Titan (moon)

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Titanium in visible light, captured by the Cassini spacecraft from a distance of 174,000 km, 2009
Titan in visible light, captured from a distance of 174,000 kilometers
( Cassini space probe , 2009)
Central body Saturn
Properties of the orbit
Major semi-axis 1,221,830 km
Periapsis 1,186,150 km
Apoapsis 1,257,510 km
eccentricity 0.0292
Orbit inclination 0.33 °
Orbital time 15,945 d
Mean orbital velocity 5.57 km / s
Physical Properties
Albedo 0.22
Apparent brightness 8.4 mag
Medium diameter 5150 km
Dimensions 1.345 x 10 23 kg
surface 8.3 · 10 7 km 2
Medium density 1.88 g / cm 3
Sidereal rotation 15,945 days
Axis inclination 1.942 °
Acceleration of gravity on the surface 1.35 m / s 2
Escape speed 2600 m / s
Surface temperature 94 K

Christiaan Huygens

Date of discovery March 25, 1655
Remarks Titan has a pronounced gas envelope:
Size comparison between Titan, Moon and Earth
Size comparison between Titan (bottom left), Earth's moon (top left) and Earth

Titan (also Saturn VI ) is the largest moon on the planet Saturn with a diameter of 5150 kilometers , which is why it was named after the gods of the titans . It is an ice moon , the second largest moon in the solar system after Ganymede and the only one with a dense gas envelope .

Titan was discovered by the Dutch astronomer Christiaan Huygens in 1655 . Observations from Earth and the Hubble Space Telescope expanded our knowledge of him, but especially the flyby of some space probes since 1979. The most informative images and measurement data were recorded when the Huygens probe landed in 2005.

Although the surface temperature of Titan is far lower than that of Earth, it is considered to be the most Earth-like celestial body in the solar system in terms of the dense, nitrogen-rich atmosphere and the occurrence of liquid . Its gas envelope is about five times as dense on the surface and the pressure about 50% higher than on earth. It consists mainly of nitrogen and contains hydrocarbons and traces of other organic compounds . The surface and the top layer of the mantle are made of ice and methane hydrate .

For life on the surface, which is based on water, Saturn orbits with its moons in an area that is much too cold around the sun (outside the habitable zone ). However, precursors to life are not excluded. Below the surface there may be an ocean of liquid water, although temperatures there are below 0 ° C.

Physical data

Titan accounts for over 95% of the total mass of all of Saturn's moons . This enormous concentration of mass among Saturn's satellites in a single body has raised questions about its formation.

It is still unclear whether Titan originated as a native moon in a material accumulation of the solar nebula that formed Saturn or whether it formed in another place and was later captured by Saturn's gravity and thus got into its orbit, as did Neptune's moon Triton was the case. The latter hypothesis could explain the unequal mass distribution of Saturn's moons.

At 1.88 g / cm³, titanium also has the highest density of all of Saturn's larger moons, although its composition is similar to these.


Titan's orbit (red) compared to the orbits of six other large Saturn moons: from the outside in, the moons outside of its orbit are Iapetus and Hyperion ; Rhea , Dione , Tethys and
Enceladus follow inside

Titan orbits Saturn at an average distance of 1,221,850 kilometers (20.3 Saturn radii) and thus outside the Saturn rings , which in the visible part (E-ring) end at around 480,000 kilometers, but reach a radius of around 960,000 kilometers. The orbits of its two nearest neighboring moons are 242,000 kilometers further out ( Hyperion ) and 695,000 kilometers further in ( Rhea ). There is a close-to- resonance orbital ratio to Hyperion, which orbits the planet almost three times (2.998) during four titanium orbits.

One circuit takes 15 days, 22 hours and 41 minutes at an average orbit speed of 5.57 km / s. The orbit does not form an exact circle, but has a numerical eccentricity of 0.029, which is a relatively high value for a large moon. The orbital plane of Titan deviates by 0.33 ° from the equatorial plane of Saturn and its rings.

Saturn's axis of rotation is inclined by 26.73 ° compared to the ecliptic (for comparison: Earth's axis 23.4 °). This creates the four seasons on the planet and all its satellites in their respective northern and southern hemispheres, but there for 7½ earth years each, as a Saturn year (orbit around the sun) lasts almost 30 years. The first summer of the third millennium came to an end in the southern hemisphere of Titan in August 2009.

The barycenter of Saturn and Titan is only 290 kilometers away from the Saturn center due to the 4227 times the mass of the planet.

Ring of hydrogen around orbit

The orbit of Titan lies within an approx. 1 million km wide ring of uncharged hydrogen; oxygen also occurs in this ring. The hydrogen most likely comes from the titanium atmosphere, which would require 1 kg / s of hydrogen.


Titanium has a rotation linked to the orbit . This means that the same side is always facing the planet, as is the case with the Earth's moon and all other large satellites. It rotates around its axis in the same time and with the same direction of rotation of its Saturn orbit - from west to east - in 15 days, 22 hours and 41 minutes. At the equator this corresponds to a speed of almost 12 m / s. Its axis of rotation deviates from the plane of the orbit by 1.942 °.

Size, mass and albedo

With an average diameter of 5150 kilometers, Titan is the second largest moon in the solar system and lies between the Jupiter moons Ganymede and Callisto in terms of mass, density and diameter .

Image of Saturn's moon Titan from NASA 's
Cassini probe

For a long time, it appeared from Earth that Titan was about 5550 kilometers in diameter, larger than Ganymede. However, the exploration by Voyager 1 in 1980 revealed the pronounced and opaque gas envelope, which is why the diameter of the solid body had previously been overestimated. Titan is about 50% larger and 80% more massive than the Earth's moon . It is also larger than the smallest planet Mercury , but far less massive than this. The surface of titanium is as large as that of the continents of Europe, Asia and Africa combined.

There is a gravitational acceleration of 1.35 m / s² on its surface . This is almost a seventh of the gravitational acceleration on the earth's surface ( 9.81 m / s² ) and slightly less than that on the surface of the earth's moon (1.62 m / s²).

Titanium is big enough not to have lost its heat of origin and has internal heat sources in the form of radioactive nuclides in minerals (e.g. potassium-40 , uranium ), so that its temperature increases with increasing depth ( temperature gradient ). It is very likely that the interior of Titan is geologically active.

The vapor layer of the gas envelope gives it a low geometric albedo of 0.22; that is, only 22% of the incident sunlight is reflected. The spherical albedo is 0.21.


About half of the solid body of titanium is composed of a mantle of water ice and the other half of a core of silicate rock. It should therefore have a similar structure to the moons of Jupiter Ganymede and Callisto and the Neptune moon Triton and possibly also the dwarf planet Pluto . However, it differs from them by its gas envelope.

the atmosphere

Comparison of the atmospheres of Earth (left) and Titan (right)
Layer structure of the atmosphere

In the solar system, Titan is the only celestial body among the planets and moons of its size class with a dense and cloud-rich atmosphere .

The Spanish astronomer Josep Comas i Solà , following his telescopic observations in 1908, was the first to suggest that the moon is surrounded by a gas envelope, since the tiny titanium disc is darker at the edge than at the center. However, evidence of this was only provided in 1944 by the American astronomer Gerard Kuiper by means of spectroscopic studies. The partial pressure of the hydrocarbon methane was determined to be 100  mbar .

Investigations using the Voyager probes have shown that the atmospheric pressure on Titan’s surface is around 1.5 bar, which is around 50% higher than that on the earth's surface . Taking into account the low force of gravity and surface temperature, this means that there is ten times as much gas above every square meter of titanium surface as there is on earth and its density near the ground is five times as great. The total mass of the gas envelope is about 1.19 times that of the - much larger - earth.

Interaction with the solar wind

Titan is protected from the solar wind by Saturn's magnetosphere. However, when the solar wind is particularly strong, Saturn's magnetosphere on the sun-facing side of the planet is compressed under the orbit of Titan, so that Titan is exposed to the solar wind when it passes this point. Since titanium does not have its own magnetic field, the solar wind, which penetrates the titanium atmosphere unhindered, can blow away small amounts of the titanium atmosphere, among other things.

Origin and supply of matter

The nitrogen atmosphere was created from ammonia (NH 3 ), which was gassed from the moon and split into nitrogen and hydrogen atoms by the high- energy UV components of solar radiation below 260 nm (corresponding to the binding energy of 460 kJ / mol), which immediately form nitrogen molecules (N 2 ) and hydrogen molecules (H 2 ) connected. The heavy nitrogen sank under the lighter ammonia, the extremely light hydrogen escaped into space; it cannot accumulate on Titan because of its low pull.

According to a new theory, the atmosphere was created when the impacts of the great bombardment shattered the surface of ammonia ice and released nitrogen from it. This is indicated by the low internal differentiation of titanium and the isotopic composition of argon in the atmosphere.

The Huygens probe also measured the proportions of the isotopes of N and C. The isotope ratio of 14 N to 15 N suggests that originally five times the amount of nitrogen was present and the slightly lighter 14 N mainly diffused into space . The ratio of 12 C to 13 C suggests that methane is continuously being formed in the gas envelope.

The titanium atmosphere consists of many superimposed layers of vapor that extend hundreds of kilometers high: UV image from the night side (Cassini, 2004). In order to achieve almost natural colors, the picture was colored.

Layer structure

Titan's atmosphere extends about ten times as far into space as that of Earth. The limit of the troposphere is at an altitude of about 44 kilometers. The temperature minimum of the atmosphere of −200 ° C was also determined here. Then the temperature rises again and is at a height of 500 kilometers at −121 ° C. The ionosphere of Titan is more complex than that of the earth. The main zone is at an altitude of 1200 kilometers, but with a further zone of charged particles at 63 kilometers. This divides the titanium atmosphere to a certain extent into two chambers that reflect radio waves .

Chemistry of the atmosphere

The only bodies in the solar system whose atmosphere consists mainly of nitrogen are Earth and Titan. In the latter case, it is 98.4% nitrogen and about 1.6% argon, as well as methane, which is predominant in the upper atmosphere due to its low density (57% of nitrogen). There are also traces of at least a dozen other organic compounds , including ethane , propane , ethyne and hydrogen cyanide . Helium , carbon dioxide and water were also found, but practically no free oxygen .

Since titanium has no magnetic field worth mentioning , its atmosphere is directly exposed to the solar wind , especially at its outer edge . In addition, it is subject to the effects of cosmic radiation and solar radiation, of which the UV component mentioned above is of chemical importance. Nitrogen and methane molecules hit by such high-energy particles of matter or photons are split into ions or very reactive radicals . These fragments form new bonds with other molecules, forming complex organic nitrogen compounds, the carbon compounds mentioned above, and various polycyclic aromatic hydrocarbons . In this way, polyynes with triple bonds are also formed in the upper titanium atmosphere . The polycyclic aromatic hydrocarbons can also contain nitrogen and clump together to form aerosols.


Formation of tholins in Titan's upper atmosphere

The heavier molecules slowly sink into deeper layers of the atmosphere and form the orange nebula that envelops Saturn's moon. The astrophysicist Carl Sagan coined the term “ tholin ” for this mixture of compounds with an as yet unknown composition . He also suspected a layer of such molecules on the surface of titanium, in which chemical reactions could take place when energized that are similar to those in primeval times of the earth and have contributed to the development of earthly life. These assumptions made Titan one of the most interesting locations in the solar system.

During its descent to the titanium surface, instruments of the Huygens probe examined the atmosphere. With the ion neutral mass spectrometer (INMS) it could be shown that the orange-colored fog contains small and medium-sized molecules. More informative were the data from the Cassini plasma spectrometer (CAPS), which was carried along especially for investigating the orange color of the atmosphere and which for the first time provided an explanation for the formation of tholins. It detected large, positively and negatively charged ions. In particular, the negatively charged ions probably play an unexpected role in the formation of tholins from carbon and nitrogen-containing compounds.

Glow in the atmosphere

Cassini was able to detect a faint glow in the atmosphere on a 560 second exposure of Titan in Saturn's shadow. The glow in the upper part of the atmosphere, 1000 km above the surface, probably originates from the collision of the atmospheric molecules with the solar wind or with particles from Saturn's magnetosphere. However, at a height of only 300 km above the ground, the atmosphere is more luminous. This glow cannot arise in the same way as in higher layers, but can only be triggered by cosmic radiation or chemical reactions of the components of the atmosphere.


Temperature and pressure curve in Titan’s lower atmosphere with representation of the layers as a function of the altitude

At the landing site of Lander Huygens the temperature measured on site was 93.7  K (−179.45 ° C), the air pressure was 1.47 bar. From the data from Voyager 1 (November 1980) it could be deduced that the temperatures on the surface change by only a few degrees Kelvin, daily and over a Titan year (29.5 Earth years). There are several fairly similar estimates, with the maximum temperature being 94 K and the minimum being ∼90 K. The temperatures that Cassini measured from 2004 to 2014 were initially 91.7 ± 0.3 K at the South Pole and finally decreased to 89.7 ± 0.5 K. At the North Pole, on the other hand, temperatures rose from 90.7 during the same period ± 0.5 K to 91.5 ± 0.2 K. At these temperature and pressure conditions, water ice does not sublime , so that only very small traces of water are present in the atmosphere.


In the uppermost atmosphere, the methane components lead to a greenhouse effect , so that it would be considerably colder without this gas.

The orange-colored fog also has climatic effects on the parts of the atmosphere below it, but these are interpreted differently ( paradox of the weak young sun ). In relation to the solid surface, in contrast to the earth, there is talk of an anti- greenhouse effect .

Wind systems and super rotation

In the upper troposphere , Titan's atmosphere rotates from east to west faster than the moon itself. This phenomenon is called “ super rotation ”; it can also be observed on Venus , for example . In the upper part of the troposphere, which reaches up to 50 kilometers high, there is strong turbulence. The wind speed there is around 30 m / s and decreases steadily downwards. The speed of the gas masses is low below 7 kilometers.

The Huygens lander measured air currents while crossing the atmosphere. A simulation based on these wind data was able to show that Titan's atmosphere in the stratosphere circulates in a single giant Hadley cell . Warm air rises in the southern hemisphere and sinks again in the northern hemisphere. As a result, the air flows from south to north at high altitudes in the stratosphere and back again from north to south at lower altitudes. In contrast to Earth, whose Hadley cell and the intertropical convergence zone are limited to the area between the tropics due to the oceans, the area of influence of Titan extends from pole to pole. About every 15 years (including a three-year transition period), i.e. twice every Saturn year, this cell reverses its wind direction. Such a Hadley cell is only possible on a slowly rotating body like titanium.

There are large, short-lived dust storms on Titan, which showed up as bright spots on infrared images from the Visual and Infrared Mapping Spectrometer (VIMS) of NASA's Cassini spacecraft. They only last a few hours to days. The source of the dust is the hydrocarbon sand of the titanium dunes. All three dust storms observed so far lay over one of the large dune fields of Titan and had an area of ​​180,000 to 420,000 km².

Huge swirl of clouds over the North Pole (Cassini 2006)

Patterns of clouds can be seen in the atmosphere, which are mainly composed of methane, but also ethane and other hydrocarbons, and which rain down on the surface.

At the end of 2006 a huge cloud vortex was discovered by means of Cassini, which covers a large part of the North Pole region with a diameter of around 2,400 kilometers. A cloud was expected, but not a structure of this size and structure. When the probe approached titanium again two weeks later, the vortex could be observed again. Presumably it had existed for several years in 2006 and will only dissolve a decade or two later and emerge again at the South Pole. According to the researchers' models, its formation is subject to a cycle that corresponds to a Saturn year, as is the case with the Hadley cell. These are unexpectedly polar stratospheric clouds of methane ice at a height of 30 to 50 km. It was previously assumed that there was nothing analogous to the glowing night clouds of the earth on Titan.

Toxic cloud vortex over the South Pole (Cassini 2012)

In mid-2012, Cassini observed a new cloud vortex of aerosols over Titan's South Pole, approx. 300 km above the surface. It is believed that its formation is related to the beginning of summer in Titan's southern hemisphere. At the end of September 2014, analyzes of Cassini's data showed that the atmosphere over the South Pole had cooled much more than expected. The cloud vortex is about 300 km above the surface of the moon, at an altitude that was previously thought to be too warm for clouds to form. In the cloud vortex observed in 2012, toxic frozen hydrogen cyanide particles (hydrocyanic acid) could be detected, which can only form at temperatures below –148 ° C, which corresponds to a deviation of around 100 ° C compared to the current theoretical model of the upper atmosphere. Since the change of seasons in 2009, atmospheric circulations have been pushing large amounts of gases southwards, and the rise in hydrogen cyanide concentration in the atmosphere has led to a significant cooling.

Lightning bolts

It is still unclear whether there are lightning bolts in Titan's atmosphere, the existence of which could catalyze the formation of organic molecules. Cassini's measurements have not measured any significant signal from lightning. However, this does not preclude the existence of lightning; alone, if lightning flashes did occur in Titan's atmosphere, they were too weak to measure a noticeable signal of lightning. However, the latest computer simulations have shown that under certain conditions streamer discharges can form, which form the precursor to lightning discharges.

Solid surface

Due to the haze-rich atmosphere, previous observations in visible light and the Voyager missions did not reveal any details on Titan’s surface.

Surface map of the two poles, created from data from the Cassini Imaging Science Subsystem (January 2009)

The global and regional surface features visible on the first radar images are, according to initial evaluations by NASA scientists, mistaken for tectonic structures such as trenches and crustal faults, which would indicate continued significant geological activity of the celestial body.

The surface is generally very flat. Height differences of more than 150 m and mountain-like features rarely occur. A bright region 4500 kilometers along the equator, called Xanadu, is particularly striking. NASA and ESA researchers interpret it, after closer observation, as a landscape with a surprisingly earth-like overall appearance. Xanadu, which is roughly the size of Australia in terms of area, is criss-crossed by mountain ridges up to 2000 m high (as of August 2010). According to radar data, these consist of porous water ice that was formed by leaching from suspected methane rain. This could have created cave systems . This corresponds completely to the landscapes of the earth shaped by wind and water. The highest peak on Titan is located in the Mithrim Montes and rises in the southernmost of the three approximately parallel mountain ridges 3337 meters above zero level . The Mithrim Montes are located in the north of Xanadu, near the equator.

In contrast to the plate tectonics on Earth, the mountains of Titans were most likely formed by the shrinking of the moon and the associated folding and shortening of the crust. A research team led by Giuseppe Mitri at Caltech came to the conclusion through computer simulations that titanium has been shrinking continuously since its formation 4.5 billion years ago. They assumed that the core of titanium was never very hot and therefore represents a relatively homogeneous mixture of ice and rock. The slow cooling of the moon ensures that parts of the ocean below the ice crust gradually freeze and the thickness of the outer ice crust grows as does the mantle of a high pressure ice below the ocean. This causes the ocean to shrink in volume and folds the surface of the ice crust. According to the simulation by Mitri's team, the moon's radius should have decreased by seven kilometers within 4.5 billion years; With today's radius of 2575 kilometers, that is around 0.3 percent.

The composition of the surface is complex. Cassini-Huygens found water ice there, which at low temperatures has the consistency of silicate rock and is partially covered by pools or lakes of liquid methane . It is believed that there are hydrocarbons on the surface that could not be produced in the laboratory before.

Cassini's radar found far fewer impact craters on Titan than (in an equal area) on other moons and planets in the solar system. Many objects burst and burn up in the dense atmosphere, so that the number of impacting objects is roughly halved from the outset. Also, fresh impact craters are very soon blown by the constant wind with particles that have been created by the agglomeration of tholines that have fallen out of the atmosphere. This effect and the methane rain form the surface comparatively quickly and lead to a geologically young surface. Menrva, by far the largest of the eleven named craters, has a diameter of 392 km and is located in the northern part of the leading hemisphere, north of Xanadu.

sand dunes

Dunes on Titan surface (below) compared to sand dunes on Earth (above)
Artist's impression of the titanium surface with sand dunes and methane lakes
Possible lakes in a 140 km area at 80 ° N and 35 ° W in false colors (radar image by Cassini, 2006)

More recent Cassini data show that in the dark equatorial areas, where initially hydrocarbon oceans were suspected, there are large desert areas with 150 m high and hundreds of kilometers long dunes , for which the steady wind on Titan is responsible. According to computer-aided simulations, a wind speed of two kilometers per hour is sufficient. However, recent images of the Cassini probe, which recently passed through Titan again, raised some questions. The shadows show that large, up to 300 m high dunes were mostly created by westerly winds, whereas the prevailing wind at Titan's equator is the east wind. One possible reason would be that these dunes only arise in the rare phases of an equinox when a possible, sufficiently strong west wind blows.

The dunes consist of particles up to 0.3 mm in size, the composition of which has not yet been clarified. Water ice or organic solids are suitable. According to a hypothesis by Donald Hunten at the University of Arizona, they could consist of ethane, which is bound to the finest dust particles. That would also explain why no hydrocarbon oceans have formed here.

Liquid hydrocarbons

In the two polar regions, the radar images show larger methane lakes that are fed by rivers. Numerous radar-dark spots, which are regarded as clear evidence of such "waters", were found around the North Pole. It was a polar night in this region at the time of the Cassini Mission .

Titans South Pole. At the bottom right there are clouds, at the center left the Ontario Lacus ; recorded by VIMS (Visual and Infrared Imaging Mapping Spectrometer) from Cassini, December 2007.

The three largest lakes Kraken Mare , Ligeia Mare and Punga Mare are referred to as “Mare” and with areas of over 100,000 square kilometers, they reach the dimensions of large terrestrial inland lakes and seas (for comparison: Upper Lake 82,100 square kilometers). At the beginning of the mission, the largest "lake" Ontario Lacus was discovered at the South Pole as the only methane lake in the southern hemisphere to date and named after Lake Ontario , which is about 20,000 square kilometers in size . DLR researchers announced on July 30, 2008 that ethane had been detected in it and that it probably also contained other alkanes . Evaluations of radar measurements in 2009 showed that the Ontario Lacus appears to be as smooth as a mirror. The variations in height were less than 3 mm at the time of measurement. This supports the suspicion that the Ontario Lacus is actually made of liquid hydrocarbons and not dried mud. It also showed that there was relatively no wind on the surface. The depth of the "Ligeia Mare", which is mostly filled with methane, was determined to be 170 m with the help of Cassini's radar. The smaller lakes include the Feia Lacus , the Kivu Lacus , the Koitere Lacus and the Neagh Lacus .

The liquids in the lake-like structures are relatively transparent, so that a person - if he were to stand on such a bank - could look into these "waters" like a clear earthly lake. According to NASA calculations, the supply of liquid hydrocarbons on Titan exceeds that on Earth by a hundred times. The atmospheric cycle, the raining down, collecting and flowing of hydrocarbons shaped the icy surface in a surprisingly similar way to how water forms the silicate rocks on earth. Even at first glance, entire river systems can be seen from a height of a few kilometers, liquid methane cuts erosively into the surface of the ice and forms a hilly, mountainous relief. On earth this would imply a (tectonic) uplift of the eroded areas above the average surface height; it shouldn't be any different on Titan.

Cassini recordings of the disappearing and reappearing structure from different years

An island in the Ligaeia Mare is a riddle for the scientists . The approximately 260 km² object first appeared on Cassini images in July 2013, but then disappeared again on later images. However, during a Cassini passage on August 21, 2014, the object reappeared. Possible explanations would be, for example, rising bubbles, suspended matter or waves on the surface of Ligeia Mare . A connection with the current change of seasons on Titan is suspected.

Seasonal fluctuations

The researchers suspect that the lakes form mainly during the 7.5 year Titan winter and mostly dry out again in summer. This discovery went well with that of the vast north polar cloud vortex a few weeks later. Both confirm the assumption of a precipitation cycle of methane, similar to the water cycle on earth - with evaporation, cloud formation and renewed precipitation (methane is only liquid in the temperature range from −182 ° C to −162 ° C, at higher temperatures it is gaseous).

The lakes are unevenly distributed. After about half of Titan had been scanned by radar by 2009, the proportion of lake areas in the southern hemisphere seems to be only 0.4%, in the northern hemisphere about 10%, i.e. over twenty times more than in the southern hemisphere. This could be related to the fact that Saturn's orbit around the sun is elliptical and therefore its distance to the sun fluctuates by around 11%. As a result, the summers in the southern hemisphere are shorter and warmer, while the winters there are longer and colder. At the point closest to the sun, the perihelion , the planet moves faster around the sun than the point of its orbit furthest away from the sun, the aphelion, and the greater proximity also increases its angular velocity around the sun. As a result, the methane rains off in the northern hemisphere, which evaporated in the southern hemisphere in the warmer summer. Even if the sea level only drops by a few decimeters per year, over the course of many years this leads to many lakes in the southern hemisphere drying up. Since the ellipse of Saturn's orbit rotates slowly around the sun, the distribution of the lake cover is likely to oscillate between the northern and southern hemispheres every 45,000 years.

According to further research, the level of the liquid in the Ontario Lacus, as in the Etosha Pan, is said to change with the level of the liquid in the ground.

This is what ice could look like on methane lakes

The existence of icebergs on the surface of the lakes is possible in principle, but has not yet been proven. Because mixtures of solid methane and ethane are heavier than liquid mixtures, only ice can float that contains inclusions of at least 5% nitrogen from the atmosphere, which increase its volume. If the temperature drops only slightly, the nitrogen contracts so much that the ice sinks to the bottom. When the temperature rises again, the ground ice can also rise to the surface of the lake.


Cryovolcano on Titan (Cassini, 2004)

There are clear signs of volcanic activity on Titan. Even if the recognized volcanoes are similar in shape and size to those on Earth, it is not silicate volcanism as on the Earth-like planets Mars or Venus , but rather so-called cryovolcanoes , i.e. ice volcanoes.

The viscous mass that occurs on the titanium surface in this cryovolcanism could consist of water and z. B. ammonia or water with other hydrocarbon-containing mixtures whose freezing points are far below that of water and which could thus briefly flow on the surface. These areas with higher temperatures are also called "hotbeds" . It is believed that volcanism on Titan, similar to that on Earth, is driven by the release of energy from the decay of radioactive elements in the mantle of Titan.

With the help of Cassini, methane emissions were discovered from a suspected cryovolcano. Cryovolcanism is believed to be a major source of methane replenishment in the atmosphere.

internal structure

Sectional drawing to illustrate the internal structure

Titanium has a large core of silicate rock surrounded by several layers of water ice. The outer layer of the mantle consists of ice and methane hydrate , the inner layer of high pressure ice (see the ice phase diagram ). According to model calculations from 2005, there could be an ocean of liquid water between these two layers .

As with other moons of the gas planets  - Jupiter's moon Io and Saturn's moon Enceladus  - tidal forces of the mother planet could also play a role in the heating and subsequent mobilization of the interior of the moon, which is necessary for tectonic movements.

Due to the relatively high eccentricity of the titanium orbit and the resulting libration in length, the tidal deformation of the moon oscillates accordingly in the course of its bound rotation and could lead to tectonic shifts in the interior of Titan with this to-and-fro movement.

Hypothetical ocean under the ice crust

Radar measurements from the Cassini probe indicate that an ocean of liquid water exists beneath the ice crust . The thickness of the ice crust is estimated to be around 80 kilometers.

According to a model that was transferred from Jupiter's moon Europa to Titan, the heat generated by tidal friction may also have led to the formation of this molten layer under its ice crust. It would have to be detectable with the Cassini probe by means of gravitational field measurements .

Ammonia, which is about 10% in the water, would act as an antifreeze (see lowering of the freezing point ), so that despite the temperature of −20 ° C to be expected at this depth, a liquid ocean could have formed - especially in connection with the high pressure there.

From a geological point of view, the existence of an ocean in the depths means that the crust above can be much more mobile than on celestial bodies that are continuously solid, such as the Earth's moon. The crustal mobility leads to the observed large tectonic structures and also to cryovolcanism , whereby it can be assumed that water from the subterranean ocean is also directly involved in ice volcanism, as is the case with the earth with magma from the mantle. As has already been demonstrated on Enceladus , the crustal movements alone can generate so much heat locally that significant amounts of ice in the movement zones are liquefied and create cryovolcanism.

Cassini discovered that the attraction over titanium mountains is weaker than over flat areas. The researchers therefore suspect that the ice under mountains extends deeper into the ocean than under plains. The evaluation of gravity field measurements by Cassini showed that the presumed ocean must be very salty. He is about to freeze, which is why the outermost layer of ice above him should be very rigid.


The observation and exploration of titanium was only possible to a very limited extent before the space age. In 1907 the Spanish astronomer Josep Comas i Solà announced that he had observed a darkening on the edge of the titanium disk and two white, round spots in the center. Gerard Kuiper's discovery of the atmosphere in the 1940s was the next discovery worth mentioning. As the first space probe, Pioneer 11 finally reached the moons of Saturn in 1979.

Discovery and naming

Titan's discoverer Christiaan Huygens after a painting by Caspar Netscher (1671)

See also: List of discoveries of planets and their moons .

The Dutch mathematician, author, physicist and astronomer Christiaan Huygens discovered titanium on March 25, 1655. He succeeded in making this first find of a moon of Saturn with a self-made telescope , the lenses of which he and his brother Constantijn jr. had sanded; it had an objective diameter of 57 mm and magnified 50 times.

With this, Huygens first observed the planets Venus , Mars , Jupiter and finally Saturn, near which he noticed a bright celestial body that orbited Saturn once in the course of 16 days. After four revolutions in June he was sure that it must be a Saturn moon. At this time the inclination of the rings of Saturn with respect to the earth was slight and hardly affected the observations. This was the second moon discovery since the invention of the telescope (1608), 45 years after the four moons of Jupiter first observed by Galileo .

The satellite remained nameless for almost two centuries. At first it was called, among other things, the Huygens moon of Saturn . Huygens himself only called him "Saturni Luna" (or "Luna Saturni" , Latin for "Saturn's moon"). The satellites of a planet that became known in the course of time were first numbered in the order of their orbital sizes; so it became the sixth Saturn moon around 1800 . In the 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, the astronomer John Herschel suggested names for the then known eight moons of Saturn after the Titans , a generation of giants from Greek mythology . As Saturn's largest moon, it was then named Titan.


Infrared image of the surface of Titan ( Hubble Space Telescope , 1998)
Titan's atmosphere from a distance of 435,000 kilometers (Voyager 1, 1980)

After more than 300 years of observing only from Earth, the first rough details of the surface of Titan were obtained in the 1990s by the Hubble orbital telescope with images in the near infrared spectral range , which penetrates the methane clouds and the organic “smog” . The striking contrasts visible on it between light and dark areas of the surface are in clear contrast to the structures known from the surfaces of other moons of this size. Concentric structures such as craters and impact basins could not be recognized at first. It made sense to consider the dark zones to be deeper than the light ones, as well as to suspect a materially different composition of these surfaces: in the light zones possibly water ice, as is common on Jupiter's moons, and in the dark areas possibly silicate rocks or organic material.

Pioneer and Voyager

As the first space probe and simple scout, the American flyby probe Pioneer 11 passed the planet Saturn on September 1, 1979 and also transmitted five images of Titan. The probe came within 353,950 kilometers of Titan. However, the scanned images were too blurred for any surface details.

Extensive investigations of the moon were carried out by Voyager 1 , which passed Saturn on November 12, 1980 and approached Titan within 4,000 kilometers. Her photos of the moon, however, weren't much better because of the opaque haze of its atmosphere. Voyager 1 was therefore only able to examine the composition of the atmosphere and determine basic data such as size, mass and orbital time more precisely.

On August 25, 1981, the sister probe Voyager 2 passed the Saturn system. Since she was supposed to swing-by at Saturn in the direction of Uranus , her trajectory could not be close to Titan's.

The small, dented moon Epimetheus , the smog-shrouded Titan, and Saturn's A and F rings (Cassini, 2006)


On October 15, 1997, the Cape Canaveral Air Force Station launched the Cassini-Huygens double probe , a joint project of ESA , NASA and Agenzia Spaziale Italiana (ASI), to explore Saturn, its satellites and Titan in particular. Cassini is the first orbiter around Saturn and was originally intended to orbit the planet for at least four years.

Cassini has orbited Saturn since its arrival on July 1, 2004. As planned, the spacecraft flew past Titan for the first time on October 26, 2004 at a distance of only 1200 kilometers. The radar photos taken of the surface show complex structures.

On June 1, 2008, the primary goal was achieved with a total of 74 Saturn orbits and 45 pre-calculated flybys at Titan. The subsequent mission phase was called " Cassini Equinox ", which provided for 21 more titanium flybys until June 30, 2010. In the end, Cassini's mission was extended to 2017. During the "Solstice" mission phase, which began on September 27, 2010, another 56 flybys were carried out at Titan. The Cassini mission ended with a controlled crash and burn up in the atmosphere of Saturn on September 15, 2017.

Huygens' descent and landing

Artist's impression: Huygens leaves Cassini for Titan.

On December 25, 2004, Huygens was disconnected and landed on Titan on January 14, 2005. The landing site is at the coordinates 10 ° S, 192 ° W, in the central area of ​​the side facing away from Saturn. Huygens is the first lander on a moon other than Earth.

During the descent, images were sent of the approaching and finally reached surface. The probe, rotating under stormy conditions, transmitted not only physical, chemical and meteorological measured values ​​but also wind noise. Particles on the order of a micrometer in the atmosphere could also be detected during the descent maneuver.

Only 20 kilometers above the surface did the haze reveal Titan. On some photos of the landing approach, a black area could be seen with short drainage channels. It was interpreted as a possible lake made of a tarry liquid.

At the end of the 2.5 hour descent through the atmosphere, the probe impacted at a speed of 4.5 m / s. After that, Cassini could receive their signals for another hour and ten minutes. At first glance, the image of the surface resembles earlier images of the Viking probes that landed on Mars : on a gray-orange plain, numerous boulders lie under a yellow-orange sky up to the horizon. According to the first analyzes, however, they are not made of rock, but, like the ground, of ice and hydrocarbons. The rounded chunks in the immediate vicinity of the camera are up to 15 cm in diameter and resemble pebbles.

Due to the much greater distance from the sun and the haze in the atmosphere, daylight on Titan is only about a thousandth as bright as that on Earth. Shortly before landing, a searchlight turned on, in whose light the ice on the titanium floor could be identified spectroscopically.

On March 14, 2007, the landing site was named “Hubert Curien Memorial” in honor of Hubert Curien , one of the founding fathers of European space travel.

Scheduled missions

Around the year 2030, the TandEM mission's lander could fall on Kraken Mare .

As part of the Titan and Enceladus exploration mission TandEM , a dedicated orbiter is planned for Titan, which is intended to place various landers and penetrators on it for closer exploration and to maintain radio contact with a balloon moving freely in the Titan atmosphere. In early 2009, the ESA Council of Ministers meeting decided that the Europa Jupiter System Mission would have priority. The EJSM has since been canceled due to NASA's withdrawal. Its European share will, however, be realized as the JUICE space probe . TandEM could not start until well after 2020.

Titan Mare Explorer (TiME) was a planned project that was supposed to set down a lander for the first time on an extraterrestrial body of water, Lake Ligeia Mare or, alternatively, on Kraken-Mare. This mission was originally proposed as a stand-alone mission by Proxemy Research. But it could also have been part of the Titan Saturn System Mission (TandEM). This low-cost mission was also to directly measure the organic constituents on Titan's surface. A possible start date would have been January 2016. The mission was subject to the InSight Mars probe in the selection process on August 20, 2012 .

NASA's Dragonfly probe is scheduled to launch in 2026 Template: future / in 5 yearsas part of the New Frontiers program and Template: future / in 5 yearsarrive at Titan in 2034 . A quadrocopter is supposed to land on the surface of Titan, fly there and explore several places on the moon.

In addition, Titan is one of several possible candidates for future colonization in the outer solar system. The American aerospace engineer and author Robert Zubrin describes Saturn as the most important and valuable of the gas planets for research due to its relative proximity, low radiation exposure and the distribution of the moons.

Speculation about (precursors to) life

Titan could hold a key to understanding the origin of life on earth, as it is believed that primordial earth had a similar atmosphere and thus similar conditions prevailed. Since Saturn and its satellites orbit far outside the habitable zone , the emergence of life is unlikely, but preliminary stages are not excluded. Overall, despite the low temperatures for cosmochemistry, very interesting processes are to be assumed on this moon, perhaps also precursors for a kind of chemical evolution . Due to the dense atmosphere of nitrogen and organic compounds, it is an important research object in astrobiology , as these conditions could be similar to those on primeval earth . A prebiotic development towards carbon-based life, comparable to terrestrial life, would be prevented by the surface temperatures.

The Cassini space probe discovered that hydrogen disappears near the ground and the acetylene expected there could not be detected. This corresponds to the hypothetical model of astrobiologist Chris McKay , according to which methane-based life could cause this effect. As a non-biological cause, previously unknown chemical processes in the atmosphere or the formation of methane from hydrogen and acetylene with the help of an unknown mineral catalyst would be conceivable.

In 2010, researchers from the University of Arizona had simulated the conditions in the titanic gas envelope in the laboratory. In doing so, they mixed nitrogen, methane and carbon monoxide, the main constituents of Titan's atmosphere, together. In this environment without water, exposed to strong radio radiation, the amino acids glycine and alanine , the basic building blocks of earthly proteins, were created. In addition, all five basic components of the nucleic acids RNA and DNA were formed - cytosine , adenine , thymine , guanine and uracil . The reactions took place completely within a gaseous environment. Sarah Hörst and Roger Yelle from the University of Arizona think it is possible that the basic building blocks on earth could not necessarily form in a primordial soup, but also in the atmosphere and then rain down on the surface.

Amateur astronomical observation

With an apparent magnitude of 8.4 and a maximum angular distance of around 3 arc minutes to Saturn, good binoculars are enough to see the great moon Titan when visibility is good . With relatively small telescopes it can already be observed very well and its orbit around the planet can be easily followed. In a larger telescope, titanium can be seen as a tiny disc. Its orbit has the apparent diameter of about a fifth of the Earth's moon disk. Titan's surface cannot be seen because of the dense gas envelope. The components of the atmosphere and their proportions can be determined with a spectrometer .

Reception in literature and culture

The first part of Stanisław Lem's 1986 work Fiasko takes place on Titan, where people from the earth mine. The 1997 novel Titan by Stephen Baxter is about a manned mission to Titan by NASA, in which the crew has to fight for survival after an unscheduled landing. In Philip K. Dick's The Game Players of Titan (1963, dt. The Globus game ), that is titanium beings via telepathic have skills and passionate players.

In the 2009 film Star Trek , the crew used Titan's dense atmosphere to hide from the Romulans . There is also a series of novels about the ship USS Titan , which is under the command of William T. Riker . The film Oblivion , released four years later, mentions the moon as a haven for humans after an alien invasion. In Gattaca (1997), Titan is the target of a space mission.

In the futuristic world of the horror game Dead Space 2 , the main plot takes place in a large human colony complex that was built on the moon of Saturn. Titan is also one of the locations in the action strategy game Battlezone .

In the 41st millennium of the Warhammer 40,000 universe, titanium is provided with a core of super-dense matter and serves as the basis for the Space Marines Order of the Gray Knights.

After a devastating attack on the last bastion of mankind in the video game Destiny 2 , the Titan provides a retreat for some of the refugees. Here the player visits an area of ​​oil rig-like structures that were built on a vast sea.

See also


Web links

Commons : Titan  album with pictures, videos and audio files
Wiktionary: Titan  - explanations of meanings, origins of words, synonyms, translations

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  111. Destiny 2: Europa, Titan, Nessus and Io - These are the four new "worlds" . In: Mein-MMO.de . May 19, 2017 ( mein-mmo.de [accessed September 11, 2017]).
further inside Saturn moons further outside
Semi- major axis  (km) Titan 1,221,900
This article was added to the list of excellent articles on November 10, 2009 in this version .