One of Saturn’s moons, Titan, has a number of similarities to Earth: it has a dense atmosphere, mountains, and deserts, it rains, snows, and storms, with precipitation collecting in lakes and flowing in rivers into oceans. One of the differences is that it is much colder there, with temperatures around negative 180 °C. Therefore, methane and ethane, which are gases on Earth, play the role of terrestrial water on Titan; they form ice and even the sand grains in the deserts consist of frozen methane mixed with water ice.
Most of our knowledge about Titan’s hydrogeology originates from NASA’s Cassini probe and ESA’s Huygens Lander. Even though Cassini long ago went out in a blaze of glory in Saturn’s atmosphere, its data is still delivering new discoveries. In its last fly-by past Titan on April 22, 2017, the probe delivered especially interesting information on the small lakes of Titan’s western hemisphere.