Titan's lakes in close-up
Titan is the only body in the solar system apart from Earth with surface lakes and seas, but at roughly –180°C, the surface of Titan is very cold and liquid methane and ethane, rather than water, dominate the "hydrological" cycle.
As the ESA reports:
Indeed, methane and ethane-filled topographic depressions are distinctive features near the moon’s poles. Two forms have been identified by Cassini. There are vast seas several hundred kilometres across and up to several hundred metres deep, fed by river-like dendritic channels. Then there are numerous smaller, shallower lakes, with rounded edges and steep walls, and generally found in flat areas. Many empty depressions are also observed.
The lakes are not associated with rivers, and are thought fill up by rainfall and liquids flooding up from underneath. Some of the lakes fill and dry out again during the 30-year seasonal cycle on Saturn and Titan.
However, no one is quite sure how the depressions that create the lakes came about.
The latest thinking is that they are the result of erosion of soluble rocks such as limestone and gypsum, which happens on Earth through groundwater and rainfall percolating through rocks.
The rate of erosion depends on factors such as the chemistry of the rocks, the rainfall rate and the surface temperature. While all of these aspects clearly differ between Titan and Earth, the underlying process may be surprisingly similar.
A team lead by ESA’s Thomas Cornet calculated that it would take around 50 million years to create a 100 metre-deep depression at Titan’s relatively rainy high polar latitudes, consistent with the youthful age of the moon’s surface.
“We compared the erosion rates of organics in liquid hydrocarbons on Titan with those of carbonate and evaporite minerals in liquid water on Earth,” says Cornet.
“We found that the dissolution process occurs on Titan some 30 times slower than on Earth due to the longer length of Titan’s year and the fact it only rains during Titan summer.
“Nevertheless, we believe that dissolution is a major cause of landscape evolution on Titan, and could be the origin of its lakes.”