From Wikipedia, the free encyclopedia
Titan is the largest
moon of Saturn. It is the only
moon known to have a dense
atmosphere, and the only object in space, other than
Earth, where clear evidence of stable bodies of surface liquid have been found.
Titan is the sixth
gravitationally rounded moon from
Saturn. Frequently described as a planet-like moon, Titan is 50% larger 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 smallest 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 would appear 11.4 times larger in
the sky than the Moon from Earth.
Titan is primarily composed of ice and rocky material. Much as with
Venus before the
Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until new information from the
Cassini–Huygens mission in 2004, including the discovery of
liquid hydrocarbon lakes in Titan's polar regions. 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 nitrogen-rich organic smog. 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 is analogous to Earth's
water cycle, at the much lower temperature of about 94 K (−179.2 °C; −290.5 °F).
History
Discovery
Titan was discovered on March 25, 1655, by the Dutch astronomer
Christiaan Huygens.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 older brother
Constantijn Huygens, Jr.,
began building telescopes around 1650 and discovered the first observed
moon orbiting Saturn with one of the telescopes they built. It was the sixth moon ever discovered, after Earth's
Moon and the
Galilean moons of Jupiter.
Naming
Huygens named his discovery
Saturni Luna (or
Luna Saturni, Latin for "Saturn's moon"), publishing in the 1655 tract
De Saturni Luna Observatio Nova (
A New Observation of Saturn's Moon). After
Giovanni Domenico Cassini
published his discoveries of four more moons of Saturn between 1673 and
1686, astronomers fell into the habit of referring to these and Titan
as Saturn I through V (with Titan then in fourth position). Other early
epithets for Titan include "Saturn's ordinary satellite". Titan is officially numbered
Saturn VI
because after the 1789 discoveries the numbering scheme was frozen to
avoid causing any more confusion (Titan having borne the numbers II and
IV as well as VI). Numerous small moons have been discovered closer to
Saturn since then.
The name
Titan, and the names of all seven satellites of Saturn then known, 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. He suggested the names of the mythological
Titans (
Ancient Greek:
Τῑτᾶνες), brothers and sisters of
Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful
deities, descendants of
Gaia and
Uranus, that ruled during the legendary
Golden Age.
Orbit and rotation
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 the
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, so Titan's "day" is equal to its orbit period. Because of
this, there is a sub-Saturnian point on its surface, from which the
planet would always appear to hang directly overhead. Longitudes on
Titan are measured westward, starting from the meridian passing through
this point. Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator.
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.
The small, irregularly shaped satellite
Hyperion is locked in a 3:4
orbital resonance
with Titan. A "slow and smooth" evolution of the resonance—in which
Hyperion migrated from a chaotic orbit—is considered unlikely, based on
models. Hyperion probably formed in a stable orbital island, whereas the
massive Titan absorbed or ejected bodies that made close approaches.
Bulk characteristics
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, 1.06 times that of the planet
Mercury, 1.48 that of the Moon, and 0.40 that of Earth. 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, which extends many kilometres above its surface and
increases its apparent diameter. Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and
Callisto. Based on its bulk density of 1.88 g/cm
3, Titan's composition is half water 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.
Titan is likely differentiated into several layers with a
3,400-kilometer (2,100 mi) rocky center surrounded by several layers
composed of different crystalline forms of ice. Its 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
presence of ammonia allows water to remain liquid even at a temperature
as low as 176 K (−97 °C) (for
eutectic mixture with water). The
Cassini probe discovered the evidence for the layered structure in the form of natural
extremely-low-frequency
radio waves in Titan's atmosphere. Titan's surface is thought to be a
poor reflector of extremely-low-frequency radio waves, so they may
instead be reflecting off the liquid–ice boundary of a
subsurface ocean. Surface features were observed by the
Cassini
spacecraft to systematically shift by up to 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.
Further supporting evidence for a liquid layer and ice shell decoupled
from the solid core comes from the way the gravity field varies as
Titan orbits Saturn. Comparison of the gravity field with the RADAR-based topography observations also suggests that the ice shell may be substantially rigid.
Formation
The moons of 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.
In 2014, analysis of Titan's atmospheric nitrogen suggested that
it has possibly been sourced from material similar to that found in the
Oort cloud and not from sources present during co-accretion of materials around Saturn.
Atmosphere
True-color image of layers of haze in Titan's atmosphere
Titan is the only known moon with a significant
atmosphere,
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. 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, 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 obscures Titan's surface features. Titan's lower gravity means that its atmosphere is far more extended than Earth's. 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. It was not until the arrival of the
Cassini–Huygens spacecraft in 2004 that the first direct images of Titan's surface were obtained.
Titan's South Pole Vortex—a swirling HCN gas cloud (November 29, 2012).
Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), hydrogen (0.1–0.2%) with trace amounts of other gases. 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. 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. Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the
solar wind.
Energy from the Sun should have converted all traces of methane
in Titan's atmosphere into more complex hydrocarbons within 50 million
years—a short time compared to the age of the Solar System. This
suggests that methane must be replenished by a reservoir on or within
Titan itself. The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from
cryovolcanoes.
On September 30, 2013,
propene was detected in the atmosphere of Titan by
NASA's
Cassini spacecraft, using its composite infrared spectrometer (CIRS).
This is the first time propene has been found on any moon or planet
other than Earth and is the first chemical found by the CIRS. The
detection of propene fills a mysterious gap in observations that date
back to NASA's
Voyager 1 spacecraft's first close
planetary flyby
of Titan in 1980, during which it 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.
On October 24, 2014,
methane was found in polar clouds on Titan.
Polar clouds, made of methane, on Titan (left) compared with polar clouds on Earth (right), which are made of water or water ice.
Climate
Titan's surface temperature is about 94 K (−179.2 °C). At this temperature, water ice has an extremely low
vapor pressure, so the little
water vapor present appears limited to the stratosphere. Titan receives about 1% as much sunlight as Earth.
Before sunlight reaches the surface, about 90% has been absorbed by the
thick atmosphere, leaving only 0.1% of the amount of light Earth
receives.
Atmospheric methane creates a
greenhouse effect on Titan's surface, without which Titan would be far colder. Conversely,
haze in Titan's atmosphere contributes to an
anti-greenhouse effect
by reflecting sunlight back into space, cancelling a portion of the
greenhouse effect and making its surface significantly colder than its
upper atmosphere.
Methane clouds (animated; July 2014).
Titan's clouds, probably composed of methane, ethane or other simple
organics, are scattered and variable, punctuating the overall haze. The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface.
Clouds typically cover 1% of Titan's disk, though outburst events
have been observed in which the cloud cover rapidly expands to as much
as 8%. One hypothesis asserts that the southern clouds are formed when
heightened
levels of sunlight during the southern summer generate uplift in the atmosphere, resulting in
convection.
This explanation is complicated by the fact that cloud formation has
been observed not only after the southern summer solstice but also
during mid-spring. Increased methane humidity at the south pole possibly
contributes to the rapid increases in cloud size.
It was summer in Titan's southern hemisphere until 2010, when Saturn's
orbit, which governs Titan's motion, moved Titan's northern hemisphere
into the sunlight. When the seasons switch, it is expected that ethane will begin to condense over the south pole.
Surface features
Titan − the surface under the haze (December 2018)
Titan – infrared views (2004 – 2017)
Global map of Titan – with IAU labels (August 2016).
The surface of Titan has been described as "complex, fluid-processed, [and] geologically young".
Titan has been around since the Solar System's formation, but its
surface is much younger, between 100 million and 1 billion years old.
Geological processes may have reshaped Titan's surface.
Titan's atmosphere is twice as thick as Earth's, making it difficult
for astronomical instruments to image its surface in the visible light
spectrum. The
Cassini spacecraft used infrared instruments, radar altimetry and
synthetic aperture radar
(SAR) imaging to map portions of Titan during its close fly-bys. The
first images revealed a diverse geology, with both rough and smooth
areas. There are features that may be
volcanic
in origin, disgorging water mixed with ammonia onto the surface. There
is also evidence that Titan's ice shell may be substantially rigid, which would suggest little geologic activity.
There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles.
Examination has also shown the surface to be relatively smooth; the few
objects that seem to be impact craters appeared to have been filled in,
perhaps by raining hydrocarbons or volcanoes. Radar altimetry suggests
height variation is low, typically no more than 150 meters. Occasional
elevation changes of 500 meters have been discovered and Titan has
mountains that sometimes reach several hundred meters to more than 1
kilometer in height.
Titan's surface is marked by broad regions of bright and dark terrain. These include
Xanadu, a large,
reflective 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.
It is criss-crossed in places by dark lineaments—sinuous topographical
features resembling ridges or crevices. These may represent
tectonic
activity, which would indicate that Xanadu is geologically young.
Alternatively, the lineaments may be liquid-formed channels, suggesting
old terrain that has been cut through by stream systems. There are dark areas of similar size elsewhere on Titan, observed from the ground and by
Cassini; at least one of these,
Ligeia Mare, Titan's second-largest sea, is almost a pure methane sea.
Titan mosaic from a Cassini flyby. The large dark region is Shangri-La.
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Titan in false color showing surface details and atmosphere. Xanadu is the bright region at the bottom-center.
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Titan globe, a mosaic of infrared images with nomenclature.
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Titan composite image in infrared. It features the dark, dune-filled regions Fensal (north) and Aztlan (south).
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Lakes
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. 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
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.
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. Near Titan's south pole, an enigmatic dark feature named
Ontario Lacus was identified (and later confirmed to be a lake). A possible shoreline was also identified near the pole via radar imagery. 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.
Based on the observations, scientists announced "definitive evidence of
lakes filled with methane on Saturn's moon Titan" in January 2007. The
Cassini–Huygens
team concluded that the imaged features are almost certainly the
long-sought hydrocarbon lakes, the first stable bodies of surface liquid
found outside Earth. Some appear to have channels associated with liquid and lie in topographical depressions.
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. Overall, the
Cassini radar observations have shown that lakes cover only a small percentage of the surface, making Titan much drier than Earth.
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.
In June 2008, the
Visual and Infrared Mapping Spectrometer on
Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus. On December 21, 2008,
Cassini
passed directly over Ontario Lacus and observed specular reflection in
radar. The strength of the reflection saturated the probe's receiver,
indicating that the lake level did not vary by more than 3 mm (implying
either that surface winds were minimal, or the lake's hydrocarbon fluid
is viscous).
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.
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). 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.
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".
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 liquid filled. 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.
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.
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. On July 2, 2014, NASA reported the ocean inside Titan may be as salty as the
Dead Sea. 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.
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, range in depth from 240 to
570 m and have sides as steep as 40°. They are believed to have formed
either by crustal uplifting, like Earth's
Grand Canyon,
or 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.
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| Photo of infrared specular reflection off Jingpo Lacus, a lake in the north polar region
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Perspective radar view of Bolsena Lacus (lower right) and other northern hemisphere hydrocarbon lakes
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| Contrasting images of the number of lakes in Titan's northern hemisphere (left) and southern hemisphere (right)
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Two images of Titan's southern hemisphere acquired one year apart, showing changes in south polar lakes
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Impact craters
Radar image of a 139 km-diameter 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. These impacts appear to be relatively young, compared to Titan's age. The few impact craters discovered include a 440-kilometer-wide (270 mi) two-ring impact basin named Menrva seen by
Cassini's ISS as a bright-dark concentric pattern. A smaller, 60-kilometer-wide (37 mi), flat-floored crater named Sinlap and a 30 km (19 mi) crater with a central peak and dark floor named Ksa have also been observed. 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.
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.
Many of Titan's craters or probable craters display evidence of extensive erosion, and all show some indication of modification.
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. 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.
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).
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".
Cryovolcanism and mountains
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.
Global maps of the lake distribution on Titan's surface revealed that
there is not enough surface methane to account for its continued
presence in its atmosphere, and thus that a significant portion must be
added through volcanic processes.
Still, there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes. 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. In 2004
Cassini also detected an unusually bright feature (called
Tortola Facula), which was interpreted as a cryovolcanic dome. No similar features have been identified as of 2010.
In December 2008, astronomers announced the discovery of two transient
but unusually long-lived "bright spots" in Titan's atmosphere, which
appear too persistent to be explained by mere weather patterns,
suggesting they were the result of extended cryovolcanic episodes.
In March 2009, structures resembling lava flows were announced in
a region of Titan called Hotei 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 been erupted
from beneath the surface.
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. Prior to
Cassini,
scientists assumed that most of the topography on Titan would be impact
structures, yet these findings reveal that similar to Earth, the
mountains were formed through geological processes. In December 2010, the
Cassini mission team announced the most compelling possible cryovolcano yet found. Named
Sotra 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.
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.
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.
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. 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.
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.
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.
Dark equatorial terrain
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. Prior to the arrival of
Cassini, these regions were thought to be seas of liquid hydrocarbons. 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 about a kilometer wide, and tens to hundreds of kilometers long. 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). 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 theorized, 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.
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.
The "sand" on Titan is likely not made up of small grains of silicates like the sand on Earth,
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.
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. Calculations indicate the sand on Titan has a density of one-third that of terrestrial sand.
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.
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.
Titan - three dust storms detected in 2009-2010.
Observation and exploration
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. Titan has a maximum
apparent magnitude of +8.2, and mean opposition magnitude 8.4. This compares to +4.6 for the similarly sized Ganymede, in the Jovian system.
Cassini's Titan flyby radio signal studies (artist's concept)
The first probe to visit the Saturnian system was
Pioneer 11 in 1979, which revealed that Titan was probably too cold to support life. It took images of Titan, including Titan and Saturn together in mid to late 1979. The quality was soon surpassed by the two
Voyagers.
Titan was examined by both
Voyager 1 and
2 in 1980 and 1981, respectively.
Voyager 1's
trajectory was designed to provide an optimized Titan flyby, during
which the spacecraft was able to determine the density, composition, and
temperature of the atmosphere, and obtain a precise measurement of
Titan's mass. Atmospheric haze prevented direct imaging of the surface, though in 2004 intensive digital processing of images taken through
Voyager 1's orange filter did reveal hints of the light and dark features now known as
Xanadu and
Shangri-la, which had been observed in the infrared by the Hubble Space Telescope.
Voyager 2, which would have been diverted to perform the Titan flyby if
Voyager 1 had been unable to, did not pass near Titan and continued on to Uranus and Neptune.
Cassini–Huygens
Cassini image of Titan, behind Epimetheus and the rings
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 mystery that had surrounded Titan
since the 17th-century observations of Christiaan Huygens and Giovanni
Cassini was revealed by a spacecraft named in their honor.
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.
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. 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.
Huygens landing
Huygens in situ image from Titan's surface—the only image from the surface of a body farther away than Mars
Same image with contrast enhanced
Huygens landed on Titan on January 14, 2005, discovering that many of its surface features seem to have been formed by fluids at some point in the past. Titan is the most distant body from Earth to have a space probe land on its surface.
The
Huygens probe landed just off 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.
After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.
The two rocks just below the middle of the image on the right are
smaller than they may appear: the left-hand one is 15 centimeters
across, and the one in the center is 4 centimeters across, at a distance
of about 85 centimeters from Huygens. There is evidence of
erosion at the base of the rocks, indicating possible fluvial activity.
The surface is darker than originally expected, consisting of a mixture
of water and hydrocarbon ice. The "soil" visible in the images is
interpreted to be precipitation from the hydrocarbon haze above.
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.
Proposed or conceptual missions
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, the
ESA and
JPL. At present, none of these proposals have become funded missions.
The
Titan Saturn System Mission (TSSM) was a joint NASA/
ESA proposal for exploration of
Saturn's moons. It envisions a hot-air balloon floating in Titan's atmosphere for six months. It was competing against the
Europa Jupiter System Mission
(EJSM) proposal for funding. In February 2009 it was announced that
ESA/NASA had given the EJSM mission priority ahead of the TSSM.
The proposed
Titan Mare Explorer
(TiME) was a low-cost lander that would splash down in a lake in
Titan's northern hemisphere and float on the surface of the lake for
three to six months. It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA
Discovery Program opportunity, but was not selected for flight.
A conceptual design for another lake lander was proposed in late 2012 by the Spanish-based private engineering firm
SENER and the Centro de Astrobiología in
Madrid. The concept probe is called
Titan Lake In-situ Sampling Propelled Explorer (TALISE).
The major difference compared to 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.
Prebiotic conditions and life
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 theorized for the primordial Earth.
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.
Formation of complex molecules
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.
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.
On April 3, 2013, NASA reported that complex
organic chemicals could arise on Titan based on studies simulating the
atmosphere of Titan.
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.
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.
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.
Possible subsurface habitats
Laboratory
simulations have led to the suggestion that enough organic material
exists on Titan to start a chemical evolution analogous to what is
thought to have started life on Earth. The analogy assumes the presence
of liquid water for longer periods than is currently observable; several
theories suggest that liquid water from an impact could be preserved
under a frozen isolation layer. It has also been theorized that liquid-ammonia oceans could exist deep below the surface.
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.
Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life. Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined.
Methane and life at the surface
It has been suggested that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water. Such organisms would inhale H
2 in place of O
2, metabolize it with
acetylene instead of
glucose, and exhale methane instead of carbon dioxide.
All living things 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. Water is a stronger solvent than methane. Water is also more chemically reactive, and can break down large organic molecules through
hydrolysis. A life-form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.
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.
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 10
28 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.
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.
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.
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.
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."
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."
In February 2015, a hypothetical
cell membrane capable of functioning in liquid
methane
in Titan 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".
Obstacles
Despite
these biological possibilities, there are formidable obstacles to life
on Titan, and any analogy to Earth is inexact. At a vast distance from
the
Sun, Titan is frigid, and its atmosphere lacks CO
2. 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 theories on the conditions that prevailed prior
to the appearance of life on Earth.
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.
Using Titan as a prebiotic experiment involves not only observation
through spacecraft, but laboratory experiments, and chemical and
photochemical modeling on Earth.
Panspermia hypothesis
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
transpermia. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan.
On the other hand, Jonathan Lunine has argued that any living things in
Titan's cryogenic hydrocarbon lakes would need to be so different
chemically from Earth life that it would not be possible for one to be
the ancestor of the other.
Future conditions
Conditions on Titan could become far more
habitable in the far future. Five billion years from now, as the Sun becomes a
red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable.
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 presence of ammonia on Titan would cause
chemical reactions to proceed more slowly.
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