Lakes and rivers of Titan
Lakes of liquid ethane and methane exist on the surface of Titan, Saturn's largest moon. This was confirmed by the Cassini–Huygens space probe, as had been suspected since the 1980s. The large bodies of liquid are known as and the small ones as .
History and discovery
The possibility that there are seas on Titan was first suggested based on data from the Voyager 1 and 2 space probes, which flew past Titan in 1980. The data showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support liquid hydrocarbons. Direct evidence was obtained in 1995 when data from the Hubble Space Telescope and other observations suggested the existence of liquid methane on Titan, either in disconnected pockets or on the scale of satellite-wide oceans, similar to water on Earth.The Cassini mission affirmed the former hypothesis, although not immediately. When the probe arrived in the Saturnian system in 2004, it was hoped that hydrocarbon lakes or oceans might be detectable by reflected sunlight from the surface of any liquid bodies, but no specular reflections were initially observed.
The possibility remained that liquid ethane and methane might be found on Titan's polar regions, where they were expected to be abundant and stable. In Titan's south polar region, an enigmatic dark feature named Ontario Lacus was the first suspected lake identified, possibly created by clouds that are observed to cluster in the area. A possible shoreline was also identified near the pole via radar imagery. Following a flyby on July 22, 2006, in which the Cassini spacecraft's radar imaged the northern latitudes, which were at the time in winter. A number of large, smooth patches were seen dotting the surface near the pole. Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007. The Cassini–Huygens team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found off Earth. Some appear to have channels associated with liquid and lie in topographical depressions. Channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms. Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface and are concentrated near the poles, making Titan much drier than Earth. The high relative humidity of methane in Titan's lower atmosphere could be maintained by evaporation from lakes covering only 0.002–0.02% of the whole surface.
During a Cassini flyby in late February 2007, radar and camera observations revealed several large features in the north polar region interpreted as large expanses of liquid methane and/or ethane, including one, Ligeia Mare, with an area of, slightly larger than Lake Michigan–Huron, the largest freshwater lake on Earth; and another, Kraken Mare, that would later prove to be three times that size. A flyby of Titan's southern polar regions in October 2007 revealed similar, though far smaller, lake-like features.
During a close Cassini flyby in December 2007 the visual and mapping instrument observed a lake, Ontario Lacus, in Titan's south polar region. This instrument identifies chemically different materials based on the way they absorb and reflect infrared light. Radar measurements made in July 2009 and January 2010 indicate that Ontario Lacus is extremely shallow, with an average depth of, and a maximum depth of. It may thus resemble a terrestrial mudflat. In contrast, the northern hemisphere's Ligeia Mare has depths of.
Chemical composition and surface roughness of the lakes
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. It has been estimated that the visible lakes and seas of Titan contain about 300 times the volume of Earth's proven oil reserves. In June 2008, Cassini Visible and Infrared Mapping Spectrometer confirmed the presence of liquid ethane beyond doubt in a lake in Titan's southern hemisphere. 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. 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. Salt-like compounds composed of ammonia and acetylene are also predicted to form. However, the chemical composition and physical properties of the lakes probably varies from one lake to another.No waves were initially detected by Cassini as the northern lakes emerged from winter darkness. This may be either due to low seasonal winds or solidification of hydrocarbons. Titan has several lakes that reside near its northern pole that vary in size, the area these lakes cover and lower wind speeds could as well explain why there were no surface waves being detected. The area over a liquid that wind blows across is known as fetch. The larger this area is, the larger waves become as wind has more area to blow across to transfer energy. The smaller the area of fetch, the smaller waves will be. The optical properties of solid methane surface 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. 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. Temperatures close to the freezing point of methane 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 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. Bubble outburst events may also influence the formation of Titan's river deltas. An alternative explanation is the transient features in Cassini VIMS near-infrared data may be shallow, wind-driven capillary waves moving at about and at heights of about. Post-Cassini analysis of VIMS data suggests tidal currents may also be responsible for the generation of persistent waves in narrow channels of Kraken Mare.
Cyclones driven by evaporation and involving rain as well as gale-force winds of up to are expected to form over the large northern seas only in northern summer during 2017, lasting up to ten days. However, a 2017 analysis of Cassini data from 2007 to 2015 indicates waves across these three seas were diminutive, reaching only about high and 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. 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.
Observation of specular reflections
On 21 December 2008, Cassini passed directly over Ontario Lacus at an altitude of 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 over a first Fresnel zone reflecting area only wide. 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.On 8 July 2009, Cassini Visual and Infrared Mapping Spectrometer observed a specular reflection in 5 μm infrared light off a northern hemisphere body of liquid at This has been described as at the southern shoreline of Kraken Mare, but on a combined radar-VIMS image the location is shown as a separate lake. 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°.