Go with the flow

Titan is Saturn’s largest moon. Its landscape was explored remotely by the NASA/ESA Cassini-Huygens mission. It revealed mountains reaching towards the sky and an astonishing sight in the valleys between: liquid rivers flowing into vast seas. These rivers are not streams of water…

… instead, they are flowing with liquid methane! Kraken Mare is Titan’s largest sea. This radar image covers an area of 1.4 million square kilometres, showing liquid methane in dark brown and black and icy mountain regions in yellow.

Earth and Titan are the only known places in the solar system with pools of liquid at the surface. A delicate balance of Earth’s atmospheric pressure and temperature allows water to flow, evaporate and fall as rain. Titan is so cold that all water freezes solid and methane is chilled to a liquid. The solid ice behaves like a rock, forming mountains that erode into sediment, creating dunes and deltas. Titan’s dense atmosphere holds liquid methane on its surface, just as Earth’s holds liquid water in the form of rivers, seas and oceans.

Water is vital to all life on Earth, making it an obvious starting point in the search for extra-terrestrial life. Geological landforms on Earth trace the history of ancient river systems, and these features are easily recognised on Mars’s surface. Jezero Crater, the landing site for Perseverance Rover, contains the remains of an ancient delta, suggesting traces of life could exist within its base.