Lakes of Titan

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.

[7] 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.

[10] Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007.

[7][11] 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.

[12] 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.

[14] 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 126,000 km2 (49,000 sq mi), 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.

[15] During a close Cassini flyby in December 2007 the visual and mapping instrument observed a lake, Ontario Lacus, in Titan's south polar region.

The desert sand dunes along the equator, while devoid of open liquid, nonetheless hold more organics than all of Earth's coal 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.

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.

Laboratory experiments suggest these features (e.g. RADAR-bright "magic islands")[28] 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.

[30] 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).

[31][32][33] 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.

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.

[37][38] 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,[39] but on a combined radar-VIMS image the location is shown as a separate lake (later named Jingpo Lacus).

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.

[42] 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.

[45] 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.

[46] 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.

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."