Titan (moon)

Lakes and seas

Following the Voyager flybys, Titan was confirmed to have an atmosphere capable of supporting liquid hydrocarbons on its surface. However, the first tentative detection only came in 1995, when data from the Hubble Space Telescope and radar observations suggested expansive hydrocarbon lakes, seas, or oceans.^[91] The existence of liquid hydrocarbons on Titan was finally confirmed in situ by the Cassini orbiter, with the Cassini mission team announcing "definitive evidence of the presence of lakes filled with liquid methane on Saturn's moon Titan" in January 2007.^[92][93]

The observed lakes and seas of Titan are largely restricted to its polar regions, where colder temperatures allow the presence of permanent liquid hydrocarbons.^[94]: 58 Near Titan's north pole are Kraken Mare, the largest sea; Ligeia Mare, the second-largest sea; and Punga Mare, each filling broad depressions and cumulatively representing roughly 80% of Titan's sea and lake coverage— 691,000 km² (267,000 sq mi) combined.^[a] All three maria's sea levels are similar, suggesting that they may be hydraulically connected. The southern polar region, meanwhile, hosts four dry broad depressions, potentially representing dried-up seabeds. Additional smaller lakes occupy Titan's polar regions, covering a cumulative surface area of 215,000 km² (83,000 sq mi). Lakes in Titan's lower-latitude and equatorial regions have been proposed, though none have been confirmed; seasonal or transient equatorial lakes may pool following large rainstorms.^[94]: 60 Cassini RADAR data has been used to conduct bathymetry of Titan's seas and lakes. Using detected subsurface reflections, the measured maximum depth of Ligeia Mare is roughly 200 metres (660 ft), and that of Ontario Lacus is roughly 90 metres (300 ft).^{[94]: 67–70}

Titan's lakes and seas are dominated by methane (CH₄), with smaller amounts of ethane (C₂H₆) and dissolved nitrogen (N₂). The fraction of these components varies across different bodies: observations of Ligeia Mare are consistent with 71% CH₄, 12% C₂H₆, and 17% N₂ by volume; whilst Ontario Lacus is consistent with 49% CH₄, 41% C₂H₆, and 10% N₂ by volume. As Titan is synchronously locked with Saturn, there exists a permanent tidal bulge of roughly 100 metres (330 ft) at the sub- and anti-Saturnian points. Titan's orbital eccentricity means that tidal acceleration varies by 9%, though the long orbital period means that these tidal cycles are very gradual.^{[94]: 70–71} A team of researchers led by Ralph D. Lorenz evaluated that the tidal range of Titan's major seas are around 0.2–0.8 metres (0.66–2.62 ft).^[95]: 12

Tectonics and cryovolcanism

Through Cassini RADAR mapping of Titan's surface, numerous landforms have been interpreted as candidate cryovolcanic and tectonic features by multiple authors.^[96]: 14 A 2016 analysis of mountainous ridges on Titan revealed that ridges are concentrated in Titan's equatorial regions, implying that ridges either form more frequently in or are better preserved in low-latitude regions. The ridges—primarily oriented east to west—are linear to arcuate in shape, with the authors of the analysis comparing them to terrestrial fold belts indicative of horizontal compression or convergence. They note that the global distribution of Titan's ridges could be indicative of global contraction, with a thickened ice shell causing regional uplift.^{[96]: 23–25}

The identification of cryovolcanic features on Titan remains controversial and inconclusive, primarily due to limitations of Cassini imagery and coverage. Cassini RADAR and VIMS imagery revealed several candidate cryovolcanic features, particularly flow-like terrains in western Xanadu and steep-sided lakes in the northern hemisphere that resemble maar craters on Earth, which are created by explosive subterranean eruptions. The likeliest cryovolcano features is a complex of landforms that includes two mountains, Doom Mons and Erebor Mons; a large depression, Sotra Patera; and a system of flow-like features, Mohini Fluctus. Between 2005 and 2006, parts of Sotra Patera and Mohini Fluctus became significantly brighter whilst the surrounding plains remained unchanged, potentially indicative of ongoing cryovolcanic activity.^{[97]: 21–23} Indirect lines of evidence for cryovolcanism include the presence of Argon-40 in Titan's atmosphere. Radiogenic ⁴⁰Ar is sourced from the decay of ⁴⁰K, and has likely been produced within Titan over the course of billions of years within its rocky core. ⁴⁰Ar's presence in Titan's atmosphere is thus supportive of active geology on Titan, with cryovolcanism being one possible method of bringing the isotope up from the interior.^[98]

Impact craters

Titan's surface has comparatively few impact craters, with erosion, tectonics, and cryovolcanism possibly working to erase them over time.^[80] Compared to the craters of similarly sized and structured Ganymede and Callisto, those of Titan are much shallower. Many have dark floors of sediment; geomorphological analysis of impact craters largely suggests that erosion and burial are the primary mechanisms of crater modification.^[99]: 2 Titan's craters are also not evenly distributed, as the polar regions are almost devoid of any identified craters whilst the majority are located in the equatorial dune fields. This inequality may be the result of oceans that once occupied Titan's poles, polar sediment deposition by past rainfall, or increased rates of erosion in the polar regions.^[97]: 19

Plains and dunes

The majority of Titan's surface is covered by plains. Of the several types of plains observed, the most extensive are the Undifferentiated Plains that encompass vast, radar-dark uniform regions.^[97]: 15 These mid-latitude plains—located largely between 20 and 60° north or south—appear younger than all major geological features except dunes and several craters.^[100]: 177 The Undifferentiated Plains likely were formed by wind-driven processes and composed of organic-rich sediment.^[100]: 180

Another extensive type of terrain on Titan are sand dunes, grouped together into vast dune fields or "sand seas" located within 30° north or south. Titanian dunes are typically 1–2 km (0.62–1.24 mi) wide and spaced 1–4 (0.62–2.49 mi) apart, with some individual dunes over 100 km (62 mi) in length. Limited radar-derived height data suggests that the dunes are 80–130 metres (260–430 ft) tall, with the dunes appearing dark in Cassini SAR imagery. Interactions between the dunes and obstacle features, such as mountains, indicate that sand is generally transported in a west-to-east direction. The sand that constructs the dunes is dominated by organic material, probably from Titan's atmosphere; possible sources of sand include river channels or the Undifferentiated Plains.^{[97]: 16–18}

Pioneer and Voyager

The first probe to visit the Saturnian system was Pioneer 11 in 1979, which revealed 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.^[106]

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.^[107] 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,^[108] 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.^[107]: 94

Cassini–Huygens

The Cassini–Huygens spacecraft reached Saturn on July 1, 2004,^[109] and began the process of mapping Titan's surface by radar.^[110] A joint project of the European Space Agency (ESA) and NASA, Cassini–Huygens proved a very successful mission.^[111] 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 km (750 mi) , discerning patches of light and dark that would be invisible to the human eye.^[35]

On July 22, 2006, Cassini made its first targeted, close fly-by at 950 km (590 mi) from Titan; the closest flyby was at 880 km (550 mi) on June 21, 2010.^[112] 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.^[113]

Huygens landing

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

Same image with contrast enhanced

Huygens was an atmospheric probe that touched down on Titan on January 14, 2005,^[114] discovering that many of its surface features seem to have been formed by fluids at some point in the past.^[115] Titan is the most distant body from Earth to have a space probe land on its surface.^[116]

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.^[117]

After landing, Huygens photographed a dark plain covered in small rocks and pebbles, which are composed of water ice.^[117] 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.^[118]

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.^[119]

Dragonfly

The Dragonfly mission, developed and operated by the Johns Hopkins Applied Physics Laboratory, will launch in July 2028.^[120] It consists of a large drone powered by an RTG to fly in the atmosphere of Titan as New Frontiers 4.^[121][122] Its instruments will study how far prebiotic chemistry may have progressed.^[123] The mission is planned to arrive at Titan in the mid-2030s.^[122]

Proposed or conceptual missions

There have been several conceptual missions proposed in recent years for returning a robotic space probe to Titan. Initial conceptual work has been completed for such missions by NASA (and JPL), and ESA. At present, none of these proposals have become funded missions. The Titan Saturn System Mission (TSSM) was a joint NASA/ESA proposal for exploration of Saturn's moons.^[124] 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.^[125] The proposed Titan Mare Explorer (TiME) was a low-cost lander that would splash down in Ligeia Mare in Titan's northern hemisphere. The probe would float whilst investigating Titan's hydrocarbon cycle, sea chemistry, and Titan's origins.^[126] It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity,^[127] but was not selected for flight.^[128]

Another mission to Titan proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho, is the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR): an uncrewed plane (or drone) that would fly through Titan's atmosphere and take high-definition images of the surface of Titan. NASA did not approve the requested $715 million, and the future of the project is uncertain.^[129][130]

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).^[131] 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.^[131]

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.^[132][133]

In 2015, the NASA Innovative Advanced Concepts program (NIAC) awarded a Phase II grant^[134] to a design study of a Titan Submarine to explore the seas of Titan.^{[135][136][137]}

Formation of complex molecules

The Miller–Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.^[145]

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.^[146]

Possible subsurface habitats

Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. The analogy assumes the presence of liquid water for longer periods than is currently observable; several hypotheses postulate that liquid water from an impact could be preserved under a frozen isolation layer.^[147] It has also been hypothesized that liquid-ammonia oceans could exist deep below the surface.^[140][148] Another model suggests an ammonia–water solution as much as 200 km (120) deep beneath a water-ice crust with conditions that, although extreme by terrestrial standards, are such that life could survive.^[141] Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life.^[140] Detection of microbial life on Titan would depend on its biogenic effects, with the atmospheric methane and nitrogen examined.^[141]

Methane and life at the surface

It has been speculated that life could exist in the lakes of liquid methane on Titan, just as organisms on Earth live in water.^[149] Such organisms would inhale H₂ in place of O₂, metabolize it with acetylene instead of glucose, and exhale methane instead of carbon dioxide.^[142][149] However, such hypothetical organisms would be required to metabolize at a deep freeze temperature of −179.2 °C (−290.6 °F; 94.0 K).^[139]

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,^[150] although water is a stronger solvent than methane.^[151] Water is also more chemically reactive, and can break down large organic molecules through hydrolysis.^[150] A life form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.^[150]

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.^[149]

In 2010, Darrell Strobel, from Johns Hopkins University, identified a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward flow at a rate of roughly 10²⁸ molecules per second and disappearance of hydrogen near Titan's surface; as Strobel noted, his findings were in line with the effects McKay had predicted if methanogenic life-forms were present.^{[149][151][152]} 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.^[151] 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.^[142] 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.^[142] With regards to the acetylene findings, Mark Allen, the principal investigator with the NASA Astrobiology Institute Titan team, provided a speculative, non-biological explanation: sunlight or cosmic rays could transform the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.^[153]

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."^[151] 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."^[151]

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".^[154][155]

Obstacles

Despite these biological possibilities, there are formidable obstacles to life on Titan, and any analogy to Earth is inexact. At a vast distance from the Sun, Titan is frigid, and its atmosphere lacks 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.^[156] 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.^[143] Using Titan as a prebiotic experiment involves not only observation through spacecraft, but laboratory experiments, and chemical and photochemical modeling on Earth.^[145]

Panspermia hypothesis

It is hypothesized that large asteroid and cometary impacts on Earth's surface may have caused fragments of microbe-laden rock to escape Earth's gravity, suggesting the possibility of panspermia. Calculations indicate that these would encounter many of the bodies in the Solar System, including Titan.^[157][158] 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.^[159]

Future conditions

Conditions on Titan could become far more habitable in the far future. Five billion years from now, as the Sun becomes a sub-red giant, its surface temperature could rise enough for Titan to support liquid water on its surface, making it habitable.^[160] As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, lessening the anti-greenhouse effect on the surface and enabling the greenhouse created by atmospheric methane to play a far greater role. These conditions together could create a habitable environment, and could persist for several hundred million years. This is proposed to have been sufficient time for simple life to spawn on Earth, though the higher viscosity of ammonia-water solutions coupled with low temperatures would cause chemical reactions to proceed more slowly on Titan.^[161]