Titan is the only celestial body besides Earth to have liquid on its surface. Although the liquid is not water, the lakes are filled with hydrocarbons such as methane and ethane. These hydrocarbons might exist as gases on Earth but, due to the sub-zero conditions, they are found in liquid form on Saturn’s moon.
The hydrocarbon lakes were thought to be made by dissolving regions of frozen water and solid organic compounds and filling the basin with liquid. Although this explains the sharp boundaries of a krastic lake, a new theory explains the formation of lakes with protruded boundaries. According to a new research published in Nature Geoscience, craters were formed by the explosive expansion of nitrogen gas due to the increase in temperature of Titan's crust. These craters were then filled with liquid hydrocarbons such as methane and ethane.
Researchers led by Giuseppe Mitri of Italy's G. d'Annunzio University used the data recorded by NASA’s Cassini Saturn Orbiter during the spacecraft's last dive into Saturn’s atmosphere. The data recorded showed a few lakes with sharp edges that did not fit the krastic lake model which made the researchers believe that these lakes were formed by a different phenomenon. As explained by Mitri, the protrusions seemed to be caused by an impact event rather than the krastic lake model.
The researchers concluded that during a colder period, the nitrogen-rich atmosphere of Titan rained liquid nitrogen, forming pools of the same over bedrock of solid compounds and liquid water. And during a warmer period, the liquid nitrogen cools rapidly expanded causing the dissolution of bedrock and forming craters. The craters then got filled by liquid hydrocarbons in another period of cold environment.
The conclusion holds up as it has been long believed that Titan has gone through multiple periods of hot and cold. The research explains the formation of the small lakes with sharp rims that has been a mystery since their discovery. Linda Spilker, Cassini Project Scientist said in a press release by NASA that, "As scientists continue to mine the treasure trove of Cassini data, we'll keep putting more and more pieces of the puzzle together. Over the next decades, we will come to understand the Saturn system better and better."