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109-529: Prime targets include moons that harbor hidden oceans beneath a shell of ice: Europa , Enceladus , and Titan . A host of other bodies in the outer Solar System are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. The US House Appropriations Committee approved the bill on May 20, 2015, and directed NASA to create the Ocean Worlds Exploration Program. The "Roadmaps to Ocean Worlds" (ROW)

218-524: A planet-like moon , Titan is 50% 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 ; yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock while much of Titan is ice, which is less dense. Discovered in 1655 by the Dutch astronomer Christiaan Huygens , Titan

327-485: A cumulative surface area of 215,000 square kilometres (83,000 sq mi). Lakes in Titan's lower-latitude and equatorial regions have been proposed, though none have been confirmed; seasonal or transient equatorial lakes may pool following large rainstorms. Cassini RADAR data has been used to conduct bathymetry of Titan's seas and lakes. Using detected subsurface reflections, the measured maximum depth of Ligeia Mare

436-559: A distance of about 85 centimeters from Huygens . There is evidence of erosion at the base of the rocks, indicating possible fluvial activity. The ground surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. In March 2007, NASA, ESA, and COSPAR decided to name the Huygens landing site the Hubert Curien Memorial Station in memory of the former president of

545-463: A dozen comparable in size to Earth's Moon: the four Galilean moons , Saturn's Titan, and Neptune 's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium , and Uranus has five. It has been suggested that some satellites may potentially harbour life . Among the objects generally agreed by astronomers to be dwarf planets, Ceres and Sedna have no known natural satellites. Pluto has

654-478: A generic sense in works of popular science and fiction, has regained respectability and is now used interchangeably with natural satellite , even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in

763-433: A global subsurface ocean of liquid water. Besides planets and dwarf planets objects within our Solar System known to have natural satellites are 76 in the asteroid belt (five with two each), four Jupiter trojans , 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers . There are also 84 known natural satellites of trans-Neptunian objects . Some 150 additional small bodies have been observed within

872-413: A high orbital eccentricity 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 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

981-466: A natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable. However, calculations performed after the 2008 detection of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd moons . However, targeted images taken by

1090-784: A planet or minor planet", and "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the Sun". There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System , some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets . Small asteroid moons (natural satellites of asteroids), such as Dactyl , have also been called moonlets. The upper limit

1199-405: A robotic space probe to Titan. Initial conceptual work has been completed for such missions by NASA (and JPL ), and ESA . At present, none of these proposals have become funded missions. The Titan Saturn System Mission (TSSM) was a joint NASA/ ESA proposal for exploration of Saturn 's moons. It envisions a hot-air balloon floating in Titan's atmosphere for six months. It was competing against

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1308-465: A se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles , meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens. The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided

1417-535: A sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite (a proper noun ), but not when referring to other natural satellites ( common nouns ). Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. A few recent authors define "moon" as "a satellite of

1526-488: A significant atmosphere was first suspected by Catalan 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

1635-447: A surface crust. The presence of ammonia allows water to remain liquid even at a temperature as low as 176 K (−97 °C) (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

1744-450: 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 similar to that found in the Oort cloud and not from sources present during the co-accretion of materials around Saturn. Titan orbits Saturn once every 15 days and 22 hours. Like Earth's Moon and many of the satellites of

1853-635: Is John Culberson , who was at the time the head of the science subcommittee in the House of Representatives . In Spring 2015 he presented a budget request, creating the possibility of an all-new NASA mission program. The House Appropriations Committee approved its version of the FY2016 House Appropriations Commerce-Justice-Science (CJS) bill on May 20, 2015. Therefore, the Committee directed NASA to create

1962-463: Is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether

2071-459: Is consistent with 49% CH 4 , 41% C 2 H 6 , and 10% N 2 by volume. As Titan is synchronously locked with Saturn, there exists a permanent tidal bulge of roughly 100 metres (330 ft) at the sub- and anti-Saturnian points. Titan's orbital eccentricity means that tidal acceleration varies by 9%, though the long orbital period means that these tidal cycles are very gradual. A team of researchers led by Ralph D. Lorenz evaluated that

2180-906: Is covered by plains. Of the several types of plains observed, the most extensive are the Undifferentiated Plains that encompass vast, radar-dark uniform regions. These mid-latitude plains—located largely between 20 and 60° north or south—appear younger than all major geological features except dunes and several craters. The Undifferentiated Plains likely were formed by wind-driven processes and composed of organic-rich sediment. Another extensive type of terrain on Titan are sand dunes, grouped together into vast dune fields or "sand seas" located within 30° north or south. Titanian dunes are typically 1–2 kilometres (0.62–1.24 mi) wide and spaced 1–4 kilometres (0.62–2.49 mi) apart, with some individual dunes over 100 kilometres (62 mi) in length. Limited radar-derived height data suggests that

2289-591: Is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle nearly resembles Earth's water cycle , albeit at a much lower temperature of about 94 K (−179 °C; −290 °F). Due to these factors, Titan is called the most Earth-like celestial object in the Solar System. 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

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2398-574: Is for the Europa Clipper orbiter and lander, with launch of the orbiter in 2025 and the potential Europa Lander shortly after. A 2017 technical analysis stated that the technical challenges are enormous, and that "Without a genuinely strategic program plan, the great promise of an OWEP [Ocean Worlds Exploration Program] is highly likely to remain unfulfilled." The report noted that development of OWEP-enabling technologies must currently compete for priority with other Solar System objectives, which

2507-474: Is four times as thick as Earth's, making it difficult for astronomical instruments to image its surface in the visible light spectrum. The Cassini spacecraft used infrared instruments, radar altimetry and synthetic aperture radar (SAR) imaging to map portions of Titan during its close fly-bys. The first images revealed a diverse geology, with both rough and smooth areas. There are features that may be volcanic in origin, disgorging water mixed with ammonia onto

2616-479: Is known to be high enough that it is probably a solid ellipsoid as well.) The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas , for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone , another of Saturn's moons,

2725-415: 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. Titan is 5,149.46 kilometres (3,199.73 mi) in diameter; it is 6% larger than the planet Mercury and 50% larger than Earth's Moon . Titan

2834-435: 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 equatorial area about the size of Australia. It was first identified in infrared images from

2943-419: Is not always permanent. According to simulations, temporary satellites should be a common phenomenon. The only observed examples are 1991 VG , 2006 RH 120 , 2020 CD 3 . 2006 RH 120 was a temporary satellite of Earth for nine months in 2006 and 2007. Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in

3052-554: Is not useful for strategic planning. The report recommends common, multi-mission technical infrastructure and secure funding to develop it. The Roadmaps to Ocean Worlds (ROW) report was submitted and it was published in January 2019. On Earth , itself an ocean world, liquid water is essential to life as we know it. A question is whether the dark, alien oceans of the outer Solar System could be habitable for simple life forms, and if so, what their biochemistry might be. The goals of

3161-458: Is only around 3 km in diameter and visibly egg-shaped . The effect is smaller on the largest natural satellites, where their gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances. Of the twenty known natural satellites in the Solar System that are large enough to be gravitationally rounded, several remain geologically active today. Io

3270-456: Is roughly 200 metres (660 ft), and that of Ontario Lacus is roughly 90 metres (300 ft). Titan's lakes and seas are dominated by methane ( CH 4 ), with smaller amounts of ethane ( C 2 H 6 ) and dissolved nitrogen ( N 2 ). The fraction of these components varies across different bodies: observations of Ligeia Mare are consistent with 71% CH 4 , 12% C 2 H 6 , and 17% N 2 by volume; whilst Ontario Lacus

3379-621: Is supporting the Europa Clipper orbiter mission to Europa, which is the first planned target of this program to be launched in 2024. The second is the Dragonfly mission to Titan. The program is also supporting concept studies for a proposed Europa Lander , and a concept to explore the moon Triton with Trident , a mission selected as a finalist in NASA's Discovery Program in 2020. Astrobiology mission concepts to water worlds in

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3488-464: Is the largest moon of Saturn and the second-largest in the Solar System . It is the only moon known to have an atmosphere denser than the Earth 's and is the only known object in space—other than Earth —on which there is clear evidence that stable bodies of liquid exist. Titan is one of seven gravitationally rounded moons of Saturn and the second-most distant among them. Frequently described as

3597-420: Is the most volcanically active body in the Solar System, while Europa , Enceladus , Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism . In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries. (This mechanism would have also operated on Triton in the past before its orbit

3706-623: 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 5,262 kilometres (3,270 mi), 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 kilometers above its surface. This increases its apparent diameter. Titan's diameter and mass (and thus its density) are similar to those of

3815-605: The Cassini spacecraft failed to detect rings around Rhea. It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge . Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon. Two natural satellites are known to have small companions at both their L 4 and L 5 Lagrangian points , sixty degrees ahead and behind

3924-482: 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 atmosphere—the methane cycle. Nitrogen is the most abundant gas, with a concentration of around 98.6% in the stratosphere that decreases to 95.1% in

4033-657: The Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of the TSSM. The proposed Titan Mare Explorer (TiME) was a low-cost lander that would splash down in Ligeia Mare in Titan's northern hemisphere. The probe would float whilst investigating Titan's hydrocarbon cycle, sea chemistry, and Titan's origins. It

4142-695: The Hubble Space Telescope 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 Titan, observed from

4251-754: The Saturn – Titan system at 0.044 (with the second mass ratio next to the Earth–Moon system, 1 to 4220), the Jupiter – Ganymede system at 0.038, and the Uranus – Titania system at 0.031. For the category of dwarf planets , Charon has the largest ratio, being 0.52 the diameter and 12.2% the mass of Pluto . The first known natural satellite was the Moon , but it was considered a "planet" until Copernicus ' introduction of De revolutionibus orbium coelestium in 1543. Until

4360-571: The Titans , a race of immortals in Greek mythology . 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

4469-625: The barycentre is below the surface of the larger body, though this is somewhat arbitrary because it depends on distance as well as relative mass. The natural satellites orbiting relatively close to the planet on prograde , uninclined circular orbits ( regular satellites ) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites (generally orbiting on distant, inclined , eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of

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4578-532: The giant planets , its rotational period (its day) is identical to its orbital period; Titan is tidally locked in synchronous rotation with Saturn, and permanently shows one face to the planet. Longitudes on Titan are measured westward, starting from the meridian passing through this point. Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator. The small and irregularly shaped satellite Hyperion

4687-520: The rings of Saturn , but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed. Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos . The giant planets have extensive systems of natural satellites, including half

4796-669: The ESA. The Dragonfly mission, developed and operated by the Johns Hopkins Applied Physics Laboratory , will launch in July 2028. It consists of a large drone powered by an RTG to fly in the atmosphere of Titan as New Frontiers 4. Its instruments will study how far prebiotic chemistry may have progressed. The mission is planned to arrive at Titan in the mid-2030s. There have been several conceptual missions proposed in recent years for returning

4905-518: The Jovian moons Ganymede and Callisto . Based on its bulk density of 1.881 g/cm , 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 3,400-kilometre (2,100 mi) 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

5014-502: The Moon from Earth, which subtends 0.48° of arc. Titan is primarily composed of ice and rocky material, with a rocky core surrounded by various layers of ice, including a crust of ice I h 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

5123-471: The Ocean Worlds Exploration Program are to "identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find." Exploring these moons could help to answer the question of how life arose on Earth and whether it exists anywhere else in the Solar System. It may also be possible to find pre-biotic chemistry occurring, which could provide clues to how life started on Earth. Any life detected at

5232-488: The Ocean Worlds Exploration Program whose primary goal is to discover extant life on another world using a mix of Discovery , New Frontiers and Flagship class missions consistent with the recommendations of current and future Planetary Science Decadal Surveys . In the FY2017 Budget Request, the committee recommended $ 348 million for "Outer Planets" and "Ocean Worlds," of which not less than $ 260 million

5341-513: The Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto , Io, and Europa ), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, Titania , Oberon , Rhea , Iapetus , Charon, Ariel , Umbriel , Dione , and Tethys,

5450-423: The Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible. Exceptions are known; one such exception is Saturn 's natural satellite Hyperion , which rotates chaotically because of

5559-491: The Solar System so far with reasonable certainty are the major moons Europa , Enceladus , Titan , Ganymede , and Callisto . Of these, Europa and Enceladus have the highest priority because their icy shells are thinner than the others (Enceladus’ is less than 10 km; Europa's is about 40 km) and there is some evidence their oceans are in contact with the rocky mantle , which could provide both energy and chemicals for life to form. Enceladus' ice crust has fractures at

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5668-587: The TiME probe would be that TALISE is envisioned with its own propulsion system and would therefore not be limited to simply drifting on the lake when it splashes down. A Discovery Program contestant for its mission #13 is Journey to Enceladus and Titan (JET), an astrobiology Saturn orbiter that would assess the habitability potential of Enceladus and Titan. In 2015, the NASA Innovative Advanced Concepts program (NIAC) awarded

5777-500: The Titan flyby if Voyager 1 had been unable to, did not pass near Titan and continued on to Uranus and Neptune. The Cassini–Huygens spacecraft reached Saturn on July 1, 2004, and began the process of mapping Titan's surface by radar . A joint project of the European Space Agency (ESA) and NASA , Cassini–Huygens proved a very successful mission. The Cassini probe flew by Titan on October 26, 2004, and took

5886-477: The Undifferentiated Plains. 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 Titan 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 of +8.2, and mean opposition magnitude 8.4. This compares to +4.6 for

5995-433: The ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite , and as a consequence, the term has become linked primarily with artificial objects flown in space. Because of this shift in meaning, the term moon , which had continued to be used in

6104-510: The atmosphere, and obtain a precise measurement of Titan's mass. Atmospheric haze prevented direct imaging of the surface, though in 2004 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 , which had been observed in the infrared by the Hubble Space Telescope. Voyager 2 , which would have been diverted to perform

6213-594: The authors of the analysis comparing them to terrestrial fold belts indicative of horizontal compression or convergence. They note that the global distribution of Titan's ridges could be indicative of global contraction, with a thickened ice shell causing regional uplift. The identification of cryovolcanic features on Titan remains controversial and inconclusive, primarily due to limitations of Cassini imagery and coverage. Cassini RADAR and VIMS imagery revealed several candidate cryovolcanic features, particularly flow-like terrains in western Xanadu and steep-sided lakes in

6322-436: The body in its orbit. These companions are called trojan moons , as their orbits are analogous to the trojan asteroids of Jupiter . The trojan moons are Telesto and Calypso , which are the leading and following companions, respectively, of the Saturnian moon Tethys ; and Helene and Polydeuces , the leading and following companions of the Saturnian moon Dione . The discovery of 243 Ida 's natural satellite Dactyl in

6431-400: 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

6540-460: The case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes . All four of the Galilean moons have atmospheres, though they are extremely thin. Four of the largest natural satellites, Europa, Ganymede, Callisto , and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus also supports

6649-407: The central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet . The capture of an asteroid from a heliocentric orbit

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6758-619: 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

6867-539: The discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis

6976-466: The dunes are 80–130 metres (260–430 ft) tall, with the dunes appearing dark in Cassini SAR imagery. Interactions between the dunes and obstacle features, such as mountains, indicate that sand is generally transported in a west-to-east direction. The sand that constructs the dunes is dominated by organic material, probably from Titan's atmosphere; possible sources of sand include river channels or

7085-611: The early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope , are double asteroids with two comparably sized components. Neptune's moon Proteus is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and Orcus ' moon Vanth are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium , i.e. are "round/rounded satellites" and are sometimes categorized as planetary-mass moons . (Dysnomia's density

7194-399: The easternmost tip of a bright region now called Adiri . 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

7303-505: The gravitational influence of Titan . Pluto's four, circumbinary small moons also rotate chaotically due to Charon's influence. In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia , Saturn's Phoebe , and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days. No "moons of moons" or subsatellites (natural satellites that orbit

7412-486: The ground and by Cassini ; at least one of these, Ligeia Mare , Titan's second-largest sea, is almost a pure methane sea. Following the Voyager flybys, Titan was confirmed to have an atmosphere capable of supporting liquid hydrocarbons on its surface. However, the first tentative detection only came in 1995, when data from the Hubble Space Telescope and radar observations suggested expansive hydrocarbon lakes, seas, or oceans. The existence of liquid hydrocarbons on Titan

7521-438: 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 I h crust and deeper ice layers made of high-pressure forms of ice. The heat flow from inside Titan may even be too high for high pressure ices to form, with the outermost layers instead consisting primarily of liquid water underneath

7630-780: 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

7739-412: The highest-resolution images ever of Titan's surface, at only 1,200 kilometres (750 mi), discerning patches of light and dark that would be invisible to the human eye. On July 22, 2006, Cassini made its first targeted, close fly-by at 950 kilometres (590 mi) from Titan; the closest flyby was at 880 kilometres (550 mi) on June 21, 2010. Liquid has been found in abundance on the surface in

7848-420: 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

7957-416: The isotope up from the interior. Titan's surface has comparatively few impact craters, with erosion, tectonics, and cryovolcanism possibly working to erase them over time. Compared to the craters of similarly sized and structured Ganymede and Callisto, those of Titan are much shallower. Many have dark floors of sediment; geomorphological analysis of impact craters largely suggests that erosion and burial are

8066-512: The largest sea; Ligeia Mare, the second-largest sea; and Punga Mare, each filling broad depressions and cumulatively representing roughly 80% of Titan's sea and lake coverage—691,000 square kilometres (267,000 sq mi) combined. All three maria's sea levels are similar, suggesting that they may be hydraulically connected. The southern polar region, meanwhile, hosts four dry broad depressions, potentially representing dried-up seabeds. Additional smaller lakes occupy Titan's polar regions, covering

8175-405: The liquid–ice boundary of a subsurface ocean . Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 kilometres (19 mi) 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

8284-488: The major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and the Moons of Pluto are exceptions among large bodies in that they are thought to have originated from the collision of two large protoplanetary objects early in the Solar System's history (see the giant impact hypothesis ). The material that would have been placed in orbit around

8393-459: The mass of any natural satellites that orbit it, with a correspondingly much larger diameter. The Earth–Moon system is a unique exception in the Solar System; at 3,474 kilometres (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about 1 ⁄ 80 of its mass. The next largest ratios are the Neptune – Triton system at 0.055 (with a mass ratio of about 1 to 4790),

8502-507: The methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes . On April 3, 2013, NASA reported that complex organic chemicals , collectively called tholins , likely arise on Titan, based on studies simulating the atmosphere of Titan. On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan. On September 30, 2013, propene

8611-558: The names of all seven satellites of Saturn then known, came from John Herschel (son of William Herschel , discoverer of two other Saturnian moons, 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

8720-498: The natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominantly Greek, except for the Uranian natural satellites , which are named after Shakespearean characters. The twenty satellites massive enough to be round are in bold in

8829-405: The north polar region, in the form of many lakes and seas discovered by Cassini . Huygens was an atmospheric probe that touched down on Titan on January 14, 2005, discovering that many of its surface features seem to have been formed by fluids at some point in the past. Titan is the most distant body from Earth to have a space probe land on its surface. The Huygens probe landed just off

8938-429: The northern hemisphere that resemble maar craters on Earth, which are created by explosive subterranean eruptions. The likeliest cryovolcano features is a complex of landforms that includes two mountains, Doom Mons and Erebor Mons ; a large depression, Sotra Patera ; and a system of flow-like features, Mohini Fluctus . Between 2005 and 2006, parts of Sotra Patera and Mohini Fluctus became significantly brighter whilst

9047-793: The outer Solar System: Natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet , dwarf planet , or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons , a derivation from the Moon of Earth . In the Solar System , there are six planetary satellite systems containing 288 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus , Pluto , Haumea , Quaoar , Makemake , Gonggong , and Eris . As of January 2022, there are 447 other minor planets known to have natural satellites . A planet usually has at least around 10,000 times

9156-417: The overall haze. The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface. Clouds typically cover 1% of Titan's disk, though 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 during

9265-435: The primary mechanisms of crater modification. Titan's craters are also not evenly distributed, as the polar regions are almost devoid of any identified craters whilst the majority are located in the equatorial dune fields. This inequality may be the result of oceans that once occupied Titan's poles, polar sediment deposition by past rainfall, or increased rates of erosion in the polar regions. The majority of Titan's surface

9374-442: The relatively large natural satellite Charon and four smaller natural satellites; Styx , Nix , Kerberos , and Hydra . Haumea has two natural satellites; Orcus , Quaoar , Makemake , Gonggong , and Eris have one each. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system. The seven largest natural satellites in

9483-537: The remote ocean worlds in the outer Solar System would likely have formed and evolved along an independent path from life on Earth, giving us a deeper understanding of the potential for life in the universe . Oceanographers , biologists and astrobiologists are part of the team developing the strategy roadmap. The planning also considers implementing planetary protection measures to avoid contaminating extraterrestrial habitable environments with resilient stowaway bacteria on their landers. Ocean worlds identified in

9592-448: The similarly sized Ganymede, in the Jovian system. Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer Josep Comas i Solà observed limb darkening of Titan, the first evidence that the body has an atmosphere. In 1944 Gerard P. Kuiper used a spectroscopic technique to detect an atmosphere of methane. The first probe to visit the Saturnian system was Pioneer 11 in 1979, which revealed that Titan

9701-408: The smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets , minor planets and other small Solar System bodies . Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites. The following is a comparative table classifying

9810-571: 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. Titan is the only moon in the Solar System with an atmosphere denser than Earth's, with a surface pressure of 1.448 atm, 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

9919-587: The south pole that allow ice and gas from the ocean to escape to space, where it has been sampled by mass spectrometers aboard the Cassini Saturn orbiter with tantalizing results. Titan's ocean is the deepest, at 50 to 100 km, and no evidence for active plumes or ice volcanism have been observed. Bodies such as Triton , Pluto , Ceres , Miranda , Ariel , and Dione are considered candidate ocean worlds, based on hints from limited spacecraft observations. The Ocean Worlds Exploration Program (OWEP)

10028-488: The southern summer generate uplift in the atmosphere, resulting in convection . This explanation is complicated by the fact that cloud formation has been observed not only after the southern summer solstice but also during mid-spring. Increased methane humidity at the south pole possibly contributes to the rapid increases in cloud size. It was summer in Titan's southern hemisphere until 2010, when Saturn's orbit, which governs Titan's motion, moved Titan's northern hemisphere into

10137-423: The sunlight. When the seasons switch, it is expected that ethane will begin to condense over the south pole. The surface of Titan has been described as "complex, fluid-processed, [and] geologically young". Titan has been around since the Solar System's formation, but its surface is much younger, between 100 million and 1 billion years old. Geological processes may have reshaped Titan's surface. Titan's atmosphere

10246-477: 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

10355-576: The surface of Titan. NASA did not approve the requested $ 715 million, and the future of the project is uncertain. A conceptual design for another lake lander was proposed in late 2012 by the Spanish-based private engineering firm SENER and the Centro de Astrobiología in Madrid . The concept probe is called Titan Lake In-situ Sampling Propelled Explorer (TALISE). The major difference compared to

10464-585: The surface, about 90% has been absorbed by the thick atmosphere, leaving only 0.1% of the amount of light Earth receives. Atmospheric methane creates a greenhouse effect on Titan's surface, without which Titan would be much colder. Conversely, haze in Titan's atmosphere contributes to an anti-greenhouse effect by absorbing sunlight, canceling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere. Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating

10573-522: The surface. There is also evidence that Titan's ice shell may be substantially rigid, which would suggest little geologic activity. 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 features that seem to be impact craters appeared to have been partially filled in, perhaps by raining hydrocarbons or cryovolcanism. Radar altimetry suggests topographical variation

10682-495: The surrounding plains remained unchanged, potentially indicative of ongoing cryovolcanic activity. Indirect lines of evidence for cryovolcanism include the presence of Argon-40 in Titan's atmosphere. Radiogenic Ar is sourced from the decay of K , and has likely been produced within Titan over the course of billions of years within its rocky core. Ar's presence in Titan's atmosphere is thus supportive of active geology on Titan, with cryovolcanism being one possible method of bringing

10791-740: The table below. Minor planets and satellites where there is disagreement in the literature on roundness are italicized in the table below. 107 Camilla and many others Solar System   → Local Interstellar Cloud   → Local Bubble   → Gould Belt   → Orion Arm   → Milky Way   → Milky Way subgroup   → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster   → Local Hole   → Observable universe   → Universe Each arrow ( → ) may be read as "within" or "part of". Titan (moon) Stratosphere : 98.4% nitrogen ( N 2 ), 1.4% methane ( CH 4 ), 0.2% hydrogen ( H 2 ); Titan

10900-553: The tidal range of Titan's major seas are around 0.2–0.8 metres (0.66–2.62 ft). Through Cassini RADAR mapping of Titan's surface, numerous landforms have been interpreted as candidate cryovolcanic and tectonic features by multiple authors. A 2016 analysis of mountainous ridges on Titan revealed that ridges are concentrated in Titan's equatorial regions, implying that ridges either form more frequently in or are better preserved in low-latitude regions. The ridges—primarily oriented east to west—are linear to arcuate in shape, with

11009-471: 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 8 kilometres (5.0 mi) 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

11118-462: Was circularized .) Many other natural satellites, such as Earth's Moon, Ganymede , Tethys, and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes , greater past orbital eccentricities (due in some cases to past orbital resonances ), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers , although in

11227-483: Was detected in the atmosphere of Titan by NASA 's Cassini spacecraft, using its composite infrared spectrometer (CIRS). This is the first time propene has been found on any moon or planet other than Earth and is the first chemical found by the CIRS. The detection of propene fills a mysterious gap in observations that date back to NASA's Voyager 1 spacecraft's first close planetary flyby of Titan in 1980, during which it

11336-489: Was discovered that many of the gases that make up Titan's brown haze were hydrocarbons, theoretically formed via the recombination of radicals created by the Sun's ultraviolet photolysis of methane. Titan's surface temperature is about 94 K (−179.2 °C). At this temperature, water ice has an extremely low vapor pressure , so the little water vapor present appears limited to the stratosphere. Titan receives about 1% as much sunlight as Earth. Before sunlight reaches

11445-420: Was finally confirmed in situ by the Cassini orbiter, with the Cassini mission team announcing "definitive evidence of the presence of lakes filled with liquid methane on Saturn's moon Titan" in January 2007. The observed lakes and seas of Titan are largely restricted to its polar regions, where colder temperatures allow the presence of permanent liquid hydrocarbons. Near Titan's north pole are Kraken Mare,

11554-430: Was probably too cold to support life. It took images of Titan, including Titan and Saturn together in mid to late 1979. The quality was soon surpassed by the two Voyagers . Titan was examined by both Voyager 1 and 2 in 1980 and 1981, respectively. Voyager 1 's trajectory was designed to provide an optimized Titan flyby, during which the spacecraft was able to determine the density, composition, and temperature of

11663-576: Was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity, but was not selected for flight. Another mission to Titan proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho , is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR): an uncrewed plane (or drone ) that would fly through Titan's atmosphere and take high-definition images of

11772-546: Was started in 2016, and was presented in January 2019. The formal program is being implemented within the agency by supporting the Europa Clipper orbiter mission to Europa, and the Dragonfly mission to Titan. The program is also supporting concept studies for a proposed Europa Lander , and concepts to explore the moon Triton . Amanda Hendrix and Terry A. Hurford are the co-leads of the NASA Roadmaps to Oceans World Group. The chief author of NASA's budget proposal

11881-416: 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 1,200,000 km above Saturn's apparent surface. From Titan's surface, Saturn subtends an arc of 5.09 degrees, and if it were visible through the moon's thick atmosphere, it would appear 11.4 times larger in the sky, in diameter, than

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