Indeed, were it not for its lack of an atmosphere

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.

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.^[93] TOSHIBA PK13AT10600 laptop keyboard

 Prior to the arrival of Cassini, these regions were thought to be seas of organic matter like tar or liquid hydrocarbons.^[94] Radar images captured by the Cassini spacecraft have instead revealed some of these regions to be extensive plains covered in longitudinal sand dunes, up to 330 meters high^[95] about a kilometer wide, and tens to hundreds of kilometers long.COMPAQ Presario CQ50-210US laptop keyboard

The longitudinal (or linear) dunes are presumed to be formed by moderately variable winds that either follow one mean direction or alternate between two different directions. 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).^[97] DELL V119525BS1 laptop keyboard

 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 causes sand dunes to build up in long parallel lines aligned west-to-east. The dunes break up around mountains, where the wind direction shifts.  TOSHIBA 9J.N9282.W01 laptop keyboard

The sand on Titan is likely not made up of small grains of silicates like the sand on Earth,^[98] but rather might have formed when liquid methane rained and eroded the ice bedrock, possibly in the form of flash floods. Alternatively, the sand could also have come from organic solids produced by photochemical reactions in Titan's atmosphere.^[ SONY KFRMBA151B laptop keyboard

Studies of dunes' composition in May 2008 revealed that they possessed less water than the rest of Titan, and are most likely to derive from organic material clumping together after raining onto the surface.

Titan is never visible to the naked eye, but can be observed through small telescopes or strong binoculars. TOSHIBA Satellite l25-s1196 laptop keyboard

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.^[101] Titan has a maximumapparent magnitude of +8.2,^[5] and mean opposition magnitude 8.4.^[102]  HP Pavilion DV6-1205ee laptop keyboard

This compares to +4.6^[102] for the similarly sized Ganymede, in the Jovian system.

Observations of Titan prior to the space age were limited. In 1907 Spanish astronomer Josep Comas Solá observed limb darkeningof 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.^[103] SAMSUNG Q320 laptop keyboard

The first probe to visit the Saturnian system was Pioneer 11 in 1979, which confirmed that Titan was probably too cold to support life.^[104] It took images of Titan, including Titan and Saturn together in mid to late 1979.^[105] The quality was soon surpassed by the two Voyagers, but Pioneer 11 provided data for everyone to prepare with. TOSHIBA Satellite M645-S4070 Laptop Keyboard

Titan was examined by both Voyager 1 and 2 in 1980 and 1981, respectively. Voyager 1's course was diverted specifically to make a closer pass of Titan. Unfortunately, the craft did not possess any instruments that could penetrate Titan's haze, an unforeseen factor. Many years later,  HP Probook 4515S Laptop Keyboard

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,^[106] but by then they had already been observed in the infrared by the Hubble Space Telescope. Voyager 2 took only a cursory look at Titan. SONY VAIO VGN-AR630E Laptop Keyboard

The Voyager 2 team had the option of steering the spacecraft to take a detailed look at Titan or to use another trajectory that would allow it to visit Uranus and Neptune. Given the lack of surface features seen by Voyager 1, the latter plan was implemented.

Even with the data provided by the Voyagers, SONY VAIO PCG-FR315M Laptop Keyboard

Titan remained a body of mystery—a planet-like satellite shrouded in an atmosphere that makes detailed observation difficult. The intrigue that had surrounded Titan since the 17th-century observations of Christiaan Huygens and Giovanni Cassini was gratified by a spacecraft named in their honor. DELL Latitude D620 Laptop Keyboard

The Cassini–Huygens spacecraft reached Saturn on July 1, 2004, and has begun the process of mapping Titan's surface by radar. A joint project of the European Space Agency (ESA) and NASA, Cassini–Huygens has 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, DELL Latitude D620 Laptop Keyboard

at only 1,200 km, discerning patches of light and dark that would be invisible to the human eye from the Earth. Huygens landed^[107] on Titan on January 14, 2005, discovering that many of its surface features seem to have been formed by flowing fluids at some point in the past.^[108] On July 22, 2006, Cassinimade its first targeted, SAMSUNG N150 Laptop Keyboard

close fly-by at 950 km from Titan; the closest flyby was at 880 km on June 21, 2010.^[109] Present liquid on the surface has been found in abundance in the north polar region, in the form of many lakes and seas discovered by Cassini.^[60]Titan is the most distant body from Earth^[110] and the second moon in the Solar System to have a space probe land on its surface. HP Pavilion G6-1B59WM Laptop Keyboard

On January 14, 2005, the Huygens probe landed on the surface of Titan, 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. HP G62-367DX Laptop Keyboard

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.^[111]

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.^[111DELL V119525BS1 Laptop Keyboard

 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. SONY VAIO VGN-N250E/B laptop keyboard

The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. The assumption is that the "soil" visible in the images is precipitation from the hydrocarbon haze above.

In March 2007, NASA, ESA, and COSPAR decided to name the Huygens landing site the Hubert Curien Memorial Stationin memory of the former president of the ESA. HP Pavilion DV7-3067nr laptop keyboard

The Titan Saturn System Mission (TSSM) is a joint NASA/ESA proposal for exploration of Saturn's moons.^[113] 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. SONY Vaio PCG-K315S laptop keyboard

In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of the TSSM,^[114] although TSSM was still considered for a later launch date. Since NASA's departure from the program in 2012, these plans were put on hold.

There has also been a proposal for a Titan Mare Explorer (TiME), TOSHIBA Satellite L755-S5216 laptop keyboard

which would be a low-cost lander that would splash down in a lake near Titan's north pole and float on the surface of the lake for 3 to 6 months. It could launch as early as 2016 and arrive in 2023.^{[115][116][117]} In 2012, however, NASA chose to fund the Mars probe InSight instead of TiME, rendering the Titan probe's future uncertain.  HP Pavilion DV6z artist edition 2 laptop keyboard

Another lake lander project was proposed in late 2012 in Europe. The concept probe is called Titan Lake In-situ Sampling Propelled Explorer (TALISE).^[118][119] The major difference with the TiME probe would be a propulsion system.

Another proposed mission to Titan is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR),  HP Pavilion dv6-3142se laptop keyboard

which is an unmanned plane (or drone) which would fly through Titan's atmosphere and take high-definition images of the surface of Titan.

Titan is thought to be a prebiotic environment rich in complex organic chemistry^[39] with a possible subsurface liquid ocean serving as a biotic environment. TOSHIBA Satellite P300-ST3712 laptop keyboard

Although the Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organic compounds, it showed an environment on Titan that is similar, in some ways, to ones theorized for the primordial Earth.^[126] 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. HP G42-265LA laptop keyboard

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.^[128] SONY VAIO VGN-FS620 laptop keyboard

In October 2010, Sarah Horst of the University of Arizona reported finding the five nucleotide bases—building blocks of DNA and RNA—among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Horst also found amino acids, the building blocks of protein. HP Pavilion dv3-2155mx laptop keyboard

She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.^[129]

On April 3, 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan. TOSHIBA PK130BH2A00 laptop keyboard

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. Although the analogy assumes the presence of liquid water for longer periods than is currently observable, several theories suggest that liquid water from an impact could be preserved under a frozen isolation layer.^[130] SONY VAIO VGN-CR123E laptop keyboard

 It has also been theorized that liquid-ammonia oceans could exist deep below the surface;^[123][131] Another model suggests an ammonia–water solution as much as 200 km deep beneath a water-ice crust with conditions that, although extreme by terrestrial standards, are such that life could indeed survive.^[124] Lenovo ThinkPad T510-4339 laptop keyboard

Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life.^[123] Detection of microbial life on Titan would depend on its biogenic effects. That the atmospheric methane and nitrogen might be of biological origin has been examined, for example. HP Pavilion dv6-2130ca laptop keyboard

It has been suggested that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water.^[132] Such creatures would inhale H₂ in place of O₂, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide.^[132][133]

Although all living things on Earth (including methanogens) use liquid water as a solvent,

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it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane.^[134] Water is a stronger solvent than methane.^[135] However, water is also more chemically reactive, and can break down large organic molecules through hydrolysis.^[134] A life-form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.^[134] HP Pavilion DV6z artist edition 2 Laptop Keyboard

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.^[132]

In 2010, Darrell Strobel, from Johns Hopkins University, identified a greater abundance of molecular HP G42-364LA Laptop Keyboard

hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward flow at a rate of roughly 10²⁵ molecules per second and disappearance of hydrogen near Titan's surface; as Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present.^[132 DELL Vostro PP37L Laptop Keyboard

The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms consuming hydrocarbons.^[135] Although restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are more likely:

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the possibilities of yet unidentified physical or chemical processes (e.g., a surface catalyst accepting hydrocarbon or hydrogen), or flaws in the current models of material flow.^[137] Composition data and transport models need to be substantiated, and, per Occam's razor, a physical or chemical explanation is preferred a priori over one of biology (given the simplicity of chemical catalysts versus the complexity of biological forms). HP G42-364LA Laptop Keyboard

Even so, McKay notes that discovery of either, including simply of a catalyst effective at 95 K (−180 °C), would be an important discovery.^[125]

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".^[135]  TOSHIBA 9J.N7482.901 Laptop Keyboard

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."

Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. TOSHIBA Satellite U305-S2804 Laptop Keyboard

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 theories on the conditions that prevailed prior to the appearance of life on Earth.^[138] HP 597635-001 Laptop Keyboard

 Although life itself may not exist, the prebiotic conditions of the Titanian environment and the associated organic chemistry remain of great interest in understanding the early history of the terrestrial biosphere.^[126] Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiment, and chemical and photochemical modeling on Earth. Compaq Presario CQ57 Series Laptop Keyboard

It is hypothesized that large asteroid and cometary impacts on Earth's surface may have caused fragments of microbe-laden rock to escape Earth's gravity, suggesting the possibility of transpermia. Calculations indicate that a number of these would encounter many of the bodies in the Solar System, including Titan.^[139][140] DELL Vostro 1014 Laptop Keyboard

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.

Conditions on Titan could become far more habitable in the far future. Lenovo 3000 N200 Laptop Keyboard

Five billion years from now, as the Sun becomes a red giant, surface temperatures could rise enough for Titan to support liquid water on its surface making it habitable.^[142] 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.  HP Pavilion zv6223ea Laptop Keyboard

These conditions together could create a habitable environment, and could persist for several hundred million years. This was sufficient time for simple life to evolve on Earth, although the presence of ammonia on Titan would cause chemical reactions to proceed more slowly.

According to Cassini data from 2008, Titan has hundreds of times more liquid hydrocarbons than all the known oil and natural gas reserves on Earth.  Lenovo Thinkpad T520 Laptop Keyboard

These hydrocarbons rain from the sky and collect in vast deposits that form lakes and dunes.^[1] "Titan is just covered in carbon-bearing material—it’s a giant factory of organic chemicals," said Ralph Lorenz, who leads the study of Titan based on radar data from Cassini. “This vast carbon inventory is an important window into the geology and climate history of Titan.” ACER Aspire 4736Z laptop keyboard

Several hundred lakes and seas have been observed, with several dozen estimated to contain more hydrocarbon liquid than Earth's oil and gas reserves. The dark dunes that run along the equator contain a volume of organics several hundred times larger than Earth's coal reserves.  HP Pavilion DV7t-3100 laptop keyboard

Radar images obtained on July 21, 2006 appear to show lakes of liquid hydrocarbon (such as methane and ethane) in Titan's northern latitudes. This is the first discovery of currently existing lakes beyond Earth. The lakes range in size from about a kilometer in width to one hundred kilometers across.^[3] HP Pavilion dv6-2150es laptop keyboard

On March 13, 2007, JPL announced that it found strong evidence of seas of methane and ethane in the northern hemisphere. At least one of these is larger than any of the Great Lakes in North America.

The American aerospace engineer and author Robert Zubrin identified Saturn as the most important and valuable of the four gas giants in the solar system, HP Pavilion dv6-3107ax laptop keyboard

because of its relative proximity, low radiation, and excellent system of moons. He also named Titan as the most important moon on which to establish a base to develop the resources of the Saturn system.

Robert Zubrin has pointed out that Titan possesses an abundance of all the elements necessary to support life, saying "In certain ways,  SONY VAIO VGN-FS415E laptop keyboard