Lakes of Titan

According to Cassini data, scientists announced on February 13, 2008, that Titan hosts within its polar lakes "hundreds of times more natural gas and other liquid hydrocarbons than all the known oil and natural gas reserves on Earth." The desert sand dunes along the equator, while devoid of open liquid, nonetheless hold more organics than all of Earth's coal reserves.[18] It has been estimated that the visible lakes and seas of Titan contain about 300 times the volume of Earth's proven oil reserves.[19] In June 2008, Cassini's Visible and Infrared Mapping Spectrometer confirmed the presence of liquid ethane beyond doubt in a lake in Titan's southern hemisphere.[20] The exact blend of hydrocarbons in the lakes is unknown. According to a computer model, 3/4 of an average polar lake is ethane, with 10 percent methane, 7 percent propane and smaller amounts of hydrogen cyanide, butane, nitrogen and argon.[21] Benzene is expected to fall like snow and quickly dissolve into the lakes, although the lakes may become saturated just as the Dead Sea on Earth is packed with salt. The excess benzene would then build up in a mud-like sludge on the shores and on the lake floors before eventually being eroded by ethane rain, forming a complex cave-riddled landscape.[22] Salt-like compounds composed of ammonia and acetylene are also predicted to form.[23] However, the chemical composition and physical properties of the lakes probably varies from one lake to another (Cassini observations in 2013 indicate Ligeia Mare is filled with a ternary mixture of methane, ethane, and nitrogen and consequently the probe's radar signals were able to detect the sea floor 170 m [560 ft] below the liquid surface).[24]

No waves were initially detected by Cassini as the northern lakes emerged from winter darkness (calculations indicate wind speeds of less than 1 meter per second [2.2 mph] should whip up detectable waves in Titan's ethane lakes but none were observed). This may be either due to low seasonal winds or solidification of hydrocarbons. The optical properties of solid methane surface (close to the melting point) are quite close to the properties of liquid surface however the viscosity of solid methane, even near the melting point, is many orders of magnitude higher, which might explain extraordinary smoothness of the surface.[25] Solid methane is denser than liquid methane so it will eventually sink. It is possible that the methane ice could float for a time as it probably contains bubbles of nitrogen gas from Titan's atmosphere.[26] Temperatures close to the freezing point of methane (90.4 K, −182.8 °C, −296.9 °F) could lead to both floating and sinking ice - that is, a hydrocarbon ice crust above the liquid and blocks of hydrocarbon ice on the bottom of the lake bed. The ice is predicted to rise to the surface again at the onset of spring before melting.

Since 2014, Cassini has detected transient features in scattered patches in Kraken Mare, Ligeia Mare and Punga Mare. Laboratory experiments suggest these features (e.g. RADAR-bright "magic islands")[27] might be vast patches of bubbles caused by the rapid release of nitrogen dissolved in the lakes. Bubble outburst events are predicted to occur as the lakes cool and subsequently warm or whenever methane-rich fluids mix with ethane-rich ones due to heavy rainfall.[28][29] Bubble outburst events may also influence the formation of Titan's river deltas.[29] An alternative explanation is the transient features in Cassini VIMS near-infrared data may be shallow, wind-driven capillary waves (ripples) moving at about 0.7 m/s (1.6 mph) and at heights of about 1.5 centimeters (0.59 in).[30][31][32] Post-Cassini analysis of VIMS data suggests tidal currents may also be responsible for the generation of persistent waves in narrow channels (Freta) of Kraken Mare.[32]

Cyclones driven by evaporation and involving rain as well as gale-force winds of up to 20 m/s (72 km/h; 45 mph) are expected to form over the large northern seas only (Kraken Mare, Ligeia Mare, Punga Mare) in northern summer during 2017, lasting up to ten days.[33] However, a 2017 analysis of Cassini data from 2007 to 2015 indicates waves across these three seas were diminutive, reaching only about 1 centimeter (0.39 in) high and 20 centimeters (7.9 in) long. The results call into question the early summer's classification as the beginning of the Titan's windy season, because high winds probably would have made for larger waves.[34] A 2019 theoretical study concluded that it is possible that the relatively dense aerosols raining down on Titan's lakes may have liquid-repelling properties, forming a persistent film on the surface of the lakes which then would inhibit formation of waves larger than a few centimetres in wavelength.[35]

Near-infrared radiation from the Sun reflecting off Titan's hydrocarbon seas.

On 21 December 2008, Cassini passed directly over Ontario Lacus at an altitude of 1,900 km (1,200 mi) and was able to observe specular reflection in radar observations. The signals were much stronger than anticipated and saturated the probe's receiver. The conclusion drawn from the strength of the reflection was that the lake level did not vary by more than 3 mm (0.12 in) over a first Fresnel zone reflecting area only 100 m (330 ft) wide (smoother than any natural dry surface on Earth). From this it was surmised that surface winds in the area are minimal at that season and/or the lake fluid is more viscous than expected.[36][37]

On 8 July 2009, Cassini's Visual and Infrared Mapping Spectrometer (VIMS) observed a specular reflection in 5 µm infrared light off a northern hemisphere body of liquid at 71° N, 337° W. This has been described as at the southern shoreline of Kraken Mare,[38] but on a combined radar-VIMS image the location is shown as a separate lake (later named Jingpo Lacus). The observation was made shortly after the north polar region emerged from 15 years of winter darkness. Because of the polar location of the reflecting liquid body, the observation required a phase angle close to 180°.[39]

The discoveries in the polar regions contrast with the findings of the Huygens probe, which landed near Titan's equator on January 14, 2005. The images taken by the probe during its descent showed no open areas of liquid, but strongly indicated the presence of liquids in the recent past, showing pale hills crisscrossed with dark drainage channels that lead into a wide, flat, darker region. It was initially thought that the dark region might be a lake of a fluid or at least tar-like substance, but it is now clear that Huygens landed on the dark region, and that it is solid without any indication of liquids. A penetrometer studied the composition of the surface as the craft impacted it, and it was initially reported that the surface was similar to wet clay, or perhaps crème brûlée (that is, a hard crust covering a sticky material). Subsequent analysis of the data suggests that this reading was likely caused by Huygens displacing a large pebble as it landed, and that the surface is better described as a "sand" made of ice grains.[40] The images taken after the probe's landing show a flat plain covered in pebbles. The pebbles may be made of water ice and are somewhat rounded, which may indicate the action of fluids.[41] Thermometers indicated that heat was wicked away from Huygens so quickly that the ground must have been damp, and one image shows light reflected by a dewdrop as it falls across the camera's field of view. On Titan, the feeble sunlight allows only about one centimeter of evaporation per year (versus one meter of water on Earth), but the atmosphere can hold the equivalent of about 10 meters (33 ft) of liquid before rain forms (versus about 2 cm [0.79 in] on Earth). So Titan's weather is expected to feature downpours of several meters (15–20 feet) causing flash floods, interspersed by decades or centuries of drought (whereas typical weather on Earth includes a little rain most weeks).[42] Cassini has observed equatorial rainstorms only once since 2004. Despite this, a number of long-standing tropical hydrocarbon lakes were unexpectedly discovered in 2012[43] (including one near the Huygens landing site in the Shangri-La region which is about half the size of Utah's Great Salt Lake, with a depth of at least 1 meter [3'4"]). As on Earth, the likely supplier is probably underground aquifers, in other words the arid equatorial regions of Titan contain "oases".[44]

Rimmed lakes of Titan
(artist concept)

Evolving feature in Ligeia Mare

Models of oscillations in Titan's atmospheric circulation suggest that over the course of a Saturnian year, liquid is transported from the equatorial region to the poles, where it falls as rain. This might account for the equatorial region's relative dryness.[45] According to a computer model, intense rainstorms should occur in normally rainless equatorial areas during Titan's vernal and autumnal equinoxes—enough liquid to carve out the type of channels that Huygens found.[46] The model also predicts energy from the Sun will evaporate liquid methane from Titan's surface except at the poles, where the relative absence of sunlight makes it easier for liquid methane to accumulate into permanent lakes. The model also apparently explains why there are more lakes in the northern hemisphere. Due to the eccentricity of Saturn's orbit, the northern summer is longer than the southern summer and consequently the rainy season is longer in the north.

However, recent Cassini observations (from 2013) suggest geology may also explain the geographic distribution of the lakes and other surface features. One puzzling feature of Titan is the lack of impact craters at the poles and mid-latitudes, particularly at lower elevations. These areas may be wetlands fed by subsurface ethane and methane springs.[47] Any crater created by meteorites is thus quickly subsumed by wet sediment. The presence of underground aquifers could explain another mystery. Titan's atmosphere is full of methane, which according to calculations should react with ultraviolet radiation from the sun to produce liquid ethane. Over time, the moon should have built up an ethane ocean hundreds of meters (1,500 to 2,500 feet) deep instead of only a handful of polar lakes. The presence of wetlands would suggest that the ethane soaks into the ground, forming a subsurface liquid layer akin to groundwater on Earth. A possibility is that the formation of materials called clathrates changes the chemical composition of the rainfall runoff that charges the subsurface hydrocarbon "aquifers." This process leads to the formation of reservoirs of propane and ethane that may feed into some rivers and lakes. The chemical transformations taking place underground would affect Titan's surface. Lakes and rivers fed by springs from propane or ethane subsurface reservoirs would show the same kind of composition, whereas those fed by rainfall would be different and contain a significant fraction of methane.[48]

All but 3% of Titan's lakes have been found within a bright unit of terrain covering about 900 by 1,800 kilometers (560 by 1,120 mi) near the north pole. The lakes found here have very distinctive shapes—rounded complex silhouettes and steep sides—suggesting deformation of the crust created fissures that could be filled up with liquid. A variety of formation mechanisms have been proposed. The explanations range from the collapse of land after a cryovolcanic eruption to karst terrain, where liquids dissolve soluble ice.[49] Smaller lakes (up to tens of miles across) with steep rims (up to hundreds of feet high) might be analogous to maar lakes, i.e. explosion craters subsequently filled with liquid. The explosions are proposed to result from fluctuations in climate, which lead to pockets of liquid nitrogen accumulating within the crust during colder periods and then exploding when warming caused the nitrogen to rapidly expand as it shifted to a gas state.[50][51][52]