Lakes of Titan

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False-color Cassini synthetic-aperture radar mosaic of Titan's north polar region, showing evidence for hydrocarbon seas, lakes and tributary networks. The false blue coloring indicates low radar reflectivity areas, likely caused by bodies of liquid ethane, methane and dissolved nitrogen. Photographs^[1] suggest that Kraken Mare, the large body at lower left, has approximately twice the extent of what can be seen here.

The lakes of Titan, a moon of Saturn, are bodies of liquid methane that have been detected by the Cassini space probe, and had been suspected long before. The large ones are known as maria (seas) and the small ones as laci (lakes).^[2]

The possibility that there were seas of liquid methane on Titan were first suggested based on Voyager 1 and 2 data that showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them, but direct evidence wasn't obtained until 1995 when data from Hubble and other observations had already 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.^[3]

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.^[4]

The possibility remained that liquid ethane and methane might be found on Titan's poles, where it was expected to be abundant and stable.^[5] At Titan's south pole, 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.^[6] A possible shoreline was also identified at the pole via radar imagery.^[7] Following a flyby on July 22, 2006, in which the Cassini spacecraft's radar imaged the northern latitudes (which are currently in winter), a number of large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole.^[8] Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007.^[5][9] 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.^[5]

Repeated coverage of these areas should prove whether they are truly liquid, as any changes that correspond with wind blowing on the surface of the liquid would alter the roughness of the surface and be visible in the radar. 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.^[10]

Size comparison of Ligeia Mare with Lake Superior.

During a Cassini flyby in late February 2007, radar and camera observations revealed several large features in the north polar region that may be large expanses of liquid methane and/or ethane, including one sea with an area of over 100,000 km² (larger than Lake Superior), and another (though less definite) region potentially the size of the Caspian Sea.^[11] A flyby of Titan's southern polar regions in October 2007 revealed similar, though far smaller, lakelike features.^[12]

Image of Titan taken during Huygens' descent, showing hills and topographical features that resemble a shoreline and drainage channels.

The discoveries at the poles 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.^[13] 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.^[14]

On February 13, 2008, scientists announced that, according to Cassini data, 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.^[15].

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.^[16]

[edit] Named lakes and seas

Titanian maria (large hydrocarbon seas) are named after sea monsters in world mythology.^[2]

Methane lakes on Titan: Cassini radar image, 2006

Features labeled 'lacus' are believed to be methane lakes. They are named after lakes on Earth.^[2]

[edit] References