribution on Titan's surface revealed that there is not enough surface methane to account for its continued presence in its atmosphere, and thus that a significant portion must be added through volcanic processes.

Still there is a paucity of surface features which can be unambiguously interpreted as cryovolcanoes. One of the first of such features revealed by Cassini radar observations in 2004, called Ganesa Macula
Ganesa Macula
Ganesa Macula is a dark feature on Saturn's moon Titan. It is named after the Hindu god Ganesha.Ganesa has been tentatively identified as a cryovolcanic dome: the result of a mixture of water and ammonia erupting from the center of the dome and spreading out to form a pancake-like deposit....

, resembles the geographic features called "pancake dome
Pancake dome
A pancake dome, also known as a lava dome, is an unusual type of volcano found on the planet Venus. They are widely scattered on that planet and often form groups or clusters, though with smaller numbers of pancake domes in each group than is typical for the more common shield volcanos...

s" found on Venus, and was thus initially believed to be cryovolcanic in origin, although the American Geophysical Union
American Geophysical Union
The American Geophysical Union is a nonprofit organization of geophysicists, consisting of over 50,000 members from over 135 countries. AGU's activities are focused on the organization and dissemination of scientific information in the interdisciplinary and international field of geophysics...

 refuted this hypothesis in December 2008. The feature was found to be not a dome at all, but appeared to result from accidental combination of light and dark patches. In 2004 Cassini also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome. No similar features have been identified as of 2010. In December 2008, astronomers announced the discovery of two transient but unusually long-lived "bright spots" in Titan's atmosphere, which appear too persistent to be explained by mere weather patterns, suggesting they were the result of extended cryovolcanic episodes.

In March, 2009, structures resembling lava flows were announced in a region of Titan called Hotei Orcus, which appears to fluctuate in brightness over several months. Though many phenomena were suggested to explain this fluctuation, the lava flows were found to rise 200 meters above Titan's surface, consistent with it having been erupted from beneath the surface.

A mountain range measuring 150 km long, 30 km wide and 1.5 km high was also 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. Prior to Cassini, scientists assumed that most of the topography on Titan would be impact structures, yet these findings reveal that similar to Earth, the mountains were formed through geological processes. In December 2010, the Cassini mission team announced the most compelling possible cryovolcano yet found. Named Sotra Facula
Sotra Facula
Sotra Facula is a prominent feature on Titan, the largest moon of Saturn. It is believed to be an ice volcano or cryovolcano, forming a roughly circular mountain measuring about across. It has two peaks standing about and high with multiple craters as much as deep...

, it is one in a chain of at least three mountains, each between 1000 and 1500 m in height, several of which are topped by large craters. The ground around their bases appears to be overlaid by frozen lava flows.

If volcanism on Titan really exists, it, like that on Earth, is believed to be driven by energy released from the decay of radioactive elements within the mantle. Magma on Earth is made of liquid rock, which is less dense than the solid rocky crust through which it erupts. Because ice is less dense than water, Titan's watery magma would be denser than its solid icy crust. This means that cryovolcanism on Titan would require a large amount of additional energy to operate, possibly via tidal flexing from nearby Saturn. Alternatively, the pressure necessary to drive the cryovolcanoes may be caused by ice Ih "underplating" Titan's outer shell. The low-pressure ice, overlaying a liquid layer of ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...

, ascends buoyantly, and the unstable system can produce dramatic plume events. Titan is resurfaced through the process by grain-sized ice and ammonium sulfate ash, which helps produce a wind-shaped
Eolian processes
Aeolian processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth and other planets. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of...

 landscape and sand dune features.

In 2008 Jeffrey Moore proposed an alternate view of Titan's geology. Noting that no volcanic features had been unambiguously identified on Titan so far, he asserted that Titan is a geologically dead world, whose surface is shaped only by impact cratering, fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...

 and eolian
Eolian processes
Aeolian processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth and other planets. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of...

 erosion, mass wasting
Mass wasting
Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place...

 and other exogenic processes. According to this hypothesis, methane is not emitted by volcanoes but slowly diffuses out of Titan's cold and stiff interior. Ganesa Macula may be an eroded impact crater with a dark dune in the center. The mountainous ridges observed in some regions can be explained as heavily degraded scarps
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...

 of large multi-ring impact structures or as a result of the global contraction due to the slow cooling of the interior. Even in this case Titan may still have an internal ocean made of the eutectic water–ammonia mixture with the temperature of 176 kelvin (-97.2 °C), which is low enough to be explained by the decay of radioactive elements in the core. The bright Xanadu terrain may be a degraded heavily cratered terrain similar to that observed on the surface of Callisto. Indeed, were it not for its lack of an atmosphere, Callisto could serve as a model for Titan's geology in this scenario. Jeffrey Moore even called Titan Callisto with weather.

Dark terrain


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. Prior to the arrival of Cassini, these regions were thought to be seas of organic matter like tar or liquid hydrocarbons. Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal sand dune
Dune
In physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...

s, up to 330 meters high about a kilometer wide, and tens to hundreds of kilometers long. The longitudinal (or linear) dunes are believed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Dunes of this type are always aligned with average wind direction. In the case of Titan, steady zonal
Zonal and meridional
The terms zonal and meridional are used to describe directions on a globe. Zonal means "along a latitude circle" or "in the west–east direction"; while meridional means "along a meridian" or "in the north–south direction"....

 (eastward) winds combine with variable tidal winds (approximately 0.5 meters per second). The tidal winds are the result of tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s from Saturn on Titan's atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth and tend to drive wind toward the equator. This wind pattern causes sand dunes to build up in long parallel lines aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.

The sand on Titan might have formed when liquid methane rained and eroded the ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids produced by photochemical reactions in Titan's atmosphere. Studies of dunes' composition in May 2008 revealed that they possessed less water than the rest of Titan, and are most likely to derive from organic material clumping together after raining onto the surface.

Observation and exploration


Titan is never visible to the naked eye, but can be observed through small telescopes or strong binoculars. Amateur observation is difficult because of the proximity of the satellite to Saturn's brilliant globe and ring system; an occulting bar, covering part of the eyepiece and used to block the bright planet, greatly improves viewing. Titan has a maximum apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of +8.2, and mean opposition magnitude 8.4. This compares to +4.6 for the similarly sized Ganymede, in the Jovian system.

Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer Josep Comas Solá
Josep Comas Solá
Josep Comas i Solà was a Catalan Spanish astronomer.He observed planets including Mars and Saturn, measuring the period of rotation of the latter...

 suggested that he had observed darkening near the edges of Titan's disk and two round, white patches in its center. The deduction of an atmosphere by Kuiper in the 1940s was the next major observational event.

The first probe to visit the Saturnian system was Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...

 in 1979, which determined that Titan was probably too cold to support life. The craft took the first images of the moon (including some of it and Saturn together), but these were of low quality; the first-ever close-up of Titan was taken on September 2, 1979.

Titan was examined by both Voyager 1 and 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 in 1980 and 1981, respectively. Voyager 1's course was diverted specifically to make a closer pass of Titan. Unfortunately, the craft 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 Shangri-la
Shangri-la (Titan)
Shangri-la is a large, dark region of Saturn's moon Titan at . It is named after Shangri-La, the mythical paradise in Tibet. It is thought to be an immense plain of dark material. It is thought that these regions of Titan were seas, but that they are now dry....

, but by then they had already been observed in the infrared by the Hubble Space Telescope. Voyager 2 took only a cursory look at Titan. The Voyager 2 team had the option of steering the spacecraft to take a detailed look at Titan or to use another trajectory which would allow it to visit Uranus and Neptune. Given the lack of surface features seen by Voyager 1, the latter plan was implemented.

Cassini–Huygens




Even with the data provided by the Voyagers, Titan remained a body of mystery—a planet-like satellite shrouded in an atmosphere that makes detailed observation difficult. The intrigue that had surrounded Titan since the 17th-century observations of Christiaan Huygens and Giovanni Cassini was gratified by a spacecraft named in their honor.

The Cassini–Huygens spacecraft reached Saturn on July 1, 2004, and has begun the process of mapping Titan's surface by radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

. A joint project of the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 (ESA) and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

, Cassini–Huygens has proved a very successful mission. The Cassini probe flew by Titan on October 26, 2004, and took the highest-resolution images ever of the moon's surface, at only 1,200 km, discerning patches of light and dark that would be invisible to the human eye from the Earth. Huygens landed on Titan on January 14, 2005, discovering that many of the moon's surface features seem to have been formed by flowing fluids at some point in the past. On July 22, 2006, Cassini made its first targeted, close fly-by at 950 km from Titan; the closest flyby was at 880 km on June 21, 2010. Present liquid on the surface may have been found near the north pole, in the form of many lakes that were recently discovered by Cassini. Titan is the most distant body from Earth that has seen a space probe landing. Titan is also the second moon in the Solar System to have a man-made object land on its surface.

Huygens landing site


On January 14, 2005, the Huygens probe landed on the surface of Titan, just off the easternmost tip of a bright region now called Adiri
Adiri (Titan)
Adiri is a large, bright albedo feature on Saturn's moon Titan. It is named after the paradise in Melanesian mythology. It is located to the west of the large, dark region of Shangri-la....

. The probe photographed pale hills with dark "rivers" running down to a dark plain. Current understanding is that the hills (also referred to as highlands) are composed mainly of water ice. Dark organic compounds, created in the upper atmosphere by the ultraviolet radiation of the Sun, may rain from Titan's atmosphere. They are washed down the hills with the methane rain and are deposited on the plains over geological time scales.

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice. The two rocks just below the middle of the image on the right are smaller than they may appear: the left-hand one is 15 centimeters across, and the one in the center is 4 centimeters across, at a distance of about 85 centimeters from Huygens. There is evidence of erosion at the base of the rocks, indicating possible fluvial activity. The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. It is believed that the "soil" visible in the images is precipitation from the hydrocarbon haze above.

In March 2007, NASA, ESA, and COSPAR
COSPAR
The Committee on Space Research was established by the International Council for Science in 1958.Among COSPAR's objectives are the promotion of scientific research in space on an international level, with emphasis on the free exchange of results, information, and opinions, and providing a forum,...

 decided to name the Huygens landing site the Hubert Curien
Hubert Curien
Hubert Curien was a French physicist and a key figure in European science politics, as the President of CERN , the first chairman of the European Space Agency , and second President of the Academia Europæa and a President of Fondation de France.-Biography:Born in Cornimont, Vosges in Lorraine,...

 Memorial Station in memory of the former president of the ESA.

Future missions



The Titan Saturn System Mission
Titan Saturn System Mission
Titan Saturn System Mission was a joint NASA/ESA proposal for an exploration of Saturn and its moons Titan and Enceladus, where many complex phenomena have been revealed by the recent Cassini–Huygens mission...

 (TSSM) is a joint NASA/ESA
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 proposal for exploration of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's moons. It envisions a hot-air balloon to float in the moon's atmosphere for six months. It was competing against the Europa Jupiter System Mission
Europa Jupiter System Mission
The Europa Jupiter System Mission – Laplace was a proposed joint NASA/ESA unmanned space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere...

 (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of the TSSM, although TSSM will continue to be studied for a later launch date. There has also been a proposal for a Titan Mare Explorer
Titan Mare Explorer
Titan Mare Explorer is a proposed spacecraft lander that, if launched, would probe Titan, the largest moon of the planet Saturn, and would perform the first exploration of an extraterrestrial sea...

 (TiME), which would be a low-cost lander that would splash down in a lake near Titan's north pole and float on the surface of the lake for at least 2 Titan days (16 Earth days). It could launch as early as 2015 and arrive in 2022 or 2023.

Prebiotic conditions and possible life


While the Cassini–Huygens mission was not equipped to provide evidence for biology or complex organics, it showed an environment on Titan that is similar, in some ways, to ones theorized for the primordial Earth. Scientists believe that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan, with the important exception of a lack of water vapor on Titan. Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution.

Formation of complex molecules


The Miller-Urey experiment
Miller-Urey experiment
The Miller and Urey experiment was an experiment that simulated hypothetical conditions thought at the time to be present on the early Earth, and tested for the occurrence of chemical origins of life. Specifically, the experiment tested Alexander Oparin's and J. B. S...

 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
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...

 of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.

In October 2010, Sarah Horst of the University of Arizona
University of Arizona
The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...

 reported finding the five nucleotide bases—building blocks of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

—among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Horst also found amino acids, the building blocks of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

. She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.

Possible subsurface habitats


Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. While the analogy assumes the presence of liquid water for longer periods than is currently observable, several theories suggest that liquid water from an impact could be preserved under a frozen isolation layer. It has also been observed that liquid ammonia oceans could exist deep below the surface; one model suggests an ammonia–water solution as much as 200 km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive". Heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 between the interior and upper layers would be critical in sustaining any sub-surface oceanic life. Detection of microbial life on Titan would depend on its biogenic effects. That the atmospheric methane and nitrogen might be of biological origin has been examined, for example.

Methane and life at the surface



It has also been suggested that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water. Such creatures would inhale H2 in place of O2, react it with acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

 instead of glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, and exhale methane instead of carbon dioxide. In 2005, astrobiologist Chris McKay predicted that if methanogenic life is consuming atmospheric hydrogen in sufficient volume, it will have a measurable effect on the mixing ratio in the troposphere.

Evidence for this form of life was identified in 2010 by Darrell Strobel of Johns Hopkins University
Johns Hopkins University
The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...

; an over-abundance of molecular hydrogen in the upper atmospheric layers, which leads to a downward flow at a rate of roughly 1025 molecules per second. Near the surface the hydrogen apparently disappears, which may imply its consumption by methanogenic life forms. Another paper released the same month showed little evidence of acetylene on Titan's surface, where scientists had expected the compound to accumulate; according to Strobel, this is consistent with the hypothesis that acetylene is being consumed by methanogens. Chris McKay, while agreeing that presence of life is a possible explanation for the findings about hydrogen and acetylene, has cautioned that other explanations are currently more likely: namely the possibility that the results are due to human error, or to the presence of some as-yet unknown catalyst in the soil. He noted that such a catalyst, effective at 95 K, would in itself be a startling discovery.

There is debate about the effectiveness of methane as a medium for life compared to water; while water is a far better solvent than methane, enabling easier transport of substances in a cell, methane's lesser chemical reactivity allows for the easier formation of large structures akin to proteins.

Obstacles


Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. At a vast distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, Titan is frigid (a fact exacerbated by the anti-greenhouse effect of its cloud cover), and its atmosphere lacks CO2. Because of these difficulties, scientists such as Jonathan Lunine
Jonathan Lunine
Jonathan I. Lunine is an American planetary scientist and physicist. Lunine teaches at Cornell University, where he is the David C. Duncan Professor in the Physical Sciences. Having published more than 200 research papers, Lunine is at the forefront of research into planet formation, evolution, and...

 have viewed Titan less as a likely habitat for life, than as an experiment for examining theories on the conditions that prevailed prior to the appearance of life on Earth. While life itself may not exist, the prebiotic conditions of the Titanian environment and the associated organic chemistry remain of great interest in understanding the early history of the terrestrial biosphere. Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiment, and chemical and photochemical modeling on Earth.

Panspermia hypothesis


An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it would be statistically more likely to have originated from Earth than to have appeared independently, a process known as panspermia. It is theorized that large asteroid and cometary impacts on Earth's surface have caused hundreds of millions of fragments of microbe-laden rock to escape Earth's gravity. Calculations indicate that a number of these would encounter many of the bodies in the Solar System, including Titan. On the other hand, Jonathan Lunine has argued that any living things in Titan's cryogenic hydrocarbon lakes would need to be so different chemically from Earth life that it would not be possible for one to be the ancestor of the other.

Future conditions


Conditions on Titan could become far more habitable in the future. Six billion years from now, as the Sun becomes a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...

, surface temperatures could rise to ~200 K (-73.2 °C), high enough for stable oceans of water/ammonia mixture to exist on the surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, lessening the anti-greenhouse effect on the surface and enabling the greenhouse created by atmospheric methane to play a far greater role. These conditions together could create an environment agreeable to exotic forms of life, and will persist for several hundred million years. This was sufficient time for simple life to evolve on Earth, although the presence of ammonia on Titan will cause the same chemical reactions to proceed more slowly.

See also


  • Colonization of Titan
    Colonization of Titan
    Saturn’s orange moon Titan is one of several candidates for a possible future colonization of the outer solar system. There are many possible reasons for colonization, one of which is mining or collecting hydrocarbons which currently power much of Earth's machinery.-Natural resources:According to...

  • Life on Titan
    Life on Titan
    Whether or not there is life on Titan, the largest moon of Saturn, is at present an open question and a topic of scientific discussion and research....

  • Lakes of Titan
    Lakes of Titan
    The Lakes of Titan, a moon of Saturn, are bodies of liquid ethane and methane that have been detected by the Cassini–Huygens space probe, and had been suspected long before. The large ones are known as maria and the small ones as lacūs .-History:The possibility that there were hydrocarbon seas on...

  • Saturn's moons in fiction
    Saturn's moons in fiction
    Several of Saturn's natural satellites have figured prominently in works of science fiction.- Saturnian system :Some works of fiction refer to several of Saturn's moons, or to no specific moon....

  • Titan in fiction
    Titan in fiction
    Titan is the largest moon of Saturn. It has a substantial atmosphere and is the most Earth-like satellite in the Solar System, making it a popular science fiction setting.- Literature :* Flight on Titan , short story by Stanley G. Weinbaum...


External links