Ligeia Mare from a false-color mosaic of synthetic aperture radar images of Titan's north polar region.
|Diameter||500 km[note 1]|
Ligeia Mare / / is a lake in the north polar region of Titan, the planet Saturn's largest moon. It is the second largest known body of liquid on Titan, after Kraken Mare. Larger than Lake Superior on Earth, it is composed of nearly pure liquid methane. It is located at 78° N, 249° W, and has been fully imaged by the Cassini spacecraft. It measures roughly 420 km (260 mi) by 350 (217 mi) km across, has a surface area of about 126,000 km2, and a shoreline over 2000 km (1240 mi) in length. The lake may be hydrologically connected to the larger Kraken Mare. Its namesake is Ligeia, one of the sirens in Greek mythology.
Ligeia Mare has two predominant types of coastline, "crenulated" and "subdued". The former is characterized by hummocky, eroded terrain, the latter by lower, smoother topography and the presence of more numerous and longer channels. Crenulated terrain predominates on the eastern and southern sides of the lake; subdued terrain to the west and north. Except in the southeast where the rough topography extends to the coast, hummocky terrain tends to be separated from the shoreline by a more subdued bench. The coast has numerous bays that appear to be flooded river mouths (rias), and unlike at Ontario Lacus there are no visible subaerial delta deposits, possible evidence of a recent sea level rise. In the northeast and northwest sections of the mare, along about a quarter of the total shoreline, there are extensive areas where the depth is less than 5 m, shallow enough for imaging radar to penetrate to the bottom. 2013 radar measurements by Cassini indicate parts of the lake are 170 m deep which implies the liquid must be very pure methane since the radar signal was able to pass right through it. The surface of the lake appears very smooth on radar; it is flat within a few millimeters.
The shorelines of Ligeia Mare and other north polar lakes and maria have been stable over the period of observation by Cassini, in contrast to south polar Ontario Lacus, where there has been significant shoreline recession. However, transient phenomena have been observed including a 260 square kilometre feature dubbed "Magic Island" by Cassini scientists. The "Magic Island" area only appeared in 2014 and may be waves, bubbles or subsurface ice rising to the surface as the lake warms up during spring, or possibly silt like material suspended in the liquid hydrocarbon sea. 
The Titan Lake In-situ Sampling Propelled Explorer (TALISE) is a proposed lander mission which is envisioned to splash down and navigate across Ligeia Mare. A similar concept, albeit without its own propulsion system, was the Titan Mare Explorer. A lake-lander mission planned to splash down on and then drift about the lake, its financial and technical support from NASA has become precarious. 
- The USGS web site gives the size as a "diameter", but it is actually the length in the longest dimension.
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