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Ligeia Mare is a
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much lar ...
in the north polar region of
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...
, the planet
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; ...
's largest moon. It is the second largest body of liquid on the surface of Titan, after Kraken Mare. Larger than
Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh w ...
on Earth, it is mostly composed of liquid
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...
, with unknown but lesser components of dissolved nitrogen and ethane, as well as other organic compounds. It is located at 78° N, 249° W, and has been fully imaged by the '' Cassini'' spacecraft. Measuring roughly 420 km (260 mi) by 350 km (217 mi) across, it has a surface area of about 126,000 km2, and a shoreline over 2,000 km (1,240 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.


Description

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, suggesting very light surface winds. The average depth is on the order of 50 m, while the maximum depth is probably > 200 m. The total volume is likely to be > 7,000 km3.


Hydrology

The reasons for Ligeia Mare's nearly pure methane composition are still not entirely clear. Until studies showed otherwise, it had been assumed that
ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petro ...
would be the primary constituent of Titan's seas, as it is produced in Titan's upper atmosphere by the following reaction: : 2 CH4 → H2 + C2H6 The lack of ethane is an open question awaiting further investigation. Theories include it migrating into the undersea crust or perhaps into Kraken Mare, whose composition has as of yet not been as well studied. In the latter case, ethane-poor liquid hydrocarbon would be the equivalent of terrestrial freshwater (Titanian precipitation is mainly methane). In addition to ethane, Titan's
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...
produces a wide range of more complex photochemical products, such as nitriles and
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen ato ...
. These are believed to precipitate out and flow into Titan's seas. Radar data suggests that Ligeia Mare's seabed is likely covered in a thick layer of these organic compounds. Temperature readings from the shoreline of Ligeia Mare suggest that it is porous and highly saturated with hydrocarbons. 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 Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel wh ...
like material suspended in the liquid hydrocarbon sea.


Proposed missions

The Titan Mare Explorer (TiME) is a lake-lander proposed in 2009 to splash down on and then drift about the lake. It was selected for a Phase-A design study in 2011 as a candidate mission for the 12th NASA Discovery Program opportunity, but was not selected for flight. The Titan Lake In-situ Sampling Propelled Explorer (TALISE) was a conceptual design study for a Spanish lander mission envisioned to splash down and navigate across Ligeia Mare. In 2015, the
NASA Innovative Advanced Concepts The NASA Institute for Advanced Concepts (NIAC) is a NASA program for development of far reaching, long term advanced concepts by "creating breakthroughs, radically better or entirely new aerospace concepts". The program operated under the name ...
program (NIAC) awarded a Phase II grant to a design study of a submarine to explore the seas of Titan.


Gallery

Titan sea map.png, Map of Ligeia Mare by an amateur cartographer PIA09184 -Titan Sea and Lake Superior.jpg, Comparison of Ligeia Mare's size with that of
Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh w ...
on Earth PIA16197 Titan river.jpg, Vid Flumina, a river over 400 km long that empties into Ligeia Mare Bathymetry Ligeia (2).png, The Cassini spacecraft’s Titan Radar Mapper produced bathymetric measurements of Titan’s Ligeia Mare


Notes


References


External links


Labelled map of the liquid bodies in the north polar region of Titan
{{Titan Lakes of Titan (moon)