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Titan (moon)

January 19, 2024

Not to be confused with Titania (moon) or Triton (moon).

Titan is the largest moon of Saturn and the second-largest in the Solar System, larger than any of the dwarf planets of the Solar System. It is the only moon known to have a dense atmosphere, and is the only known object in space other than Earth on which clear evidence of stable bodies of surface liquid has been found.[16]

Titan
Titan pictured in 2011 in natural color. The thick atmosphere is yellow due to a dense organonitrogen haze.
Discovery
Discovered byChristiaan Huygens
Discovery dateMarch 25, 1655
Designations
Designation
Saturn VI
Pronunciation/ˈtaɪtən/ ⓘ[1]
Named after
Τῑτάν Tītan
AdjectivesTitanian[2] or Titanean[3] (both /taɪˈteɪniən/)[4][5]
Orbital characteristics[6]
Periapsis1186680 km
Apoapsis1257060 km
1221870 km
Eccentricity0.0288
15.945 d
5.57 km/s (calculated)
Inclination0.34854° (to Saturn's equator)
Satellite ofSaturn
Physical characteristics
Mean radius
2574.73±0.09 km (0.404 Earth's)[7]
8.3×107 km2 (0.163 Earth's)
Volume7.16×1010 km3 (0.066 Earth's)
Mass(1.3452±0.0002)×1023 kg
(0.0225 Earth's)[8]
Mean density
1.8798±0.0044 g/cm3[8]
1.352 m/s2 (0.138 g)
0.3414±0.0005[9] (estimate)
2.641 km/s
Synchronous
Zero (to the orbital plane);
27° (to the sun)
Albedo0.22 (geometric) [10]0.265±0.03 (Bond)[11]
Temperature93.7 K (−179.5 °C)[12]
8.2[13] to 9.0
Atmosphere
Surface pressure
146.7 kPa (1.45 atm)
Composition by volumeVariable

Stratosphere:
98.4% nitrogen (N
2),
1.4% methane (CH
4),
0.2% hydrogen (H
2);

Lower troposphere:
95.0% N
2, 4.9% CH
4;[14]
97% N
2, 2.7±0.1% CH
4, 0.1–0.2% H
2[15]

Titan is one of the seven gravitationally rounded moons in orbit around Saturn, and the second most distant from Saturn of those seven. Frequently described as 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 moon Ganymede, and is larger than the planet Mercury, but only 40% as massive.

Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. 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 the Moon from Earth, which subtends 0.48° of arc.

Titan is primarily composed of ice and rocky material, which is likely differentiated into a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water.[17] Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions and the discovery of its atmospheric super-rotation. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and heavy organonitrogen haze. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle bears a striking similarity to Earth's water cycle, albeit at the much lower temperature of about 94 K (−179 °C; −290 °F). Due to these factors, Titan has been described as the most Earth-like celestial object in the Solar System.[18]

History edit

Discovery edit

 
Christiaan Huygens discovered Titan in 1655.

Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.[19][20] Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his 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.[21] It was the sixth moon ever discovered, after Earth's Moon and the Galilean moons of Jupiter.[22]

Titan is the largest and brightest moon of Saturn, and so is the easiest to observe of Saturn's moons with a standard optical telescope from Earth.

Naming edit

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).[23] After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V (with Titan then in fourth position). Other early epithets for Titan include "Saturn's ordinary satellite".[24] The International Astronomical Union officially numbers Titan as Saturn VI.[25]

The name Titan, and 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.[26][27] Numerous small moons have been discovered around Saturn since then.[28] Saturnian moons are named after mythological giants. The name Titan comes from the Titans, a race of immortals in Greek mythology.[25]

Orbit and rotation edit

Main article: Moons of Saturn
 
Titan's orbit (highlighted in red) among the other large inner moons of Saturn. The moons outside its orbit are (from the outside to the inside) Iapetus and Hyperion; those inside are Rhea, Dione, Tethys, Enceladus, and Mimas.

Titan orbits Saturn once every 15 days and 22 hours. Like Earth's Moon and many of the satellites of 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.[29] Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator,[6] and hence also about a third of a degree off of the equatorial ring plane. Viewed from Earth, Titan reaches an angular distance of about 20 Saturn radii (just over 1,200,000 kilometers (750,000 mi)) from Saturn and subtends a disk 0.8 arcseconds in diameter.[citation needed]

The small and irregularly shaped satellite Hyperion is locked in a 3:4 orbital resonance with Titan. Hyperion probably formed in a stable orbital island, whereas the massive Titan absorbed or ejected any other bodies that made close approaches.[30]

Bulk characteristics edit

 
Size comparison: Titan (lower left) with the Moon and Earth (top and right)
 
A model of Titan's internal structure showing ice-six layer

Titan is 5,149.46 kilometers (3,199.73 mi) in diameter,[7] 1.06 times that of the planet Mercury, 1.48 that of the Moon, and 0.40 that of Earth. Titan is the tenth-largest object in the solar system, including the Sun. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede (diameter 5,262 kilometers (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 kilometres above its surface. This increases its apparent diameter.[31] Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto.[32] Based on its bulk density of 1.88 g/cm3, Titan's composition is half ice and half rocky material. Though similar in composition to Dione and Enceladus, it is denser due to gravitational compression. It has a mass 1/4226 that of Saturn, making it the largest moon of the gas giants relative to the mass of its primary. It is second in terms of relative diameter of moons to a gas giant; Titan being 1/22.609 of Saturn's diameter, Triton is larger in diameter relative to Neptune at 1/18.092.[citation needed]

Titan is probably partially differentiated into distinct layers with a 3,400-kilometer (2,100 mi) rocky center.[33] This rocky center is believed to be surrounded by several layers composed of different crystalline forms of ice, and/or water.[34] The exact structure depends heavily on the heat flux from within Titan itself, which is poorly constrained. The interior may still be hot enough for a liquid layer consisting of a "magma" composed of water and ammonia between the ice Ih 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 a surface crust.[35] 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).[36] The Cassini probe discovered evidence for the layered structure in the form of natural extremely-low-frequency radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean.[37] Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 kilometers (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.[38] Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn.[39] Comparison of the gravity field with the RADAR-based topography observations[40] also suggests that the ice shell may be substantially rigid.[41][42]

Formation edit

The moons of Jupiter and Saturn are thought to have formed through co-accretion, a similar process to that 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. Whereas Jupiter possesses four large satellites in highly regular, planet-like orbits, Titan overwhelmingly dominates Saturn's system and possesses 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 satellites, but that they were disrupted by a series of giant impacts, which would go on to form Titan. Saturn's mid-sized moons, such as Iapetus and Rhea, were formed from the debris of these collisions. Such a violent beginning would also explain Titan's orbital eccentricity.[43]

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.[44]

Atmosphere edit

Main article: Atmosphere of Titan
 
True-color image of layers of haze in Titan's atmosphere

Titan is the only known moon with a significant atmosphere,[45] and its atmosphere is the only nitrogen-rich dense atmosphere in the Solar System aside from Earth's. Observations of it made in 2004 by Cassini suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.[46] Observations from the Voyager space probes have shown that Titan's atmosphere is denser than Earth's, with a surface pressure about 1.45 atm. It is also about 1.19 times as massive as Earth's overall,[47] or about 7.3 times more massive on a per surface area basis. Opaque haze layers block most visible light from the Sun and other sources and obscure Titan's surface features.[48] Titan's lower gravity means that its atmosphere is far more extended than Earth's.[49] The atmosphere of Titan is opaque at many wavelengths and as a result, a complete reflectance spectrum of the surface is impossible to acquire from orbit.[50] It was not until the arrival of the Cassini–Huygens spacecraft in 2004 that the first direct images of Titan's surface were obtained.[51]

Titan Clouds
 
Clouds (Nov 4, 2022)
 
Clouds (Nov 6, 2022)

Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), and hydrogen (0.1–0.2%), with trace amounts of other gases.[15] There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene and propane, and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium.[14] The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog.[52] Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind.[53]

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.[54] The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.[55][56][57][58]

 
Trace organic gases in Titan's atmosphere—HNC (left) and HC3N (right).

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.[59] On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan.[60][61]

On September 30, 2013, propene was detected in the atmosphere of Titan by NASA's Cassini spacecraft, using its composite infrared spectrometer (CIRS).[62] 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 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.[52]

On October 24, 2014, methane was found in polar clouds on Titan.[63][64] On December 1, 2022, astronomers reported viewing clouds, likely made of methane, moving across Titan, using the James Webb Space Telescope.[65][66]

 
Polar clouds, made of methane, on Titan (left) compared with polar clouds on Earth (right), which are made of water or water ice.

Climate edit

Main article: Climate of Titan
 
Atmospheric polar vortex over Titan's south pole

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.[67] Titan receives about 1% as much sunlight as Earth.[68] Before sunlight reaches the surface, about 90% has been absorbed by the thick atmosphere, leaving only 0.1% of the amount of light Earth receives.[69]

Atmospheric methane creates a greenhouse effect on Titan's surface, without which Titan would be much colder.[70] Conversely, haze in Titan's atmosphere contributes to an anti-greenhouse effect by absorbing sunlight, cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere.[71]

 
Methane clouds (animated; July 2014).[72]

Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze.[31] The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface.[73]

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 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.[74] 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 the sunlight.[75] When the seasons switch, it is expected that ethane will begin to condense over the south pole.[76]

Surface features edit

See also: List of geological features on Titan
 
Global geologic map of Titan (2019)[16]
  •  
    Global map of Titan – with IAU labels (August 2016).
  •  
    Titan – infrared views (2004–2017)
  •  
    Titan's North Pole (2014)
  •  
    Titan's South Pole (2014)

The surface of Titan has been described as "complex, fluid-processed, [and] geologically young".[77] 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.[78] Titan's atmosphere is four times as thick as Earth's,[79] making it difficult for astronomical instruments to image its surface in the visible light spectrum.[80] 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 the surface. There is also evidence that Titan's ice shell may be substantially rigid,[41][42] which would suggest little geologic activity.[81] There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles.[82][83] 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.[84]

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 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.[85] 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.[86] There are dark areas of similar size elsewhere on Titan, observed from the ground and by Cassini; at least one of these, Ligeia Mare, Titan's second-largest sea, is almost a pure methane sea.[87][88]

 
Titan mosaic from a Cassini flyby. The large dark region is Shangri-La.
 
Titan in false color showing surface details and atmosphere. Xanadu is the bright region at the bottom-center.
 
Titan composite image in infrared. It features the dark, dune-filled regions Fensal (north) and Aztlan (south).

Lakes edit

Main article: Lakes of Titan
 
Titan lakes (September 11, 2017)
 
False-color Cassini radar mosaic of Titan's north polar region. Blue coloring indicates low radar reflectivity, caused by hydrocarbon seas, lakes and tributary networks filled with liquid ethane, methane and dissolved N
2.[15] About half of the large body at lower left, Kraken Mare, is shown. Ligeia Mare is at lower right.
 
Mosaic of three Huygens images of channel system on Titan
 
Rimmed lakes of Titan
(artist concept)

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 was not 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.[89]

The Cassini mission confirmed the former hypothesis. When the probe arrived in the Saturnian system in 2004, it was hoped that hydrocarbon lakes or oceans would be detected from the sunlight reflected off their surface, but no specular reflections were initially observed.[90] Near Titan's south pole, an enigmatic dark feature named Ontario Lacus was identified[91] (and later confirmed to be a lake).[92] A possible shoreline was also identified near the pole via radar imagery.[93] Following a flyby on July 22, 2006, in which the Cassini spacecraft's radar imaged the northern latitudes (that were then in winter), several large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole.[94] Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007.[95][96] 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 Earth.[95] Some appear to have channels associated with liquid and lie in topographical depressions.[95] The liquid erosion features appear to be a very recent occurrence: channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms.[78] Overall, the Cassini radar observations have shown that lakes cover only a small percentage of the surface, making Titan much drier than Earth.[97] Most of the lakes are concentrated near the poles (where the relative lack of sunlight prevents evaporation), but several long-standing hydrocarbon lakes in the equatorial desert regions have also been discovered, including one near the Huygens landing site in the Shangri-La region, which is about half the size of the Great Salt Lake in Utah, USA. The equatorial lakes are probably "oases", i.e. the likely supplier is underground aquifers.[98]

 
 
Evolving feature in Ligeia Mare

In June 2008, the Visual and Infrared Mapping Spectrometer on Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus.[99] 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).[100][101]

 
Near-infrared radiation from the Sun reflecting off Titan's hydrocarbon seas

On July 8, 2009, Cassini's VIMS observed a specular reflection indicative of a smooth, mirror-like surface, off what today is called Jingpo Lacus, a lake in the north polar region shortly after the area emerged from 15 years of winter darkness. Specular reflections are indicative of a smooth, mirror-like surface, so the observation corroborated the inference of the presence of a large liquid body drawn from radar imaging.[102][103]

Early radar measurements made in July 2009 and January 2010 indicated that Ontario Lacus was extremely shallow, with an average depth of 0.4–3 m, and a maximum depth of 3 to 7 m (9.8 to 23.0 ft).[104] In contrast, the northern hemisphere's Ligeia Mare was initially mapped to depths exceeding 8 m, the maximum discernable by the radar instrument and the analysis techniques of the time.[104] Later science analysis, released in 2014, more fully mapped the depths of Titan's three methane seas and showed depths of more than 200 meters (660 ft). Ligeia Mare averages from 20 to 40 m (66 to 131 ft) in depth, while other parts of Ligeia did not register any radar reflection at all, indicating a depth of more than 200 m (660 ft). While only the second largest of Titan's methane seas, Ligeia "contains enough liquid methane to fill three Lake Michigans".[105]

In May 2013, Cassini's radar altimeter observed Titan's Vid Flumina channels, defined as a drainage network connected to Titan's second-largest hydrocarbon sea, Ligeia Mare. Analysis of the received altimeter echoes showed that the channels are located in deep (up to ~570 m), steep-sided, canyons and have strong specular surface reflections that indicate they are currently filled with liquid. Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0.7 m, consistent with the interpretation of drowned river valleys. Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare, consistent with drainage feeding into the main channel system. This is likely the first direct evidence of the presence of liquid channels on Titan and the first observation of hundred-meter deep canyons on Titan. Vid Flumina canyons are thus drowned by the sea but there are a few isolated observations to attest to the presence of surface liquids standing at higher elevations.[106]

During six flybys of Titan from 2006 to 2011, Cassini gathered radiometric tracking and optical navigation data from which investigators could roughly infer Titan's changing shape. The density of Titan is consistent with a body that is about 60% rock and 40% water. The team's analyses suggest that Titan's surface can rise and fall by up to 10 metres during each orbit. That degree of warping suggests that Titan's interior is relatively deformable, and that the most likely model of Titan is one in which an icy shell dozens of kilometres thick floats atop a global ocean.[107] The team's findings, together with the results of previous studies, hint that Titan's ocean may lie no more than 100 kilometers (62 mi) below its surface.[107][108] On July 2, 2014, NASA reported the ocean inside Titan may be as salty as the Dead Sea.[109][110] On September 3, 2014, NASA reported studies suggesting methane rainfall on Titan may interact with a layer of icy materials underground, called an "alkanofer", to produce ethane and propane that may eventually feed into rivers and lakes.[111]

In 2016, Cassini found the first evidence of fluid-filled channels on Titan, in a series of deep, steep-sided canyons flowing into Ligeia Mare. This network of canyons, dubbed Vid Flumina, ranges in depth from 240 to 570 m and has sides as steep as 40°. They are believed to have formed either by crustal uplifting, like Earth's Grand Canyon, a lowering of sea level, or perhaps a combination of the two. The depth of erosion suggests that liquid flows in this part of Titan are long-term features that persist for thousands of years.[112]

 
 
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 edit

 
Radar image of a 139 km-diameter[113] impact crater on Titan's surface, showing a smooth floor, rugged rim, and possibly a central peak.

Radar, SAR and imaging data from Cassini have revealed few impact craters on Titan's surface.[78] These impacts appear to be relatively young, compared to Titan's age.[78] The few impact craters discovered include a 392-kilometer-wide (244 mi) two-ring impact basin named Menrva seen by Cassini's ISS as a bright-dark concentric pattern.[114] A smaller, 80-kilometer-wide (50 mi), flat-floored crater named Sinlap[115] and a 30 km (19 mi) crater with a central peak and dark floor named Ksa have also been observed.[116] Radar and Cassini imaging have also revealed "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-kilometer-wide (56 mi) ring of bright, rough material known as Guabonito has been observed by Cassini.[117] 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.[118]

 
Ligeia Mare – SAR and clearer despeckled views.[119]

Many of Titan's craters or probable craters display evidence of extensive erosion, and all show some indication of modification.[113] Most large craters have breached or incomplete rims, despite the fact that some craters on Titan have relatively more massive rims than those anywhere else in the Solar System. There is little evidence of formation of palimpsests through viscoelastic crustal relaxation, unlike on other large icy moons.[113] Most craters lack central peaks and have smooth floors, possibly due to impact-generation or later eruption of cryovolcanic lava. 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.[120]

The limited high-resolution radar coverage of Titan obtained through 2007 (22%) suggested the existence of nonuniformities in its crater distribution. Xanadu has 2–9 times more craters than elsewhere. The leading hemisphere has a 30% higher density than the trailing hemisphere. There are lower crater densities in areas of equatorial dunes and in the north polar region (where hydrocarbon lakes and seas are most common).[113]

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".[121]

Cryovolcanism and mountains edit

See also: Cryovolcano
 
Near-infrared image of Tortola Facula, thought to be a possible cryovolcano

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.[122] 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.[123]

Still, there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes.[124] One of the first of such features revealed by Cassini radar observations in 2004, called Ganesa Macula, resembles the geographic features called "pancake domes" found on Venus, and was thus initially thought to be cryovolcanic in origin, until Kirk et al. refuted this hypothesis at the American Geophysical Union annual meeting 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.[125][126] In 2004 Cassini also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome.[127] No similar features have been identified as of 2010.[128] 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.[36]

A mountain range measuring 150 kilometers (93 mi) long, 30 kilometers (19 mi) wide and 1.5 kilometers (0.93 mi) 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.[129] 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.[130]

In 2008 Jeffrey Moore (planetary geologist of Ames Research Center) 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 and eolian erosion, mass wasting 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 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 a temperature of 176 K (−97 °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.[124][131]

In March 2009, structures resembling lava flows were announced in a region of Titan called Hotei Arcus, 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 (660 ft) above Titan's surface, consistent with it having erupted from beneath the surface.[132]

In December 2010, the Cassini mission team announced the most compelling possible cryovolcano yet found. Named Sotra Patera, 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.[133]

Crater-like landforms possibly formed via explosive, maar-like or caldera-forming cryovolcanic eruptions have been identified in Titan's polar regions.[134] These formations are sometimes nested or overlapping and have features suggestive of explosions and collapses, such as elevated rims, halos, and internal hills or mountains.[134] The polar location of these features and their colocalization with Titan's lakes and seas suggests volatiles such as methane may help power them. Some of these features appear quite fresh, suggesting that such volcanic activity continues to the present.[134]

Most of Titan's highest peaks occur near its equator in so-called "ridge belts". They are believed to be analogous to Earth's fold mountains such as the Rockies or the Himalayas, formed by the collision and buckling of tectonic plates, or to subduction zones like the Andes, where upwelling lava (or cryolava) from a melting descending plate rises to the surface. One possible mechanism for their formation is tidal forces from Saturn. Because Titan's icy mantle is less viscous than Earth's magma mantle, and because its icy bedrock is softer than Earth's granite bedrock, mountains are unlikely to reach heights as great as those on Earth. In 2016, the Cassini team announced what they believe to be the tallest mountain on Titan. Located in the Mithrim Montes range, it is 3,337 m tall.[135]

 
False-color VIMS image of the possible cryovolcano Sotra Patera, combined with a 3D map based on radar data, showing 1000-meter-high peaks and a 1500-meter-deep crater.

If volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, as it is on Earth.[36] 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.[36] The low-pressure ice, overlaying a liquid layer of ammonium sulfate, 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 landscape and sand dune features.[136] Titan may have been much more geologically active in the past; models of Titan's internal evolution suggest that Titan's crust was only 10 kilometers thick until about 500 million years ago, allowing vigorous cryovolcanism with low viscosity water magmas to erase all surface features formed before that time. Titan's modern geology would have formed only after the crust thickened to 50 kilometers and thus impeded constant cryovolcanic resurfacing, with any cryovolcanism occurring since that time producing much more viscous water magma with larger fractions of ammonia and methanol; this would also suggest that Titan's methane is no longer being actively added to its atmosphere and could be depleted entirely within a few tens of millions of years.[137]

Many of the more prominent mountains and hills have been given official names by the International Astronomical Union. According to JPL, "By convention, mountains on Titan are named for mountains from Middle-earth, the fictional setting in fantasy novels by J. R. R. Tolkien." Colles (collections of hills) are named for characters from the same Tolkien works.[138]

Dark equatorial terrain edit

 
Sand dunes in the Namib Desert on Earth (top), compared with dunes in Belet on Titan

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.[139] Prior to the arrival of Cassini, these regions were thought to be seas of liquid hydrocarbons.[140] Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal dunes, up to 330 ft (100 m) high,[141] about a kilometer wide, and tens to hundreds of kilometers long.[142] Dunes of this type are always aligned with average wind direction. In the case of Titan, steady zonal (eastward) winds combine with variable tidal winds (approximately 0.5 meters per second).[143] The tidal winds are the result of tidal forces 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, it was hypothesized, causes granular material on the surface to gradually build up in long parallel dunes aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.[144]

The longitudinal (or linear) dunes were initially presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. Subsequent observations indicate that the dunes point to the east although climate simulations indicate Titan's surface winds blow toward the west. At less than 1 meter per second, they are not powerful enough to lift and transport surface material. Recent computer simulations indicate that the dunes may be the result of rare storm winds that happen only every fifteen years when Titan is in equinox. These storms produce strong downdrafts, flowing eastward at up to 10 meters per second when they reach the surface.[145]

The "sand" on Titan is likely not made up of small grains of silicates like the sand on Earth,[146] but rather might have formed when liquid methane rained and eroded the water-ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids called tholins, produced by photochemical reactions in Titan's atmosphere.[141][143][147] Studies of dunes' composition in May 2008 revealed that they possessed less water than the rest of Titan, and are thus most likely derived from organic soot like hydrocarbon polymers clumping together after raining onto the surface.[148] Calculations indicate the sand on Titan has a density of one-third that of terrestrial sand.[149] The low density combined with the dryness of Titan's atmosphere might cause the grains to clump together because of static electricity buildup. The "stickiness" might make it difficult for the generally mild breeze close to Titan's surface to move the dunes although more powerful winds from seasonal storms could still blow them eastward.[150]

Around equinox, strong downburst winds can lift micron-sized solid organic particles up from the dunes to create Titanian dust storms, observed as intense and short-lived brightenings in the infrared.[151]

 
Titan – three dust storms detected in 2009–2010.[152]

Observation and exploration edit

 
Voyager 1 view of haze on Titan's limb (1980)

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.[153] Titan has a maximum apparent magnitude of +8.2,[13] and mean opposition magnitude 8.4.[154] This compares to +4.6 for the similarly sized Ganymede, in the Jovian system.[154]

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.[155]

Fly-by missions: Pioneer and Voyager edit

The first probe to visit the Saturnian system was Pioneer 11 in 1979, which revealed that Titan was probably too cold to support life.[156] It took images of Titan, including Titan and Saturn together in mid to late 1979.[157] The quality was soon surpassed by the two Voyagers.[158]

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 the atmosphere, and obtain a precise measurement of Titan's mass.[159] 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,[160] which had been observed in the infrared by the Hubble Space Telescope. Voyager 2, which would have been diverted to perform the Titan flyby if Voyager 1 had been unable to, did not pass near Titan and continued on to Uranus and Neptune.[159]: 94 

Cassini–Huygens edit

Main articles: Cassini–Huygens and Huygens (spacecraft)
 
Cassini image of Titan in front of the rings of Saturn
 
Cassini image of Titan, behind Epimetheus and the rings
 
Cassini's Titan flyby radio signal studies (artist's concept)

Even with the data provided by the Voyagers, Titan remained a body of mystery—a large satellite shrouded in an atmosphere that makes detailed observation difficult.

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 the highest-resolution images ever of Titan's surface, at only 1,200 kilometers (750 mi), discerning patches of light and dark that would be invisible to the human eye.[citation needed]

On July 22, 2006, Cassini made its first targeted, close fly-by at 950 kilometers (590 mi) from Titan; the closest flyby was at 880 kilometers (550 mi) on June 21, 2010.[161] Liquid has been found in abundance on the surface in the north polar region, in the form of many lakes and seas discovered by Cassini.[94]

Huygens landing edit

 
Huygens in situ image from Titan's surface—the only image from the surface of a body permanently farther away than Mars
 
Same image with contrast enhanced

Huygens was an atmospheric probe that touched down on Titan on January 14, 2005,[162] discovering that many of its surface features seem to have been formed by fluids at some point in the past.[163] Titan is the most distant body from Earth to have a space probe land on its surface.[164]

The Huygens probe descends by parachute and lands on Titan on January 14, 2005

The Huygens probe landed just off 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 the hills with the methane rain and are deposited on the plains over geological time scales.[165]

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.[165] 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 ground surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice.[166]

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 the ESA.[167]

Dragonfly edit

Main article: Dragonfly (spacecraft)

The Dragonfly mission, developed and operated by the Johns Hopkins Applied Physics Laboratory, will launch in July 2028.[168] It consists of a large drone powered by an RTG to fly in the atmosphere of Titan as New Frontiers 4.[169][170] Its instruments will study how far prebiotic chemistry may have progressed.[171] The mission is planned to arrive at Titan in the mid-2030s.[170]

Proposed or conceptual missions edit

 
The balloon proposed for the Titan Saturn System Mission (artistic rendition)

There have been several conceptual missions proposed in recent years for returning 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.[citation needed]

The Titan Saturn System Mission (TSSM) was a joint NASA/ESA proposal for exploration of Saturn's moons.[172] It envisions a hot-air balloon floating in Titan's atmosphere for six months. It was competing against 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.[173]

The proposed Titan Mare Explorer (TiME) was a low-cost lander that would splash down in a lake in Titan's northern hemisphere and float on the surface of the lake for three to six months.[174][175][176] It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity,[177] but was not selected for flight.[178]

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 the surface of Titan. NASA did not approve the requested $715 million, and the future of the project is uncertain.[179][180]

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).[181][182] The major difference compared to 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.[181]

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.[183][184][185]

In 2015, the NASA Innovative Advanced Concepts program (NIAC) awarded a Phase II grant[186] to a design study of a Titan Submarine to explore the seas of Titan.[187][188][189][190][191]

Prebiotic conditions and life edit

Main article: Life on Titan
See also: Planetary habitability

Titan is thought to be a prebiotic environment rich in complex organic compounds,[59][192] but its surface is in a deep freeze at −179 °C (−290.2 °F; 94.1 K) so it is currently understood that life cannot exist on the moon's frigid surface.[193] However, Titan seems to contain a global ocean beneath its ice shell, and within this ocean, conditions are potentially suitable for microbial life.[194][195][196]

The Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organic compounds; it showed an environment on Titan that is similar, in some ways, to ones hypothesized for the primordial Earth.[197] Scientists surmise 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.[198][192]

Formation of complex molecules edit

The Miller–Urey experiment 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 of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.[199]

It has been reported that when energy was applied to a combination of gases like those in Titan's atmosphere, five nucleotide bases, the building blocks of DNA and RNA, were among the many compounds produced. In addition, amino acids, the building blocks of protein were found. It was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.[200]

On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan.[59]

On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons (PAH) in the upper atmosphere of Titan.[60][61]

On July 26, 2017, Cassini scientists positively identified the presence of carbon chain anions in Titan's upper atmosphere which appeared to be involved in the production of large complex organics.[201] These highly reactive molecules were previously known to contribute to building complex organics in the Interstellar Medium, therefore highlighting a possibly universal stepping stone to producing complex organic material.[202]

On July 28, 2017, scientists reported that acrylonitrile, or vinyl cyanide, (C2H3CN), possibly essential for life by being related to cell membrane and vesicle structure formation, had been found on Titan.[203][204][205]

In October 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex polycyclic aromatic hydrocarbon (PAH) chemicals. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it.[206][207]

Possible subsurface habitats edit

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. The analogy assumes the presence of liquid water for longer periods than is currently observable; several hypotheses postulate that liquid water from an impact could be preserved under a frozen isolation layer.[208] It has also been hypothesized that liquid-ammonia oceans could exist deep below the surface.[194][209] Another model suggests an ammonia–water solution as much as 200 kilometers (120 mi) deep beneath a water-ice crust with conditions that, although extreme by terrestrial standards, are such that life could survive.[195] Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life.[194] Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined.[195]

Methane and life at the surface edit

See also: Hypothetical types of biochemistry

It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water.[210] Such organisms would inhale H2 in place of O2, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide.[196][210] However, such hypothetical organisms would be required to metabolize at a deep freeze temperature of −179.2 °C (−290.6 °F; 94.0 K).[193]

All life forms on Earth (including methanogens) use liquid water as a solvent; it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane,[211] although water is a stronger solvent than methane.[212] Water is also more chemically reactive, and can break down large organic molecules through hydrolysis.[211] A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.[211]

In 2005, astrobiologist Chris McKay argued that if methanogenic life did exist on the surface of Titan, it would likely have a measurable effect on the mixing ratio in the Titan troposphere: levels of hydrogen and acetylene would be measurably lower than otherwise expected. Assuming metabolic rates similar to those of methanogenic organisms on Earth, the concentration of molecular hydrogen would drop by a factor of 1000 on the Titanian surface solely due to a hypothetical biological sink. McKay noted that, if life is indeed present, the low temperatures on Titan would result in very slow metabolic processes, which could conceivably be hastened by the use of catalysts similar to enzymes. He also noted that the low solubility of organic compounds in methane presents a more significant challenge to any possible form of life. Forms of active transport, and organisms with large surface-to-volume ratios could theoretically lessen the disadvantages posed by this fact.[210]

In 2010, Darrell Strobel, from Johns Hopkins University, identified a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward flow at a rate of roughly 1028 molecules per second and disappearance of hydrogen near Titan's surface; as Strobel noted, his findings were in line with the effects McKay had predicted if methanogenic life-forms were present.[210][212][213] The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by McKay as consistent with the hypothesis of organisms consuming hydrocarbons.[212] Although restating the biological hypothesis, he cautioned that other explanations for the hydrogen and acetylene findings are more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a surface catalyst accepting hydrocarbons or hydrogen), or flaws in the current models of material flow.[196] Composition data and transport models need to be substantiated, etc. Even so, despite saying that a non-biological catalytic explanation would be less startling than a biological one, McKay noted that the discovery of a catalyst effective at 95 K (−180 °C) would still be significant.[196] With regards to the acetylene findings, Mark Allen, the principal investigator with the NASA Astrobiology Institute Titan team, provided a speculative, non-biological explanation: sunlight or cosmic rays could transform the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.[214]

As NASA notes in its news article on the June 2010 findings: "To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere."[212] As the NASA statement also says: "some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan's surface."[212]

In February 2015, a hypothetical cell membrane capable of functioning in liquid methane at cryogenic temperatures (deep freeze) conditions was modeled. Composed of small molecules containing carbon, hydrogen, and nitrogen, it would have the same stability and flexibility as cell membranes on Earth, which are composed of phospholipids, compounds of carbon, hydrogen, oxygen, and phosphorus. This hypothetical cell membrane was termed an "azotosome", a combination of "azote", French for nitrogen, and "liposome".[215][216]

Obstacles edit

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, Titan is frigid, and its atmosphere lacks CO2. At Titan's surface, water exists only in solid form. Because of these difficulties, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for life than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth.[217] Although life itself may not exist, the prebiotic conditions on Titan and the associated organic chemistry remain of great interest in understanding the early history of the terrestrial biosphere.[197] Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiments, and chemical and photochemical modeling on Earth.[199]

Panspermia hypothesis edit

Main article: Panspermia

It is hypothesized that large asteroid and cometary impacts on Earth's surface may have caused fragments of microbe-laden rock to escape Earth's gravity, suggesting the possibility of panspermia. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan.[218][219] 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.[220]

Future conditions edit

Conditions on Titan could become far more habitable in the far future. Five billion years from now, as the Sun becomes a sub-red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable.[221] 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 a habitable environment, and could persist for several hundred million years. This is proposed to have been sufficient time for simple life to spawn on Earth, though the higher viscosity of ammonia-water solutions coupled with low temperatures would cause chemical reactions to proceed more slowly on Titan.[222]

See also edit

References edit

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Bibliography edit

  • Coustenis, Athéna; Taylor, F. W. (2008). Titan: Exploring an Earthlike World. World Scientific. ISBN 978-981-270-501-3.

Further reading edit

  • Lorenz, Ralph; Mitton, Jacqueline (2002). Lifting Titan's Veil: Exploring the Giant Moon of Saturn. Cambridge University Press. ISBN 978-0-521-79348-3.
  • Lorenz, Ralph; Mitton, Jacqueline (2008). Titan Unveiled. Princeton University Press. ISBN 978-0-691-14633-1.
  • Lorenz, Ralph (2017). NASA/ESA/ASI Cassini-Huygens: 1997 onwards (Cassini orbiter, Huygens probe and future exploration concepts) (Owners' Workshop Manual). Haynes Manuals, UK. ISBN 978-1-78521-111-9.
  • O'Callaghan, Jonathan (November 21, 2019). "A Map of Saturn's Largest Moon" (PDF). Nature. 575 (7783): 426–427. Bibcode:2019Natur.575..426O. doi:10.1038/d41586-019-03539-8. PMID 31745360. S2CID 208171884.

External links edit

 
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  • Cassini Imaging Central Laboratory for Operations (CICLOPS) site Titan image search February 20, 2022, at the Wayback Machine
  • The Planetary Society (2005). . Retrieved March 28, 2005.
  • The Alien Noise. This recording is a laboratory reconstruction of the sounds heard by Huygens' microphones.
  • AstronomyCast: Titan October 12, 2011, at the Wayback Machine Fraser Cain and Pamela Gay, 2010.
  • Titan nomenclature and Titan map with feature names from the USGS planetary nomenclature page
  • Google Titan 3D, interactive map of the moon
  • Image album by Kevin M. Gill

January 19, 2024
titan, moon, confused, with, titania, moon, triton, moon, titan, largest, moon, saturn, second, largest, solar, system, larger, than, dwarf, planets, solar, system, only, moon, known, have, dense, atmosphere, only, known, object, space, other, than, earth, whi. Not to be confused with Titania moon or Triton moon Titan is the largest moon of Saturn and the second largest in the Solar System larger than any of the dwarf planets of the Solar System It is the only moon known to have a dense atmosphere and is the only known object in space other than Earth on which clear evidence of stable bodies of surface liquid has been found 16 TitanTitan pictured in 2011 in natural color The thick atmosphere is yellow due to a dense organonitrogen haze DiscoveryDiscovered byChristiaan HuygensDiscovery dateMarch 25 1655DesignationsDesignationSaturn VIPronunciation ˈ t aɪ t en 1 Named afterTῑtan TitanAdjectivesTitanian 2 or Titanean 3 both t aɪ ˈ t eɪ n i e n 4 5 Orbital characteristics 6 Periapsis1186 680 kmApoapsis1257 060 kmSemi major axis1221 870 kmEccentricity0 0288Orbital period sidereal 15 945 dAverage orbital speed5 57 km s calculated Inclination0 34854 to Saturn s equator Satellite ofSaturnPhysical characteristicsMean radius2574 73 0 09 km 0 404 Earth s 7 Surface area8 3 107 km2 0 163 Earth s Volume7 16 1010 km3 0 066 Earth s Mass 1 3452 0 0002 1023 kg 0 0225 Earth s 8 Mean density1 8798 0 0044 g cm3 8 Surface gravity1 352 m s2 0 138 g Moment of inertia factor0 3414 0 0005 9 estimate Escape velocity2 641 km sSynodic rotation periodSynchronousAxial tiltZero to the orbital plane 27 to the sun Albedo0 22 geometric 10 0 265 0 03 Bond 11 Temperature93 7 K 179 5 C 12 Apparent magnitude8 2 13 to 9 0AtmosphereSurface pressure146 7 kPa 1 45 atm Composition by volumeVariable Stratosphere 98 4 nitrogen N2 1 4 methane CH4 0 2 hydrogen H2 Lower troposphere 95 0 N2 4 9 CH4 14 97 N2 2 7 0 1 CH4 0 1 0 2 H2 15 Titan is one of the seven gravitationally rounded moons in orbit around Saturn and the second most distant from Saturn of those seven Frequently described as 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 moon Ganymede and is larger than the planet Mercury but only 40 as massive Discovered in 1655 by the Dutch astronomer Christiaan Huygens Titan was the first known moon of Saturn and the sixth known planetary satellite after Earth s moon and the four Galilean moons of Jupiter Titan orbits Saturn at 20 Saturn radii 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 the Moon from Earth which subtends 0 48 of arc Titan is primarily composed of ice and rocky material which is likely differentiated into a rocky core surrounded by various layers of ice including a crust of ice Ih and a subsurface layer of ammonia rich liquid water 17 Much as with Venus before the Space Age the dense opaque atmosphere prevented understanding of Titan s surface until the Cassini Huygens mission in 2004 provided new information including the discovery of liquid hydrocarbon lakes in Titan s polar regions and the discovery of its atmospheric super rotation The geologically young surface is generally smooth with few impact craters although mountains and several possible cryovolcanoes have been found The atmosphere of Titan is largely nitrogen minor components lead to the formation of methane and ethane clouds and heavy organonitrogen haze The climate including wind and rain creates surface features similar to those of Earth such as dunes rivers lakes seas probably of liquid methane and ethane and deltas and is dominated by seasonal weather patterns as on Earth With its liquids both surface and subsurface and robust nitrogen atmosphere Titan s methane cycle bears a striking similarity to Earth s water cycle albeit at the much lower temperature of about 94 K 179 C 290 F Due to these factors Titan has been described as the most Earth like celestial object in the Solar System 18 Contents 1 History 1 1 Discovery 1 2 Naming 2 Orbit and rotation 3 Bulk characteristics 4 Formation 5 Atmosphere 6 Climate 7 Surface features 7 1 Lakes 7 2 Impact craters 7 3 Cryovolcanism and mountains 7 4 Dark equatorial terrain 8 Observation and exploration 8 1 Fly by missions Pioneer and Voyager 8 2 Cassini Huygens 8 2 1 Huygens landing 8 3 Dragonfly 8 4 Proposed or conceptual missions 9 Prebiotic conditions and life 9 1 Formation of complex molecules 9 2 Possible subsurface habitats 9 3 Methane and life at the surface 9 4 Obstacles 9 5 Panspermia hypothesis 9 6 Future conditions 10 See also 11 References 12 Bibliography 13 Further reading 14 External linksHistory editDiscovery edit nbsp Christiaan Huygens discovered Titan in 1655 Titan was discovered on March 25 1655 by the Dutch astronomer Christiaan Huygens 19 20 Huygens was inspired by Galileo s discovery of Jupiter s four largest moons in 1610 and his improvements in telescope technology Christiaan with the help of his 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 21 It was the sixth moon ever discovered after Earth s Moon and the Galilean moons of Jupiter 22 Titan is the largest and brightest moon of Saturn and so is the easiest to observe of Saturn s moons with a standard optical telescope from Earth Naming edit 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 23 After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686 astronomers fell into the habit of referring to these and Titan as Saturn I through V with Titan then in fourth position Other early epithets for Titan include Saturn s ordinary satellite 24 The International Astronomical Union officially numbers Titan as Saturn VI 25 The name Titan and 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 26 27 Numerous small moons have been discovered around Saturn since then 28 Saturnian moons are named after mythological giants The name Titan comes from the Titans a race of immortals in Greek mythology 25 Orbit and rotation editMain article Moons of Saturn nbsp Titan s orbit highlighted in red among the other large inner moons of Saturn The moons outside its orbit are from the outside to the inside Iapetus and Hyperion those inside are Rhea Dione Tethys Enceladus and Mimas Titan orbits Saturn once every 15 days and 22 hours Like Earth s Moon and many of the satellites of 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 29 Its orbital eccentricity is 0 0288 and the orbital plane is inclined 0 348 degrees relative to the Saturnian equator 6 and hence also about a third of a degree off of the equatorial ring plane Viewed from Earth Titan reaches an angular distance of about 20 Saturn radii just over 1 200 000 kilometers 750 000 mi from Saturn and subtends a disk 0 8 arcseconds in diameter citation needed The small and irregularly shaped satellite Hyperion is locked in a 3 4 orbital resonance with Titan Hyperion probably formed in a stable orbital island whereas the massive Titan absorbed or ejected any other bodies that made close approaches 30 Bulk characteristics edit nbsp Size comparison Titan lower left with the Moon and Earth top and right nbsp A model of Titan s internal structure showing ice six layer Titan is 5 149 46 kilometers 3 199 73 mi in diameter 7 1 06 times that of the planet Mercury 1 48 that of the Moon and 0 40 that of Earth Titan is the tenth largest object in the solar system including the Sun Before the arrival of Voyager 1 in 1980 Titan was thought to be slightly larger than Ganymede diameter 5 262 kilometers 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 kilometres above its surface This increases its apparent diameter 31 Titan s diameter and mass and thus its density are similar to those of the Jovian moons Ganymede and Callisto 32 Based on its bulk density of 1 88 g cm3 Titan s composition is half ice and half rocky material Though similar in composition to Dione and Enceladus it is denser due to gravitational compression It has a mass 1 4226 that of Saturn making it the largest moon of the gas giants relative to the mass of its primary It is second in terms of relative diameter of moons to a gas giant Titan being 1 22 609 of Saturn s diameter Triton is larger in diameter relative to Neptune at 1 18 092 citation needed Titan is probably partially differentiated into distinct layers with a 3 400 kilometer 2 100 mi rocky center 33 This rocky center is believed to be surrounded by several layers composed of different crystalline forms of ice and or water 34 The exact structure depends heavily on the heat flux from within Titan itself which is poorly constrained The interior may still be hot enough for a liquid layer consisting of a magma composed of water and ammonia between the ice Ih 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 a surface crust 35 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 36 The Cassini probe discovered evidence for the layered structure in the form of natural extremely low frequency radio waves in Titan s atmosphere Titan s surface is thought to be a poor reflector of extremely low frequency radio waves so they may instead be reflecting off the liquid ice boundary of a subsurface ocean 37 Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 kilometers 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 38 Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn 39 Comparison of the gravity field with the RADAR based topography observations 40 also suggests that the ice shell may be substantially rigid 41 42 Formation editThe moons of Jupiter and Saturn are thought to have formed through co accretion a similar process to that 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 Whereas Jupiter possesses four large satellites in highly regular planet like orbits Titan overwhelmingly dominates Saturn s system and possesses 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 satellites but that they were disrupted by a series of giant impacts which would go on to form Titan Saturn s mid sized moons such as Iapetus and Rhea were formed from the debris of these collisions Such a violent beginning would also explain Titan s orbital eccentricity 43 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 44 Atmosphere editMain article Atmosphere of Titan nbsp True color image of layers of haze in Titan s atmosphereTitan is the only known moon with a significant atmosphere 45 and its atmosphere is the only nitrogen rich dense atmosphere in the Solar System aside from Earth s Observations of it made in 2004 by Cassini suggest that Titan is a super rotator like Venus with an atmosphere that rotates much faster than its surface 46 Observations from the Voyager space probes have shown that Titan s atmosphere is denser than Earth s with a surface pressure about 1 45 atm It is also about 1 19 times as massive as Earth s overall 47 or about 7 3 times more massive on a per surface area basis Opaque haze layers block most visible light from the Sun and other sources and obscure Titan s surface features 48 Titan s lower gravity means that its atmosphere is far more extended than Earth s 49 The atmosphere of Titan is opaque at many wavelengths and as a result a complete reflectance spectrum of the surface is impossible to acquire from orbit 50 It was not until the arrival of the Cassini Huygens spacecraft in 2004 that the first direct images of Titan s surface were obtained 51 Titan Clouds nbsp Clouds Nov 4 2022 nbsp Clouds Nov 6 2022 Titan s atmospheric composition is nitrogen 97 methane 2 7 0 1 and hydrogen 0 1 0 2 with trace amounts of other gases 15 There are trace amounts of other hydrocarbons such as ethane diacetylene methylacetylene acetylene and propane and of other gases such as cyanoacetylene hydrogen cyanide carbon dioxide carbon monoxide cyanogen argon and helium 14 The hydrocarbons are thought to form in Titan s upper atmosphere in reactions resulting from the breakup of methane by the Sun s ultraviolet light producing a thick orange smog 52 Titan spends 95 of its time within Saturn s magnetosphere which may help shield it from the solar wind 53 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 54 The ultimate origin of the methane in its atmosphere may be its interior released via eruptions from cryovolcanoes 55 56 57 58 nbsp Trace organic gases in Titan s atmosphere HNC left and HC3N right 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 59 On June 6 2013 scientists at the IAA CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan 60 61 On September 30 2013 propene was detected in the atmosphere of Titan by NASA s Cassini spacecraft using its composite infrared spectrometer CIRS 62 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 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 52 On October 24 2014 methane was found in polar clouds on Titan 63 64 On December 1 2022 astronomers reported viewing clouds likely made of methane moving across Titan using the James Webb Space Telescope 65 66 nbsp Polar clouds made of methane on Titan left compared with polar clouds on Earth right which are made of water or water ice Climate editMain article Climate of Titan nbsp Atmospheric polar vortex over Titan s south poleTitan 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 67 Titan receives about 1 as much sunlight as Earth 68 Before sunlight reaches the surface about 90 has been absorbed by the thick atmosphere leaving only 0 1 of the amount of light Earth receives 69 Atmospheric methane creates a greenhouse effect on Titan s surface without which Titan would be much colder 70 Conversely haze in Titan s atmosphere contributes to an anti greenhouse effect by absorbing sunlight cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere 71 nbsp Methane clouds animated July 2014 72 Titan s clouds probably composed of methane ethane or other simple organics are scattered and variable punctuating the overall haze 31 The findings of the Huygens probe indicate that Titan s atmosphere periodically rains liquid methane and other organic compounds onto its surface 73 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 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 74 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 the sunlight 75 When the seasons switch it is expected that ethane will begin to condense over the south pole 76 Surface features editSee also List of geological features on Titan nbsp Global geologic map of Titan 2019 16 nbsp Global map of Titan with IAU labels August 2016 nbsp Titan infrared views 2004 2017 nbsp Titan s North Pole 2014 nbsp Titan s South Pole 2014 The surface of Titan has been described as complex fluid processed and geologically young 77 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 78 Titan s atmosphere is four times as thick as Earth s 79 making it difficult for astronomical instruments to image its surface in the visible light spectrum 80 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 the surface There is also evidence that Titan s ice shell may be substantially rigid 41 42 which would suggest little geologic activity 81 There are also streaky features some of them hundreds of kilometers in length that appear to be caused by windblown particles 82 83 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 84 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 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 85 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 86 There are dark areas of similar size elsewhere on Titan observed from the ground and by Cassini at least one of these Ligeia Mare Titan s second largest sea is almost a pure methane sea 87 88 nbsp Titan mosaic from a Cassini flyby The large dark region is Shangri La nbsp Titan in false color showing surface details and atmosphere Xanadu is the bright region at the bottom center nbsp Titan composite image in infrared It features the dark dune filled regions Fensal north and Aztlan south Lakes edit Main article Lakes of Titan nbsp Titan lakes September 11 2017 nbsp False color Cassini radar mosaic of Titan s north polar region Blue coloring indicates low radar reflectivity caused by hydrocarbon seas lakes and tributary networks filled with liquid ethane methane and dissolved N2 15 About half of the large body at lower left Kraken Mare is shown Ligeia Mare is at lower right nbsp Mosaic of three Huygens images of channel system on Titan nbsp Rimmed lakes of Titan artist concept 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 was not 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 89 The Cassini mission confirmed the former hypothesis When the probe arrived in the Saturnian system in 2004 it was hoped that hydrocarbon lakes or oceans would be detected from the sunlight reflected off their surface but no specular reflections were initially observed 90 Near Titan s south pole an enigmatic dark feature named Ontario Lacus was identified 91 and later confirmed to be a lake 92 A possible shoreline was also identified near the pole via radar imagery 93 Following a flyby on July 22 2006 in which the Cassini spacecraft s radar imaged the northern latitudes that were then in winter several large smooth and thus dark to radar patches were seen dotting the surface near the pole 94 Based on the observations scientists announced definitive evidence of lakes filled with methane on Saturn s moon Titan in January 2007 95 96 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 Earth 95 Some appear to have channels associated with liquid and lie in topographical depressions 95 The liquid erosion features appear to be a very recent occurrence channels in some regions have created surprisingly little erosion suggesting erosion on Titan is extremely slow or some other recent phenomena may have wiped out older riverbeds and landforms 78 Overall the Cassini radar observations have shown that lakes cover only a small percentage of the surface making Titan much drier than Earth 97 Most of the lakes are concentrated near the poles where the relative lack of sunlight prevents evaporation but several long standing hydrocarbon lakes in the equatorial desert regions have also been discovered including one near the Huygens landing site in the Shangri La region which is about half the size of the Great Salt Lake in Utah USA The equatorial lakes are probably oases i e the likely supplier is underground aquifers 98 nbsp nbsp Evolving feature in Ligeia Mare In June 2008 the Visual and Infrared Mapping Spectrometer on Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus 99 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 100 101 nbsp Near infrared radiation from the Sun reflecting off Titan s hydrocarbon seasOn July 8 2009 Cassini s VIMS observed a specular reflection indicative of a smooth mirror like surface off what today is called Jingpo Lacus a lake in the north polar region shortly after the area emerged from 15 years of winter darkness Specular reflections are indicative of a smooth mirror like surface so the observation corroborated the inference of the presence of a large liquid body drawn from radar imaging 102 103 Early radar measurements made in July 2009 and January 2010 indicated that Ontario Lacus was extremely shallow with an average depth of 0 4 3 m and a maximum depth of 3 to 7 m 9 8 to 23 0 ft 104 In contrast the northern hemisphere s Ligeia Mare was initially mapped to depths exceeding 8 m the maximum discernable by the radar instrument and the analysis techniques of the time 104 Later science analysis released in 2014 more fully mapped the depths of Titan s three methane seas and showed depths of more than 200 meters 660 ft Ligeia Mare averages from 20 to 40 m 66 to 131 ft in depth while other parts of Ligeia did not register any radar reflection at all indicating a depth of more than 200 m 660 ft While only the second largest of Titan s methane seas Ligeia contains enough liquid methane to fill three Lake Michigans 105 In May 2013 Cassini s radar altimeter observed Titan s Vid Flumina channels defined as a drainage network connected to Titan s second largest hydrocarbon sea Ligeia Mare Analysis of the received altimeter echoes showed that the channels are located in deep up to 570 m steep sided canyons and have strong specular surface reflections that indicate they are currently filled with liquid Elevations of the liquid in these channels are at the same level as Ligeia Mare to within a vertical precision of about 0 7 m consistent with the interpretation of drowned river valleys Specular reflections are also observed in lower order tributaries elevated above the level of Ligeia Mare consistent with drainage feeding into the main channel system This is likely the first direct evidence of the presence of liquid channels on Titan and the first observation of hundred meter deep canyons on Titan Vid Flumina canyons are thus drowned by the sea but there are a few isolated observations to attest to the presence of surface liquids standing at higher elevations 106 During six flybys of Titan from 2006 to 2011 Cassini gathered radiometric tracking and optical navigation data from which investigators could roughly infer Titan s changing shape The density of Titan is consistent with a body that is about 60 rock and 40 water The team s analyses suggest that Titan s surface can rise and fall by up to 10 metres during each orbit That degree of warping suggests that Titan s interior is relatively deformable and that the most likely model of Titan is one in which an icy shell dozens of kilometres thick floats atop a global ocean 107 The team s findings together with the results of previous studies hint that Titan s ocean may lie no more than 100 kilometers 62 mi below its surface 107 108 On July 2 2014 NASA reported the ocean inside Titan may be as salty as the Dead Sea 109 110 On September 3 2014 NASA reported studies suggesting methane rainfall on Titan may interact with a layer of icy materials underground called an alkanofer to produce ethane and propane that may eventually feed into rivers and lakes 111 In 2016 Cassini found the first evidence of fluid filled channels on Titan in a series of deep steep sided canyons flowing into Ligeia Mare This network of canyons dubbed Vid Flumina ranges in depth from 240 to 570 m and has sides as steep as 40 They are believed to have formed either by crustal uplifting like Earth s Grand Canyon a lowering of sea level or perhaps a combination of the two The depth of erosion suggests that liquid flows in this part of Titan are long term features that persist for thousands of years 112 nbsp nbsp 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 nbsp nbsp 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 lakesImpact craters edit nbsp Radar image of a 139 km diameter 113 impact crater on Titan s surface showing a smooth floor rugged rim and possibly a central peak Radar SAR and imaging data from Cassini have revealed few impact craters on Titan s surface 78 These impacts appear to be relatively young compared to Titan s age 78 The few impact craters discovered include a 392 kilometer wide 244 mi two ring impact basin named Menrva seen by Cassini s ISS as a bright dark concentric pattern 114 A smaller 80 kilometer wide 50 mi flat floored crater named Sinlap 115 and a 30 km 19 mi crater with a central peak and dark floor named Ksa have also been observed 116 Radar and Cassini imaging have also revealed 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 kilometer wide 56 mi ring of bright rough material known as Guabonito has been observed by Cassini 117 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 118 nbsp Ligeia Mare SAR and clearer despeckled views 119 Many of Titan s craters or probable craters display evidence of extensive erosion and all show some indication of modification 113 Most large craters have breached or incomplete rims despite the fact that some craters on Titan have relatively more massive rims than those anywhere else in the Solar System There is little evidence of formation of palimpsests through viscoelastic crustal relaxation unlike on other large icy moons 113 Most craters lack central peaks and have smooth floors possibly due to impact generation or later eruption of cryovolcanic lava 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 120 The limited high resolution radar coverage of Titan obtained through 2007 22 suggested the existence of nonuniformities in its crater distribution Xanadu has 2 9 times more craters than elsewhere The leading hemisphere has a 30 higher density than the trailing hemisphere There are lower crater densities in areas of equatorial dunes and in the north polar region where hydrocarbon lakes and seas are most common 113 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 121 Cryovolcanism and mountains edit See also Cryovolcano nbsp Near infrared image of Tortola Facula thought to be a possible cryovolcanoScientists 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 122 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 123 Still there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes 124 One of the first of such features revealed by Cassini radar observations in 2004 called Ganesa Macula resembles the geographic features called pancake domes found on Venus and was thus initially thought to be cryovolcanic in origin until Kirk et al refuted this hypothesis at the American Geophysical Union annual meeting 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 125 126 In 2004 Cassini also detected an unusually bright feature called Tortola Facula which was interpreted as a cryovolcanic dome 127 No similar features have been identified as of 2010 128 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 36 A mountain range measuring 150 kilometers 93 mi long 30 kilometers 19 mi wide and 1 5 kilometers 0 93 mi 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 129 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 130 In 2008 Jeffrey Moore planetary geologist of Ames Research Center 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 and eolian erosion mass wasting 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 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 a temperature of 176 K 97 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 124 131 In March 2009 structures resembling lava flows were announced in a region of Titan called Hotei Arcus 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 660 ft above Titan s surface consistent with it having erupted from beneath the surface 132 In December 2010 the Cassini mission team announced the most compelling possible cryovolcano yet found Named Sotra Patera 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 133 Crater like landforms possibly formed via explosive maar like or caldera forming cryovolcanic eruptions have been identified in Titan s polar regions 134 These formations are sometimes nested or overlapping and have features suggestive of explosions and collapses such as elevated rims halos and internal hills or mountains 134 The polar location of these features and their colocalization with Titan s lakes and seas suggests volatiles such as methane may help power them Some of these features appear quite fresh suggesting that such volcanic activity continues to the present 134 Most of Titan s highest peaks occur near its equator in so called ridge belts They are believed to be analogous to Earth s fold mountains such as the Rockies or the Himalayas formed by the collision and buckling of tectonic plates or to subduction zones like the Andes where upwelling lava or cryolava from a melting descending plate rises to the surface One possible mechanism for their formation is tidal forces from Saturn Because Titan s icy mantle is less viscous than Earth s magma mantle and because its icy bedrock is softer than Earth s granite bedrock mountains are unlikely to reach heights as great as those on Earth In 2016 the Cassini team announced what they believe to be the tallest mountain on Titan Located in the Mithrim Montes range it is 3 337 m tall 135 nbsp False color VIMS image of the possible cryovolcano Sotra Patera combined with a 3D map based on radar data showing 1000 meter high peaks and a 1500 meter deep crater If volcanism on Titan really exists the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle as it is on Earth 36 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 36 The low pressure ice overlaying a liquid layer of ammonium sulfate 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 landscape and sand dune features 136 Titan may have been much more geologically active in the past models of Titan s internal evolution suggest that Titan s crust was only 10 kilometers thick until about 500 million years ago allowing vigorous cryovolcanism with low viscosity water magmas to erase all surface features formed before that time Titan s modern geology would have formed only after the crust thickened to 50 kilometers and thus impeded constant cryovolcanic resurfacing with any cryovolcanism occurring since that time producing much more viscous water magma with larger fractions of ammonia and methanol this would also suggest that Titan s methane is no longer being actively added to its atmosphere and could be depleted entirely within a few tens of millions of years 137 Many of the more prominent mountains and hills have been given official names by the International Astronomical Union According to JPL By convention mountains on Titan are named for mountains from Middle earth the fictional setting in fantasy novels by J R R Tolkien Colles collections of hills are named for characters from the same Tolkien works 138 Dark equatorial terrain edit nbsp Sand dunes in the Namib Desert on Earth top compared with dunes in Belet on TitanIn 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 139 Prior to the arrival of Cassini these regions were thought to be seas of liquid hydrocarbons 140 Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal dunes up to 330 ft 100 m high 141 about a kilometer wide and tens to hundreds of kilometers long 142 Dunes of this type are always aligned with average wind direction In the case of Titan steady zonal eastward winds combine with variable tidal winds approximately 0 5 meters per second 143 The tidal winds are the result of tidal forces 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 it was hypothesized causes granular material on the surface to gradually build up in long parallel dunes aligned west to east The dunes break up around mountains where the wind direction shifts 144 The longitudinal or linear dunes were initially presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions Subsequent observations indicate that the dunes point to the east although climate simulations indicate Titan s surface winds blow toward the west At less than 1 meter per second they are not powerful enough to lift and transport surface material Recent computer simulations indicate that the dunes may be the result of rare storm winds that happen only every fifteen years when Titan is in equinox These storms produce strong downdrafts flowing eastward at up to 10 meters per second when they reach the surface 145 The sand on Titan is likely not made up of small grains of silicates like the sand on Earth 146 but rather might have formed when liquid methane rained and eroded the water ice bedrock possibly in the form of flash floods Alternatively the sand could also have come from organic solids called tholins produced by photochemical reactions in Titan s atmosphere 141 143 147 Studies of dunes composition in May 2008 revealed that they possessed less water than the rest of Titan and are thus most likely derived from organic soot like hydrocarbon polymers clumping together after raining onto the surface 148 Calculations indicate the sand on Titan has a density of one third that of terrestrial sand 149 The low density combined with the dryness of Titan s atmosphere might cause the grains to clump together because of static electricity buildup The stickiness might make it difficult for the generally mild breeze close to Titan s surface to move the dunes although more powerful winds from seasonal storms could still blow them eastward 150 Around equinox strong downburst winds can lift micron sized solid organic particles up from the dunes to create Titanian dust storms observed as intense and short lived brightenings in the infrared 151 nbsp Titan three dust storms detected in 2009 2010 152 Observation and exploration edit nbsp Voyager 1 view of haze on Titan s limb 1980 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 153 Titan has a maximum apparent magnitude of 8 2 13 and mean opposition magnitude 8 4 154 This compares to 4 6 for the similarly sized Ganymede in the Jovian system 154 Observations of Titan prior to the space age were limited In 1907 Spanish astronomer Josep Comas i Sola 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 155 Fly by missions Pioneer and Voyager edit The first probe to visit the Saturnian system was Pioneer 11 in 1979 which revealed that Titan was probably too cold to support life 156 It took images of Titan including Titan and Saturn together in mid to late 1979 157 The quality was soon surpassed by the two Voyagers 158 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 the atmosphere and obtain a precise measurement of Titan s mass 159 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 160 which had been observed in the infrared by the Hubble Space Telescope Voyager 2 which would have been diverted to perform the Titan flyby if Voyager 1 had been unable to did not pass near Titan and continued on to Uranus and Neptune 159 94 Cassini Huygens edit Main articles Cassini Huygens and Huygens spacecraft nbsp Cassini image of Titan in front of the rings of Saturn nbsp Cassini image of Titan behind Epimetheus and the rings nbsp Cassini s Titan flyby radio signal studies artist s concept Even with the data provided by the Voyagers Titan remained a body of mystery a large satellite shrouded in an atmosphere that makes detailed observation difficult 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 the highest resolution images ever of Titan s surface at only 1 200 kilometers 750 mi discerning patches of light and dark that would be invisible to the human eye citation needed On July 22 2006 Cassini made its first targeted close fly by at 950 kilometers 590 mi from Titan the closest flyby was at 880 kilometers 550 mi on June 21 2010 161 Liquid has been found in abundance on the surface in the north polar region in the form of many lakes and seas discovered by Cassini 94 Huygens landing edit nbsp Huygens in situ image from Titan s surface the only image from the surface of a body permanently farther away than Mars nbsp Same image with contrast enhanced Huygens was an atmospheric probe that touched down on Titan on January 14 2005 162 discovering that many of its surface features seem to have been formed by fluids at some point in the past 163 Titan is the most distant body from Earth to have a space probe land on its surface 164 source source source source source source The Huygens probe descends by parachute and lands on Titan on January 14 2005The Huygens probe landed just off 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 the hills with the methane rain and are deposited on the plains over geological time scales 165 After landing Huygens photographed a dark plain covered in small rocks and pebbles which are composed of water ice 165 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 ground surface is darker than originally expected consisting of a mixture of water and hydrocarbon ice 166 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 the ESA 167 Dragonfly edit Main article Dragonfly spacecraft The Dragonfly mission developed and operated by the Johns Hopkins Applied Physics Laboratory will launch in July 2028 168 It consists of a large drone powered by an RTG to fly in the atmosphere of Titan as New Frontiers 4 169 170 Its instruments will study how far prebiotic chemistry may have progressed 171 The mission is planned to arrive at Titan in the mid 2030s 170 Proposed or conceptual missions edit nbsp The balloon proposed for the Titan Saturn System Mission artistic rendition There have been several conceptual missions proposed in recent years for returning 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 citation needed The Titan Saturn System Mission TSSM was a joint NASA ESA proposal for exploration of Saturn s moons 172 It envisions a hot air balloon floating in Titan s atmosphere for six months It was competing against 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 173 The proposed Titan Mare Explorer TiME was a low cost lander that would splash down in a lake in Titan s northern hemisphere and float on the surface of the lake for three to six months 174 175 176 It was selected for a Phase A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity 177 but was not selected for flight 178 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 the surface of Titan NASA did not approve the requested 715 million and the future of the project is uncertain 179 180 A conceptual design for another lake lander was proposed in late 2012 by the Spanish based private engineering firm SENER and the Centro de Astrobiologia in Madrid The concept probe is called Titan Lake In situ Sampling Propelled Explorer TALISE 181 182 The major difference compared to 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 181 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 183 184 185 In 2015 the NASA Innovative Advanced Concepts program NIAC awarded a Phase II grant 186 to a design study of a Titan Submarine to explore the seas of Titan 187 188 189 190 191 Prebiotic conditions and life editMain article Life on Titan See also Planetary habitability Titan is thought to be a prebiotic environment rich in complex organic compounds 59 192 but its surface is in a deep freeze at 179 C 290 2 F 94 1 K so it is currently understood that life cannot exist on the moon s frigid surface 193 However Titan seems to contain a global ocean beneath its ice shell and within this ocean conditions are potentially suitable for microbial life 194 195 196 The Cassini Huygens mission was not equipped to provide evidence for biosignatures or complex organic compounds it showed an environment on Titan that is similar in some ways to ones hypothesized for the primordial Earth 197 Scientists surmise 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 198 192 Formation of complex molecules edit The Miller Urey experiment 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 of nitrogen and methane forming hydrogen cyanide and acetylene Further reactions have been studied extensively 199 It has been reported that when energy was applied to a combination of gases like those in Titan s atmosphere five nucleotide bases the building blocks of DNA and RNA were among the many compounds produced In addition amino acids the building blocks of protein were found It was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present 200 On April 3 2013 NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan 59 On June 6 2013 scientists at the IAA CSIC reported the detection of polycyclic aromatic hydrocarbons PAH in the upper atmosphere of Titan 60 61 On July 26 2017 Cassini scientists positively identified the presence of carbon chain anions in Titan s upper atmosphere which appeared to be involved in the production of large complex organics 201 These highly reactive molecules were previously known to contribute to building complex organics in the Interstellar Medium therefore highlighting a possibly universal stepping stone to producing complex organic material 202 On July 28 2017 scientists reported that acrylonitrile or vinyl cyanide C2H3CN possibly essential for life by being related to cell membrane and vesicle structure formation had been found on Titan 203 204 205 In October 2018 researchers reported low temperature chemical pathways from simple organic compounds to complex polycyclic aromatic hydrocarbon PAH chemicals Such chemical pathways may help explain the presence of PAHs in the low temperature atmosphere of Titan and may be significant pathways in terms of the PAH world hypothesis in producing precursors to biochemicals related to life as we know it 206 207 Possible subsurface habitats edit 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 The analogy assumes the presence of liquid water for longer periods than is currently observable several hypotheses postulate that liquid water from an impact could be preserved under a frozen isolation layer 208 It has also been hypothesized that liquid ammonia oceans could exist deep below the surface 194 209 Another model suggests an ammonia water solution as much as 200 kilometers 120 mi deep beneath a water ice crust with conditions that although extreme by terrestrial standards are such that life could survive 195 Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life 194 Detection of microbial life on Titan would depend on its biogenic effects with the atmospheric methane and nitrogen examined 195 Methane and life at the surface edit See also Hypothetical types of biochemistry It has been speculated that life could exist in the lakes of liquid methane on Titan just as organisms on Earth live in water 210 Such organisms would inhale H2 in place of O2 metabolize it with acetylene instead of glucose and exhale methane instead of carbon dioxide 196 210 However such hypothetical organisms would be required to metabolize at a deep freeze temperature of 179 2 C 290 6 F 94 0 K 193 All life forms on Earth including methanogens use liquid water as a solvent it is speculated that life on Titan might instead use a liquid hydrocarbon such as methane or ethane 211 although water is a stronger solvent than methane 212 Water is also more chemically reactive and can break down large organic molecules through hydrolysis 211 A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way 211 In 2005 astrobiologist Chris McKay argued that if methanogenic life did exist on the surface of Titan it would likely have a measurable effect on the mixing ratio in the Titan troposphere levels of hydrogen and acetylene would be measurably lower than otherwise expected Assuming metabolic rates similar to those of methanogenic organisms on Earth the concentration of molecular hydrogen would drop by a factor of 1000 on the Titanian surface solely due to a hypothetical biological sink McKay noted that if life is indeed present the low temperatures on Titan would result in very slow metabolic processes which could conceivably be hastened by the use of catalysts similar to enzymes He also noted that the low solubility of organic compounds in methane presents a more significant challenge to any possible form of life Forms of active transport and organisms with large surface to volume ratios could theoretically lessen the disadvantages posed by this fact 210 In 2010 Darrell Strobel from Johns Hopkins University identified a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers arguing for a downward flow at a rate of roughly 1028 molecules per second and disappearance of hydrogen near Titan s surface as Strobel noted his findings were in line with the effects McKay had predicted if methanogenic life forms were present 210 212 213 The same year another study showed low levels of acetylene on Titan s surface which were interpreted by McKay as consistent with the hypothesis of organisms consuming hydrocarbons 212 Although restating the biological hypothesis he cautioned that other explanations for the hydrogen and acetylene findings are more likely the possibilities of yet unidentified physical or chemical processes e g a surface catalyst accepting hydrocarbons or hydrogen or flaws in the current models of material flow 196 Composition data and transport models need to be substantiated etc Even so despite saying that a non biological catalytic explanation would be less startling than a biological one McKay noted that the discovery of a catalyst effective at 95 K 180 C would still be significant 196 With regards to the acetylene findings Mark Allen the principal investigator with the NASA Astrobiology Institute Titan team provided a speculative non biological explanation sunlight or cosmic rays could transform the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature 214 As NASA notes in its news article on the June 2010 findings To date methane based life forms are only hypothetical Scientists have not yet detected this form of life anywhere 212 As the NASA statement also says some scientists believe these chemical signatures bolster the argument for a primitive exotic form of life or precursor to life on Titan s surface 212 In February 2015 a hypothetical cell membrane capable of functioning in liquid methane at cryogenic temperatures deep freeze conditions was modeled Composed of small molecules containing carbon hydrogen and nitrogen it would have the same stability and flexibility as cell membranes on Earth which are composed of phospholipids compounds of carbon hydrogen oxygen and phosphorus This hypothetical cell membrane was termed an azotosome a combination of azote French for nitrogen and liposome 215 216 Obstacles edit 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 Titan is frigid and its atmosphere lacks CO2 At Titan s surface water exists only in solid form Because of these difficulties scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for life than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth 217 Although life itself may not exist the prebiotic conditions on Titan and the associated organic chemistry remain of great interest in understanding the early history of the terrestrial biosphere 197 Using Titan as a prebiotic experiment involves not only observation through spacecraft but laboratory experiments and chemical and photochemical modeling on Earth 199 Panspermia hypothesis edit Main article Panspermia It is hypothesized that large asteroid and cometary impacts on Earth s surface may have caused fragments of microbe laden rock to escape Earth s gravity suggesting the possibility of panspermia Calculations indicate that these would encounter many of the bodies in the Solar System including Titan 218 219 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 220 Future conditions edit Conditions on Titan could become far more habitable in the far future Five billion years from now as the Sun becomes a sub red giant its surface temperature could rise enough for Titan to support liquid water on its surface making it habitable 221 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 a habitable environment and could persist for several hundred million years This is proposed to have been sufficient time for simple life to spawn on Earth though the higher viscosity of ammonia water solutions coupled with low temperatures would cause chemical reactions to proceed more slowly on Titan 222 See also edit nbsp Solar System portal nbsp Outer space portal nbsp Astronomy portalColonization of Titan Lakes of Titan Atmosphere of Titan Life on Titan List of natural satellites Saturn s moons in fiction The sky of Titan Titan in fiction Titan Winged Aerobot Vid Flumina a river of methane and ethane on TitanReferences edit Titan Oxford English Dictionary Online ed Oxford University Press 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microphones AstronomyCast Titan Archived October 12 2011 at the Wayback Machine Fraser Cain and Pamela Gay 2010 Titan nomenclature and Titan map with feature names from the USGS planetary nomenclature page Google Titan 3D interactive map of the moon Image album by Kevin M Gill Portals nbsp Stars nbsp Spaceflight nbsp Outer space Retrieved from https en wikipedia org w index php title Titan moon amp oldid 1195463471, wikipedia, wiki, book, books, library,

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