# Why does Titan have lower surface gravity than the Moon when Titan is more massive?

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Reading on Wikipedia I saw that Titan is 80% more massive than the earth's moon but has only 85% the surface gravity. Why is this?

Surface gravitational acceleration on an object with mass $$M$$ and radius $$R$$ is given by $$g = frac{GM}{R^2} propto G ho R$$ where $$ho propto M/R^3$$ is the density of the object. If one body has smaller surface $$g$$ than another, it must have smaller density $$ho$$, smaller radius $$R$$, or both. Titan is larger than Earth's Moon, so your observation about its surface gravity means Titan must be less dense than the Moon. Wikipedia confirms:

• $$R_ ext{Titan} = 1.5 R_ ext{Moon}$$, but
• $$ho_ ext{Moon} = 3.34 m,g/cm^3$$ while Titan has only $$ho_ ext{Titan} = 1.88 m,g/cm^3$$.

## Titan's Seas Are Sand

Detail from a Cassini radar image of sand dunes on Titan.

New radar evidence shows they are seas -- but seas of sand dunes like those in the Arabian or Namibian Deserts, a University of Arizona member of the Cassini radar team and colleagues report in Science (May 5).

Radar images taken when the Cassini spacecraft flew by Titan last October show dunes 330 feet (100 meters) high that run parallel to each other for hundreds of miles at Titan's equator. One dune field runs more than 930 miles (1500 km) long, said Ralph Lorenz of UA's Lunar and Planetary Laboratory.

"It's bizarre," Lorenz said. "These images from a moon of Saturn look just like radar images of Namibia or Arabia. Titan's atmosphere is thicker than Earth's, its gravity is lower, its sand is certainly different -- everything is different except for the physical process that forms the dunes and resulting landscape."

Ten years ago, scientists believed that Saturn's moon Titan is too far from the sun to have solar-driven surface winds powerful enough to sculpt sand dunes. They also theorized that the dark regions at Titan's equator might be liquid ethane oceans that would trap sand.

But researchers have since learned that Saturn's powerful gravity creates significant tides in Titan's atmosphere. Saturn's tidal effect on Titan is roughly 400 times greater than our moon's tidal pull on Earth.

As first seen in circulation models a couple of years ago, Lorenz said, "Tides apparently dominate the near-surface winds because they're so strong throughout the atmosphere, top to bottom. Solar-driven winds are strong only high up."

The dunes seen by Cassini radar are a particular linear or longitudinal type that is characteristic of dunes formed by winds blowing from different directions. The tides cause wind to change direction as they drive winds toward the equator, Lorenz said.

And when the tidal wind combines with Titan's west-to-east zonal wind, as the radar images show, it creates dunes aligned nearly west-east except near mountains that influence local wind direction.

"When we saw these dunes in radar it started to make sense," he said. "If you look at the dunes, you see tidal winds might be blowing sand around the moon several times and working it into dunes at the equator. It's possible that tidal winds are carrying dark sediments from higher latitudes to the equator, forming Titan's dark belt."

The researchers' model of Titan suggests tides can create surface winds that reach about one mile per hour (a half-meter per second). "Even though this is a very gentle wind, this is enough to blow grains along the ground in Titan's thick atmosphere and low gravity," Lorenz said. Titan's sand is a little coarser but less dense than typical sand on Earth or Mars. "These grains might resemble coffee grounds."

The variable tidal wind combines with Titan's west-to-east zonal wind to create surface winds that average about one mile per hour (a half meter per second). Average wind speed is a bit deceptive, because sand dunes wouldn't form on Earth or Mars at their average wind speeds.

Whether the grains are made of organic solids, water ice, or a mixture of both is a mystery. Cassini's Visual and Infrared Mapping Spectrometer, led by UA's Robert Brown, may get results on sand dune composition.

How the sand formed is another peculiar story.

Sand may have formed when liquid methane rain eroded particles from ice bedrock. Researchers previously thought that it doesn't rain enough on Titan to erode much bedrock, but they thought in terms of average rainfall.

Observations and models of Titan show that clouds and rain are rare. That means that individual storms could be large and still yield a low average rainfall, Lorenz explained.

When the UA-led Descent Imager/Spectral Radiometer (DISR) team produced images taken during the Huygens probe landing on Titan in January 2005, the world saw gullies, streambeds and canyons in the landscape. These same features on Titan have been seen with radar.

These features show that when it does rain on Titan, it rains in very energetic events, just as it does in the Arizona desert, Lorenz said.

Energetic rain that triggers flash floods may be a mechanism for making sand, he added.

Alternatively, the sand may come from organic solids produced by photochemical reactions in Titan's atmosphere.

"It's exciting that the radar, which is mainly to study the surface of Titan, is telling us so much about how winds on Titan work," Lorenz said. "This will be important information for when we return to Titan in the future, perhaps with a balloon."

An international group of scientists are co-authors on the Science article, "The Sand Seas of Titan: Cassini Observations of Longitudinal Dunes." They are from the Jet Propulsion Laboratory, California Institute of Technology, U.S. Geological Survey - Flagstaff, Planetary Science Institute, Wheeling Jesuit College, Proxemy Research of Bowie, Md., Stanford University, Goddard Institute for Space Studies, Observatoire de Paris, International Research School of Planetary Sciences, Universita' d'Annunzio, Facolt di Ingegneria, Universit La Sapienza, Politecnico di Bari and Agenzia Spaziale Italiana. Jani Radebaugh and Jonathan Lunine of UA's Lunar and Planetary Laboratory are among the co-authors.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL.

## Why does Titan have lower surface gravity than the Moon when Titan is more massive? - Astronomy

### From Effective Temperature to Actual Temperature:

Effective temperature: The ideal temperature at which a planet precisely re-radiates all of the energy it receives from the sun. For an idealized black-body, it is:

• L = Solar luminosity = 3.846*10 26 W m -2 K -4
• D = distance from Sun
• σ = the Stefan-Boltzman constant = 5.6704 * 10 -8 W

Thus, in our solar System, the only real variable in computing effective temperature is distance from the Sun.

Iapetus: High and low albedo on a single world
• Reflective icy worlds like Enceladus have high albedo.
• Dark worlds like Mercury have low albedo.
• Iapetus (right) offers a contrast of high and low albedo. (Which side do you think is warmer?)
• Mercury - 0.068
• Moon - 0.136
• Mars - 0.250
• Earth - 0.306
• Venus - 0.900

Again, makes sense. Mars is farther from the sun than Earth, and Venus has much greater albedo. For Mars, it also approximates mean surface temperature well.

So, how is this heat distributed vertically in the atmosphere?

### Atmospheric Structure

The troposphere is warmed by infrared radiation from the planet's surface (which has been directly illuminated by visible sunlight) With increasing elevation, heat from this source diminishes rapidly, causing rapid cooling.

With two notable exceptions, the atmospheres of the Solar System conform to this schematic:

• It emits infrared radiation that warms the air above it.
• This warm parcel of air then rises.
• As it rises into regions of lower atmospheric pressure, it expands to equalize its pressure with that of the air around it.
• As a result of its expansion, the air cools.

Because this cooling occurs even if there is no transfer of heat to surrounding air, it is termed adiabatic cooling. (Adiabatic means "without transfer.") This is the minimum rate at which rising air cools. In practice, there is usually some heat transfer to surrounding air, also.

Adiabatic lapse rate: The rate at which rising air cools adiabatically. As long as the surrounding air cools faster with increasing height than the adiabatic lapse rate, that parcel will continue to rise. Typically, the adiabatic lapse rate is slightly less than general cooling in the lower atmosphere. Once the adiabatic lapse rate exceeds the general rate of cooling, the parcel will no longer rise. This places an upper limit on convection and defines the top of the troposphere.

The Mesosphere: Here, the transfer of heat is relatively straightforward. There is no convection, only conduction - the absorption and re-radiation of energy.

The Thermosphere: In its thin outer reaches, an atmosphere mixes and interacts with the solar wind with the result that it is heated and ionized. (Regions of extensive ionization are called the ionosphere). Heated molecules re-radiate energy at different rates depending on their physical characteristics, such that some retain their heat longer than others.

### Atmospheric circulation:

Latitudinal differential in incoming sunlight : The amount of solar energy received per unit area of Earth's surface is a function of the angle at which the light strikes. The most concentrated energy is delivered to equatorial regions whereas polar regions receive very little. Resulting circulation is, in part, the atmosphere's attempt to equalize that heat distribution.

The Coriolis effect : The greatest solar heating occurs near the equator. Picture a rising parcel of air in Earth's equatorial troposphere. It reaches the top of the troposphere then circulates toward higher latitudes. At the equator, its motion equalled that of the Earth's equator. At it moves north, this momentum is conserved even though it is now moving over a surface that is not moving as fast. As a result, to conserve momentum, it seems to accelerate with respect to Earth's surface. Viewed from the surface, it appears to follow a curved path, as it is deflected toward the east.

• Hadley cells: form adjacent to the equator as strong sunlight warms equatorial air, which rises to the top of the troposphere in the intertropical convergence zone (ICZ) and spreads north and south. Because it carries moist air into the colder upper atmosphere, the ICZ is a place of frequent rainfall.
• Polar cells: form as very cold air descends near the poles and spreads southward.
• Ferrel cells: occur in the middle latitudes as a result of interactions between Hadley and polar circulation.
• Venus: Because its rotation is slow, the Coriolis force is weak and surface winds are weak.
• Mars: Mars shows proper tropical Hadley cells, but at higher latitudes global circulation is overwhelmed by the effects of polar ice.

### Planetary idiosyncrasies:

• Global dust storms: Mars' winds are sufficient to raise significant dust storms. When large dust clouds are raised, they tend to absorb heat, causing a temperature differential that increases wind speeds that pump more dust into the atmosphere, in a positive feedback loop. One or twice a year, Mars is enveloped in a global dust storm that:
• elevates the temperature of the upper troposphere by up to 45 K (in effect, the dust creates a stratosphere-like temperature inversion.)
• shades the lower troposphere, lowering surface temperatures.

Titan in visible light from Wikipedia

### Titan:

• Previously we said that two worlds lacked the typical troposphere-mesosphere-thermosphere stratification. One was earth, with its stratosphere. Fulchignone et al., 2005 revealed that Titan also has a stratosphere, caused by the absorption of visible light by the dense photochemical smog above its troposphere.
• Titan's modeled effective temperature (with albedo factored in): 82 K
• Titan's modeled temperature with greenhouse effect: 105 K
• Titan's observed mean surface temperature: 93.7 K

### Processes that alter atmospheric composition over time:

• Why does Earth have an atmosphere and the moon not, even though they are roughly the same average temperature?
• Why does the icy moon Titan have a thick atmosphere while the physically similar icy moon Ganymede is essentially airless?
• Gravity: determines its escape velocity.
• Temperature: determines the speed with which gas molecules actually move.
• G = gravitational constant
• M = mass of planet
• R = radius at which molecule escapes (top of atmosphere)

Notice that the escape velocity depends only on the ratio M/R of the planet. It has nothing to do with characteristics of the object that may escape. The escape velocity is the same for a rocket and for a gas molecule.

These are the speeds that a gas molecule must attain to escape from a planet's atmosphere. How do they attain those speeds?

Thermal energy: So, what is the velocity of a gas molecule? It depends on the temperature and the molecular mass of the gas. Hot gas molecules move at higher velocities than cold gases, and heavy molecules move more slowly than light ones. Note that this is an average velocity. Gas molecules do not all move at one speed there is a wide distribution of speeds at any temperature.

• Differences in atmospheric retention between bodies that are roughly the same temperature, like:
• Earth and moon
• Titan and Rhea

Artist's impression of the Moon, 3.5 Ga. From Phys.org

For an atmosphere to remain bound to its planet over the age of the solar system (4.56 Ga), the average speed of gas molecules should be less than 1/6 of the escape velocity.

Note: The temperatures we use here are temperatures of the upper atmosphere - the region from which gas molecules actually have a clear shot at escaping. Nevertheless a glance at the x-axis figures of this graph from your text shows that something is horribly wrong.

### Good Graph!

Here is a correct version. To learn more, play with the applet from the University of Nebraka Lincoln

## Contents

### Discovery Edit

Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. [17] [18] Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his elder brother Constantijn Huygens, Jr., began building telescopes around 1650 and discovered the first observed moon orbiting Saturn with one of the telescopes they built. [19] It was the sixth moon ever discovered, after Earth's Moon and the Galilean moons of Jupiter. [20]

### Naming Edit

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). [21] 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". [22] The International Astronomical Union officially numbers Titan as Saturn VI. [23]

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. [24] [25] Numerous small moons have been discovered around Saturn since then. [26] Saturnian moons are named after mythological giants. The name Titan comes from the Titans, a race of immortals in Greek mythology. [23]

Titan orbits Saturn once every 15 days 22 hours. Like Earth's Moon and many of the satellites of the giant planets, its rotational period (its day) is identical to its orbital period Titan is tidally locked in synchronous rotation with Saturn, and permanently shows one face to the planet. Longitudes on Titan are measured westward, starting from the meridian passing through this point. [27] Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator. [6] Viewed from Earth, Titan reaches an angular distance of about 20 Saturn radii (just over 1,200,000 kilometers (750,000 mi)) from Saturn and subtends a disk 0.8 arcseconds in diameter. [ citation needed ]

The small, 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. [28]

Titan is 5,149.46 kilometers (3,199.73 mi) in diameter, [7] 1.06 times that of the planet Mercury, 1.48 that of the Moon, and 0.40 that of Earth. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede (diameter 5,262 kilometers (3,270 mi)) and thus the largest moon in the Solar System this was an overestimation caused by Titan's dense, opaque atmosphere, with a haze layer 100-200 kilometres above its surface. This increases its apparent diameter. [29] Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto. [30] 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. [ citation needed ]

Titan is probably partially differentiated into distinct layers with a 3,400-kilometer (2,100 mi) rocky center. [31] This rocky center is surrounded by several layers composed of different crystalline forms of ice. [32] 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). [33] 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. [34] 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. [35] 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. [36] Comparison of the gravity field with the RADAR-based topography observations [37] also suggests that the ice shell may be substantially rigid. [38] [39]

The moons of Jupiter and Saturn are thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young gas giants formed, they were surrounded by discs of material that gradually coalesced into moons. Whereas Jupiter possesses four large satellites in highly regular, planet-like orbits, Titan overwhelmingly dominates Saturn's system and possesses a high orbital eccentricity not immediately explained by co-accretion alone. A proposed model for the formation of Titan is that Saturn's system began with a group of moons similar to Jupiter's Galilean satellites, but that they were disrupted by a series of giant impacts, which would go on to form Titan. Saturn's mid-sized moons, such as Iapetus and Rhea, were formed from the debris of these collisions. Such a violent beginning would also explain Titan's orbital eccentricity. [40]

A 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. [41]

Titan is the only known moon with a significant atmosphere, [42] 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. [43] 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, [44] or about 7.3 times more massive on a per surface area basis. Opaque haze layers block most visible light from the Sun and other sources and obscure Titan's surface features. [45] Titan's lower gravity means that its atmosphere is far more extended than Earth's. [46] 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. [47] It was not until the arrival of the Cassini–Huygens spacecraft in 2004 that the first direct images of Titan's surface were obtained. [48]

Titan's atmospheric composition is nitrogen (97%), methane (2.7±0.1%), hydrogen (0.1–0.2%) with trace amounts of other gases. [14] 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. [13] 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. [49] Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind. [50]

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. [51] The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes. [52] [53] [54] [55]

On April 3, 2013, NASA reported that complex organic chemicals, collectively called tholins, likely arise on Titan, based on studies simulating the atmosphere of Titan. [56]

On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan. [57]

On September 30, 2013, propene was detected in the atmosphere of Titan by NASA's Cassini spacecraft, using its composite infrared spectrometer (CIRS). [58] 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. [49]

On October 24, 2014, methane was found in polar clouds on Titan. [59] [60]

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. [61] Titan receives about 1% as much sunlight as Earth. [62] 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. [63]

Atmospheric methane creates a greenhouse effect on Titan's surface, without which Titan would be far colder. [64] 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. [65]

Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze. [29] The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto its surface. [67]

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. [68] 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. [69] When the seasons switch, it is expected that ethane will begin to condense over the south pole. [70]

Global map of Titan – with IAU labels (August 2016).

Titan – infrared views (2004–2017)

The surface of Titan has been described as "complex, fluid-processed, [and] geologically young". [71] 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. [72] Titan's atmosphere is four times as thick as Earth's, [73] making it difficult for astronomical instruments to image its surface in the visible light spectrum. [74] 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, [38] [39] which would suggest little geologic activity. [75] There are also streaky features, some of them hundreds of kilometers in length, that appear to be caused by windblown particles. [76] [77] 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. [78]

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. [79] 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. [80] 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. [81] [82]

### Lakes Edit

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. [83]

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. [84] Near Titan's south pole, an enigmatic dark feature named Ontario Lacus was identified [85] (and later confirmed to be a lake). [86] A possible shoreline was also identified near the pole via radar imagery. [87] 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. [88] Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007. [89] [90] 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. [89] Some appear to have channels associated with liquid and lie in topographical depressions. [89] 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. [72] Overall, the Cassini radar observations have shown that lakes cover only a small percentage of the surface, making Titan much drier than Earth. [91] 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. [92]

In June 2008, the Visual and Infrared Mapping Spectrometer on Cassini confirmed the presence of liquid ethane beyond doubt in Ontario Lacus. [93] 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). [94] [95]

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. [96] [97]

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). [98] 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. [98] 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". [99]

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. [100]

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. [101] 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. [101] [102] On July 2, 2014, NASA reported the ocean inside Titan may be as salty as the Dead Sea. [103] [104] 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. [105]

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. [106]

### Impact craters Edit

Radar, SAR and imaging data from Cassini have revealed few impact craters on Titan's surface. [72] These impacts appear to be relatively young, compared to Titan's age. [72] 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. [108] A smaller, 60-kilometer-wide (37 mi), flat-floored crater named Sinlap [109] and a 30 km (19 mi) crater with a central peak and dark floor named Ksa have also been observed. [110] 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. [111] 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. [112]

Many of Titan's craters or probable craters display evidence of extensive erosion, and all show some indication of modification. [107] 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. [107] 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. [114]

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). [107]

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". [115]

### Cryovolcanism and mountains Edit

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. [116] 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. [117]

Still, there is a paucity of surface features that can be unambiguously interpreted as cryovolcanoes. [118] 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. [119] [120] In 2004 Cassini also detected an unusually bright feature (called Tortola Facula), which was interpreted as a cryovolcanic dome. [121] No similar features have been identified as of 2010. [122] 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. [33]

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. [123] 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. [124]

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. [118] [125]

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. [126]

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. [127]

Crater-like landforms possibly formed via explosive, maar-like or caldera-forming cryovolcanic eruptions have been identified in Titan's polar regions. [128] These formations are sometimes nested or overlapping and have features suggestive of explosions and collapses, such as elevated rims, halos, and internal hills or mountains. [128] The polar location of these features and their colocalization with Titan's lakes and seas suggests volatiles such as methane may help power them. Some of these features appear quite fresh, suggesting that such volcanic activity continues to the present. [128]

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. [129]

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. [33] 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. [33] 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. [130] Titan may have been much more geologically active in the past models of Titan's internal evolution suggest that Titan's crust was only 10 kilometers thick until about 500 million years ago, allowing vigorous cryovolcanism with low viscosity water magmas to erase all surface features formed before that time. Titan's modern geology would have formed only after the crust thickened to 50 kilometers and thus impeded constant cryovolcanic resurfacing, with any cryovolcanism occurring since that time producing much more viscous water magma with larger fractions of ammonia and methanol this would also suggest that Titan's methane is no longer being actively added to its atmosphere and could be depleted entirely within a few tens of millions of years. [131]

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. [132]

### Dark equatorial terrain Edit

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. [133] Prior to the arrival of Cassini, these regions were thought to be seas of liquid hydrocarbons. [134] 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 [135] about a kilometer wide, and tens to hundreds of kilometers long. [136] 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). [137] The tidal winds are the result of tidal forces from Saturn on Titan's atmosphere, which are 400 times stronger than the tidal forces of the Moon on Earth and tend to drive wind toward the equator. This wind pattern, it was hypothesized, causes granular material on the surface to gradually build up in long parallel dunes aligned west-to-east. The dunes break up around mountains, where the wind direction shifts. [ citation needed ]

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. [138]

The "sand" on Titan is likely not made up of small grains of silicates like the sand on Earth, [139] 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. [135] [137] [140] 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. [141] Calculations indicate the sand on Titan has a density of one-third that of terrestrial sand. [142] 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. [143]

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. [144]

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. [146] Titan has a maximum apparent magnitude of +8.2, [12] and mean opposition magnitude 8.4. [147] This compares to +4.6 for the similarly sized Ganymede, in the Jovian system. [147]

Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer Josep Comas i Solà observed limb darkening of Titan, the first evidence that the body has an atmosphere. In 1944 Gerard P. Kuiper used a spectroscopic technique to detect an atmosphere of methane. [148]

The first probe to visit the Saturnian system was Pioneer 11 in 1979, which revealed that Titan was probably too cold to support life. [149] It took images of Titan, including Titan and Saturn together in mid to late 1979. [150] The quality was soon surpassed by the two Voyagers. [ citation needed ]

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. [151] 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, [152] 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. [151] : 94

### Cassini–Huygens Edit

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

The Cassini–Huygens spacecraft reached Saturn on July 1, 2004, and began the process of mapping Titan's surface by radar. A joint project of the European Space Agency (ESA) and NASA, Cassini–Huygens proved a very successful mission. The Cassini probe flew by Titan on October 26, 2004, and took the highest-resolution images ever of Titan's surface, at only 1,200 kilometers (750 mi), discerning patches of light and dark that would be invisible to the human eye. [ citation needed ]

On July 22, 2006, Cassini made its first targeted, close fly-by at 950 kilometers (590 mi) from Titan the closest flyby was at 880 kilometers (550 mi) on June 21, 2010. [153] 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. [88]

#### Huygens landing Edit

Huygens was an atmospheric probe that touched down on Titan on January 14, 2005, [154] discovering that many of its surface features seem to have been formed by fluids at some point in the past. [155] Titan is the most distant body from Earth to have a space probe land on its surface. [156]

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. [157]

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice. [157] The two rocks just below the middle of the image on the right are smaller than they may appear: the left-hand one is 15 centimeters across, and the one in the center is 4 centimeters across, at a distance of about 85 centimeters from Huygens. There is evidence of erosion at the base of the rocks, indicating possible fluvial activity. The ground surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. [158]

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. [159]

### Dragonfly Edit

The Dragonfly mission, developed and operated by the Johns Hopkins Applied Physics Laboratory, will launch in June 2027. [160] [161] It consists of a large drone powered by an RTG to fly in the atmosphere of Titan as New Frontiers 4. [162] [163] Its instruments will study how far prebiotic chemistry may have progressed. [164] The mission is planned to arrive at Titan in 2034. [163]

### Proposed or conceptual missions Edit

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. [ citation needed ]

The Titan Saturn System Mission (TSSM) was a joint NASA/ESA proposal for exploration of Saturn's moons. [165] 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. [166]

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. [167] [168] [169] It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity, [170] but was not selected for flight. [171]

Another mission to Titan proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho, is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR): an unmanned plane (or drone) that would fly through Titan's atmosphere and take high-definition images of the surface of Titan. NASA did not approve the requested \$715 million, and the future of the project is uncertain. [172] [173]

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). [174] [175] 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. [174]

A Discovery Program contestant for its mission #13 is Journey to Enceladus and Titan (JET), an astrobiology Saturn orbiter that would assess the habitability potential of Enceladus and Titan. [176] [177] [178]

In 2015, the NASA Innovative Advanced Concepts program (NIAC) awarded a Phase II grant [179] to a design study of a Titan Submarine to explore the seas of Titan. [180] [181] [182] [183] [184]

Titan is thought to be a prebiotic environment rich in complex organic compounds, [56] [185] but its surface is in a deep freeze at −179 °C (−290.2 °F 94.1 K) so life as we know it cannot exist on the moon's frigid surface. [186] However, Titan seems to contain a global ocean beneath its ice shell, and within this ocean, conditions are potentially suitable for microbial life. [187] [188] [189]

The Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organic compounds it showed an environment on Titan that is similar, in some ways, to ones hypothesized for the primordial Earth. [190] 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. [191] [185]

### Formation of complex molecules Edit

The Miller–Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively. [192]

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. [193]

On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan. [56]

On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons (PAH) in the upper atmosphere of Titan. [57]

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. [194] 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. [195]

On July 28, 2017, scientists reported that acrylonitrile, or vinyl cyanide, (C2H3CN), possibly essential for life by being related to cell membrane and vesicle structure formation, had been found on Titan. [196] [197] [198]

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. [199] [200]

### Possible subsurface habitats Edit

Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. The analogy assumes the presence of liquid water for longer periods than is currently observable several hypotheses postulate that liquid water from an impact could be preserved under a frozen isolation layer. [201] It has also been hypothesized that liquid-ammonia oceans could exist deep below the surface. [187] [202] 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. [188] Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life. [187] Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined. [188]

### Methane and life at the surface Edit

It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water. [203] Such organisms would inhale H2 in place of O2, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide. [189] [203] However, such hypothetical organisms would be required to metabolize at a deep freeze temperature of −179.2 °C (−290.6 °F 94.0 K). [186]

All life forms on Earth (including methanogens) use liquid water as a solvent it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane, [204] although water is a stronger solvent than methane. [205] Water is also more chemically reactive, and can break down large organic molecules through hydrolysis. [204] A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way. [204]

In 2005, astrobiologist Chris McKay argued that if methanogenic life did exist on the surface of Titan, it would likely have a measurable effect on the mixing ratio in the Titan troposphere: levels of hydrogen and acetylene would be measurably lower than otherwise expected. Assuming metabolic rates similar to those of methanogenic organisms on Earth, the concentration of molecular hydrogen would drop by a factor of 1000 on the Titanian surface solely due to a hypothetical biological sink. McKay noted that, if life is indeed present, the low temperatures on Titan would result in very slow metabolic processes, which could conceivably be hastened by the use of catalysts similar to enzymes. He also noted that the low solubility of organic compounds in methane presents a more significant challenge to any possible form of life. Forms of active transport, and organisms with large surface-to-volume ratios could theoretically lessen the disadvantages posed by this fact. [203]

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. [203] [205] [206] 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. [205] 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. [189] 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. [189]

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." [205] 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." [205]

In February 2015, a hypothetical cell membrane capable of functioning in liquid methane at cryogenic temperatures (deep freeze) conditions was modeled. Composed of small molecules containing carbon, hydrogen, and nitrogen, it would have the same stability and flexibility as cell membranes on Earth, which are composed of phospholipids, compounds of carbon, hydrogen, oxygen, and phosphorus. This hypothetical cell membrane was termed an "azotosome", a combination of "azote", French for nitrogen, and "liposome". [207] [208]

### Obstacles Edit

Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. At a vast distance from the Sun, Titan is frigid, and its atmosphere lacks CO2. At Titan's surface, water exists only in solid form. Because of these difficulties, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for life than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth. [209] 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. [190] Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiments, and chemical and photochemical modeling on Earth. [192]

### Panspermia hypothesis Edit

It is hypothesized that large asteroid and cometary impacts on Earth's surface may have caused fragments of microbe-laden rock to escape Earth's gravity, suggesting the possibility of panspermia. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan. [210] [211] 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. [212]

### Future conditions Edit

Conditions on Titan could become far more habitable in the far future. Five billion years from now, as the Sun becomes a red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable. [213] 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. [214]

## MAILBOX: Why does Titan have an atmosphere?

"I'm interested in NASA's dragonfly quadcopter mission to Saturn's moon Titan. How can a moon with such a low gravity hold a thicker atmosphere than Earth? Is this the same mechanism to give Venus its thick atmosphere?"

Izzie - Alistair, this is a brilliant question. Titan is Saturn's biggest moon and it's similar in size and mass to our own Moon. The difference is that Titan is much colder, and the colder molecules move more slowly, which makes them easier to hold on to. Titan is minus 100 degrees Celsius. Our moon, however, is about plus 100 degrees Celsius when it's lit by the Sun. So Titan is able to hold onto that atmosphere which the moon can't.

Now looking at Venus, Venus is a much bigger body and has stronger gravity. So even though it's extremely hot, and it’s got these energetic molecules, it's this gravity that means it can hold on to its thick atmosphere. I hope that answers your question.

## Why does Titan have lower surface gravity than the Moon when Titan is more massive? - Astronomy

Mars lost most of its atmosphere long ago to the solar wind due to a lack of a meaningful electromagnetic field. Saturn's Titan has an atmosphere denser than Earth's, yet Titan is a smaller body than Mars. Why hasn't the solar wind carried Titan's atmosphere away?

Roughly speaking, at the distance of Saturn, the solar electromagnetic power per unit area and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases. Titan's surface temperature is also quite low, about 90 K. Therefore, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth.

In fact, current interpretations suggest that only about 70% of Titan's mass is silicates, with the rest consisting primarily of various H2O ices and NH3-H2O (ammonia hydrates). NH3, which may be the original source of Titan's atmospheric N2, may constitute as much as 8% of the NH3-H2O mass.

Much of the original atmosphere appears to have been lost over geologic time. But since Titan began with a proportionally greater volatile budget than Earth or Mars, atmospheric pressure on its surface remains nearly 1.5 times that of Earth's. It is possible that most of the atmospheric loss was within 50 Ma of accretion, from a highly energetic escape of light atoms carrying away a large portion of the atmosphere (hydrodynamic blow off event). Such an event could be driven by heating and photolysis effects of the early Sun's higher output of X-ray and ultraviolet (XUV) photons.

We don't really know why only Titan has a thick atmosphere, while the structurally similar Ganymede and Callisto don't. Temperatures may have been too high (well above

40K) in the Jovian subnebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH3-hydrate inventory accreted by Callisto and Ganymede. The resulting N2 atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood.

Alternatively, cometary impacts may release more energy on Callisto and Ganymede than they do at Titan due to the higher gravitational field of Jupiter. The higher energies could erode the atmospheres of Callisto and Ganymede, while the cometary material would build Titan's atmosphere. Nevertheless, D/H ratios suggest that cometary input is unlikely to be the major contributor to Titan's atmosphere.

As with Mars, Titan's internal magnetic field is negligible, and perhaps even nonexistent. Furthermore, relative speed between Saturn's magnetic field and Titan actually accelerate reactions within Titan's atmosphere, instead of guarding the atmosphere from the solar wind.

If you would like to know the underlying research and more technical details, I have posted them as a co-edited entry online.

#### Suniti Karunatillake

After learning the ropes in physics at Wabash College, IN, Suniti Karunatillake enrolled in the Department of Physics as a doctoral candidate in Aug, 2001. However, the call of the planets, instilled in childhood by Carl Sagan's documentaries and Arthur C. Clarke's novels, was too strong to keep him anchored there. Suniti was apprenticed with Steve Squyres to become a planetary explorer. He mostly plays with data from the Mars Odyssey Gamma Ray Spectrometer and the Mars Exploration Rovers for his thesis project on Martian surface geochemistry, but often relies on the synergy of numerous remote sensing and surface missions to realize the story of Mars. He now works at Stonybrook.

## Why does Titan have lower surface gravity than the Moon when Titan is more massive? - Astronomy

Walter S. Kiefer, Lunar and Planetary Institute, Houston TX

Originally published in Space Science Reference Guide, Second Edition, Lunar and Planetary Institute, 2003. Document compiled July 2002.

Biologists believe that life requires the presence of some sort of liquid to serve as a medium for the chemical reactions needed to sustain life. On Earth, liquid water plays this role. Water has some chemical properties that make it particularly favorable as a medium for life, although we probably should not rule out the possibility that other types of liquid, such as organic liquids, might play this role in other types of biology. If liquids truly are necessary for life, then the potential abodes for life in the outer Solar System are quite limited. Europa and Titan both have been proposed to have oceans and are therefore the best possible candidate locations for life in the outer Solar System.

### Europa

#### 1. Global View

As this global view shows, much of Europa’s surface is covered by a series of dark bands. When studied by NASA’s Voyager spacecraft in 1979, the nature of these bands was enigmatic, but was presumed to reflect some sort of faulting or other type of surface deformation. The virtual absence of impact craters indicates that the surface of Europa is quite young.

#### 2. Wedges Region

Observations by NASA’s Galileo spacecraft since 1996 have provided a much clearer view of Europa. This Galileo image is 230 kilometers across and shows some of the dark bands in greater detail. In some cases, these structures can be seen to be low ridges or pairs of ridges. (You can tell whether a feature is high or low by the nature of the shadow it casts. In this image, the illumination is from the left.) The dark band that originates at the bottom center of the image and runs to the left center is wedge-shaped. This wedge-shaped band probably formed by the gradual spreading of Europa’s surface — think for example of the spreading as a door opens on a hinge. As in image 1, there is a noticeable absence of impact craters.

#### 3. Ice Rafts

This Galileo image is 42 kilometers across and is illuminated from the right. It shows a series of “ice rafts” that have been disrupted and jostled about. Although we saw indications of surface motions in image 2, this image is by far the clearest evidence for large motions of blocks of material across the surface of Europa. When NASA scientists reported in the spring of 1997 that they had evidence of an ocean below the surface of Europa, this image was their “smoking gun”. The ocean interpretation rests on the belief that the existence of so much lateral motion across the surface requires the presence of some sort of layer to lubricate the flow at depth. These scientists assume that this lubrication requires a liquid, and hence favor the existence of an ocean.

As a possible counter-example, consider the physics controlling plate tectonics on Earth. As a general rule, temperature increases with depth inside a planet, and as materials increase in temperature, they tend to become less viscous (less rigid, or more colloquially, softer). The Earth’s surface consists of about 12 large tectonic plates, which move about at speeds of up to 10 centimeters per year, producing all of the earthquakes, volcanic activity, and mountain belt formation that occurs on Earth. These plates move over a mantle which is solid virtually everywhere (we know this because of the way seismic waves travel through the mantle). The “lubrication” that allows all of this motion and geologic activity is actually solid rock that is simply hotter and thus less viscous than the rock above it. The Earth’s example demonstrates that we should consider the possibility that the motion which we see on Europa is lubricated by warm, soft ice rather than by a liquid ocean.

#### 4. Chaos Region

This Galileo image is 175 kilometers across and is illuminated from the left. The major feature is a mitten-shaped region of chaotically disrupted terrain in the center of the image. This chaos region is superimposed on the surrounding plains and ridges, so it must be the youngest feature in this region. Based on the pattern of sunlight and shadows around the edge of the chaos region, the chaos region is slightly elevated compared with the surrounding plains. On the west (left) side of the structure, there is a narrow trough separating the plains from the uplifted chaos terrain. Similar chaos units are found in many parts of Europa. Some scientists believe that these regions form when the subsurface ocean melts through a relatively thin outer ice shell. Other scientists believe that the chaos regions are uplifted and disrupted where a diapir (“blob”) of relatively warm ice rose through the surrounding crust of colder ice. Numerous ridges also cross this image. The relative ages of these ridges can be determined by observing the intersections between ridges (the younger ridge will appear to cut the older ridge).

#### 5. Impact Craters

This image shows four of the largest impact craters found on Europa. Because impact craters excavate into the crust of a planet, they serve as natural core samples into the structure of the upper crust. Generally, the excavation depth of a crater increases as the size of the crater increases. In other words, small craters make shallow holes and larger craters make deeper holes. If an impacting object penetrated all the way through the solid ice shell on Europa to an underlying ocean, the sudden loss of material strength in the crust would cause the crater to collapse (think about the “hole” that is made when you throw a rock into a pond!). Based on the known depths of the largest craters on Europa, it appears that the ice shell of Europa remains solid to a depth of at least 19 to 25 kilometers. The pattern of crater depths as a function of crater diameter suggests that either an ocean or a layer of warm (and thus soft and weak) ice occurs below this depth.

#### 6. Internal Structure

This image shows cross sectional views of Europa’s internal structure. Our current knowledge of the interior of Europa comes from observations of its gravitational and magnetic fields. Europa’s relatively high density of 3.04 grams per cubic centimeter implies that is composed mostly of rock and metal, with relatively little water ice. This material has probably separated into a metal-rich core and a rock-rich mantle, with the core having a radius of 500 to 1000 kilometers. The surface of Europa is known to be predominantly water ice, probably with some rock mixed in, based on spectroscopy studies. This outer shell of water ice is 100 to 200 kilometers thick.

The right side of the image highlights two fundamentally different views about the nature of the ice shell on Europa. The available gravity observations do not indicate whether this layer is entirely solid or if there is a subsurface ocean on Europa. However, magnetic field observations do indicate the presence of an ocean: the salts that would likely be dissolved in such an ocean would be good electrical conductors and hence modify Jupiter’s magnetic field in the vicinity of Europa. This effect has been observed by Galileo and is the strongest present evidence for a subsurface ocean inside Europa. This ocean must be globally distributed. Solid ice and rock can not explain the observed magnetic signature.

The magnetic evidence requires that the ocean be at least 10 kilometers thick, but does not tightly constrain the depth at which this ocean begins. As noted in the captions for other images, geological arguments have been made for both a thin ice layer and a thick ice layer. In the thin ice shell model, the ice shell might be just 1𔃀 kilometers thick. In this model, the ocean might frequently break through to the surface, and the various ridges and faults are assumed to be related to tidal forces in the ocean. In the thick ice shell model, the ocean occurs at much greater depths, at least 20 kilometers beneath the surface. There are scientists who argue with great passion for each model. In my personal view, the cratering evidence (image 5) is a strong constraint favoring a relatively thick ice shell. It is possible that heat pulses do occasionally produce regions with a thin ice shell (for example, the chaos regions shown in image 3 and image 4). However, such regions of thin ice are probably restricted to geographically limited regions and to short intervals of time.

### Future Exploration of Europa

NASA has considered a Europa Orbiter mission that might provide clearer evidence about the nature of Europa’s subsurface ocean. The orbiter would use very long-wavelength radar to attempt to see through the ice to an underlying ocean. A radar flown on the Space Shuttle in 1981 was able to “look” below the Sahara Desert and detect ancient river channels that are now buried under 1 to 2 meters of sand. On Apollo 17, a radar system was able to look through the upper kilometer of rock and image buried lava flows on the Moon. Similar radars are planned for launch to Mars in 2003 and 2005 to image the subsurface distribution of water and ice.

The Europa Orbiter would also carry an altimeter to accurately measure Europa’s shape. The shape changes over the course of an orbit about Jupiter because of tidal deformation. The amount of tidal deformation depends on whether there is an ocean just below the surface or if Europa is solid throughout. Thus, precise measurements of the shape of Europa may provide details about the structure of the subsurface ocean. The Europa Orbiter would also collect additional high resolution images and gravity observations of Europa. Jupiter is surrounded by very strong radiation belts that are dangerous to spacecraft. The Galileo spacecraft dipped deep into the radiation belts for only a few days every few months. In contrast, the Europa Orbiter would be exposed to strong radiation for a much longer period of time. Because of the high cost of designing a spacecraft to endure such radiation (perhaps one billion dollars), the proposed mission is currently on hold.

### Titan

#### 7. Voyager Image of Titan

Titan is the largest of Saturn’s moons. With a radius of 2575 kilometers, it is the second largest moon in the entire Solar System and is larger than the planets Mercury and Pluto. Titan is the only satellite in the Solar System to have a significant atmosphere. At the surface, the atmospheric pressure is 1.6 bars (60% higher than on Earth) and the temperature is a frigid 94 Kelvin. The atmosphere is composed primarily of nitrogen, as on Earth, and also includes some methane and possibly argon. Trace amounts of hydrogen and many organic molecules are also present. Some of these compounds form a thick haze layer in the upper atmosphere of Titan. At visible wavelengths, this haze makes it impossible to see down to the surface of Titan. Ultraviolet radiation from the Sun can break up methane molecules, and the resulting hydrogen atoms can be lost to space. The remnants of the methane can form heavier organic compounds, such as ethane and acetylene. Even at Titan’s cold temperature, ethane is a liquid and might form an ocean on Titan’s surface. Over the age of the Solar System, an ocean of ethane several hundred meters thick might have formed, probably with some methane dissolved in it. The actual distribution of ethane, whether in a surface ocean or in subsurface cavities, is not known at present. Infrared images of Titan obtained by the Hubble Space Telescope show a pattern of bright and dark regions that some scientists think might be related to oceans and continents. Radar observations of Titan also hint at the possibility of oceans and continents.

#### 8. Cassini Probe at Titan

The Cassini spacecraft was launched in October 1997 and will arrive at Saturn in July 2004. In early 2005, a probe will study the composition and physical properties of Titan’s atmosphere and surface. In the event that the probe lands in an ethane ocean, it is able to float. Cassini will also use radar to map parts of Titan’s surface and will also study Saturn’s atmosphere, rings, magnetic field, and other satellites between 2004 and 2008.

### For Further Study

#### Books and Articles

Beatty, J. Kelly, Carolyn Collins Peterson, and Andrew Chaikin, editors, The New Solar System, 4th Edition, Sky Publishing Corp., 1999, 421 pages.

Mackenzie, Dana, "Is There Life Under the Ice?", Astronomy, August 2001, pp. 32-37.

Pappalardo, Robert T., James W. Head, and Ronald Greeley, "The Hidden Ocean of Europa", Scientific American, October 1999, pp. 54-63.

Schenk, Paul, "Oceans, Ice Shells, and Life on Europa", The Planetary Report, November/December 2002, pp. 10-15.

Stone, Richard, "Vostok: Looing for Life Beneath an Antarctic Glacier", Smithsonian, July 2000, pp. 92-102.

## Titan

Titan is the fifteenth of Saturn’s known satellites and the largest:

In Greek mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

Discovered by Christiaan Huygens in 1655.

It was long thought that Titan was the largest satellite in the solar system but recent observations have shown that Titan’s atmosphere is so thick that its solid surface is slightly smaller than Ganymede’s. Titan is nevertheless larger in diameter than Mercury and larger and more massive than Pluto.

Surface viewOne of the principal objectives of the Voyager 1 mission was the study of Titan. Voyager 1 came within 4000 km of the surface. We learned more in the few minutes of that fly-by than in the previous 300 years. Then in late 2004, the Cassini orbiter began a series of close encounters with Titan, taking data with many instruments. And in January 2005, the Huygens probe actually landed on the surface of Titan and sent back images from the surface.

Titan is similar in bulk composition to Ganymede, Callisto, Triton and (probably) Pluto, ie about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn’s moons, it is denser because it is so large that its gravity slightly compresses its interior.

Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure is more than 1.5 bar (50% higher than Earth’s). It is composed primarily of molecular nitrogen (as is Earth’s) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan’s upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started. But it is this thick hazy atmosphere that makes it so hard to see Titan’s surface.

Titan has no magnetic field and sometimes orbits outside Saturn’s magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere. It may also drive some of Titan’s peculiar chemistry.

At the surface, Titan’s temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. Nevertheless, there appears to be a lot of chemistry going on the end result seems to be a lot like a very thick smog.

There are scattered variable clouds in Titan’s atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space. Analysis of the Huygens data will tell us a great deal about the details of the atmospheric chemistry.

Prior to Cassini’s arrival, it seemed likely that the clouds would produce a rain of ethane or methane onto the surface perhaps producing an “ocean” up to 1000 meters deep. However, this seems not to be the case at least at the present time. There is little doubt that some active processes are occuring on Titan there are few if any craters visible indicating that the surface must be very young. But it may be that the “lakes” are more slushy than liquid or that the basins are not filled with liquid at all times. Preliminary results from Huygens indicate that while Titan’s rivers and lakes appear dry at the moment, rain may have occurred not long ago. There is clear evidence for “precipitation, erosion, mechanical abrasion and other fluvial activity”. In addition, Cassini has found evidence of a peculiar kind of volcano on Titan that may account for some of the unusual features of Titan’s atmosphere.

We are beginning to get some understanding of Titan’s surface by combining the data from all the sources available. Large ground based observatories operating in the infra-red can see some details as can the Hubble Space Telescope. These show a huge bright “continent” (preliminarily called “Xanadu“) on the hemisphere of Titan that faces forward in its orbit and some darker regions that are suggestive of oceans or lakes. Cassini’s much higher resolution infrared images (below right, click for animation) show the same structures in more detail. And the close-ups from Huygens (left) show what appear to be drainage channels and shorelines.

These observations also confirm that Titan’s rotation is in fact synchronous like most of Saturn’s other moons.

Cassini’s IR camera has detected a strange and as yet unexplained bright spot on Titan’s surface.

Titan is a difficult object to study. The Cassini instruments are specifically designed to penetrate the haze, its radar mapper can see right thru it and the Huygens images show the surface clearly. But the orbiter images are still frustratingly vague and the Huygens images are few in number and cover only a tiny area. Analysis of this data will take some time Titan is a very strange place.

## Seven Hundred Leagues Beneath Titan’s Methane Seas

Mars, Shmars this voyager is looking forward to a submarine ride under the icebergs on Saturn’s strange moon.

Clouds of methane moving across the far northern regions of Saturn’s largest moon, Titan, in 2016. Video by NASA/JPL-Caltech/Space Science Institute/Univ. of Arizona Credit.

What could be more exciting than flying a helicopter over the deserts of Mars? How about playing Captain Nemo on Saturn’s large, foggy moon Titan — plumbing the depths of a methane ocean, dodging hydrocarbon icebergs and exploring an ancient, frigid shoreline of organic goo a billion miles from the sun?

Those are the visions that danced through my head recently. The eyes of humanity are on Mars these days. A convoy of robots, after a half-year in space, has been dropping, one after another, into orbit or straight to the ground on the Red Planet, like incoming jets at J.F.K. Among the cargo is a helicopter that armchair astronauts look forward to flying over the Martian sands.

But my own attention was diverted to the farther reaches of the solar system by the news that Kraken Mare, an ocean of methane on Titan, had recently been gauged for depth and probably went at least 1,000 feet down. That is as deep as nuclear submarines will admit to going. The news rekindled my dreams of what I think would be the most romantic of space missions: a voyage on, and ultimately even under, the oceans of Titan.

“The depth and composition of each of Titan’s seas had already been measured, except for Titan’s largest sea, Kraken Mare — which not only has a great name but also contains about 80 percent of the moon’s surface liquids,” said Valerio Poggiali, research associate at the Cornell Center for Astrophysics and Planetary Science. Dr. Poggiali is the lead author of a paper describing the new depth measurements in The Journal of the American Geophysical Union.

NASA recently announced that it would launch a drone called Dragonfly to the Saturnian moon in 2026. Proposals have also circulated for an orbiter, a floating probe that could splash down in a lake, even a robotic submarine.

“The Titan submarine is still going,” Dr. Poggiali said in an email, although it is unlikely to happen before Titan’s next summer, around 2047. By then, he said, there will be more ambient light and the submarine conceivably could communicate on a direct line to Earth with no need of an orbiting radio relay.

Titan is the weirdest place in the solar system, in some regards, and also the world most like our own. Like Earth, it has a thick atmosphere of mostly nitrogen (the only moon that has much of an atmosphere at all), and like Earth, it has weather, rain, rivers and seas.