Saturn’s largest moon is covered in seas and lakes of liquid hydrocarbons – and one sea has now been found to be filled with pure methane, with a seabed covered by a sludge of organic-rich material, and possibly surrounded by wetlands.

Of all the moons in the Solar System, Titan is the only one with a thick atmosphere and large liquid reservoirs on its surface – in some ways making it more like a planet such as Earth.

Both Earth and Titan have atmospheres dominated by nitrogen, over 95% in Titan’s case. However, unlike Earth, there is little oxygen: the remaining is mostly methane, with a small amount of hydrogen, and trace amounts of other gases such as ethane.

And at the low temperatures found far from the Sun, the methane and ethane can in principle sit on the surface as liquids.

There has long been speculation about possible hydrocarbon lakes and seas on Titan, and when the international Cassini–Huygens mission arrived at Saturn in 2004 they were revealed.

Since then, the Cassini orbiter has revealed that more than 1.6 million sq km of Titan – almost 2% – are covered in liquid.

There are three large seas, all close to the north pole, surrounded by dozens of smaller lakes in the northern hemisphere. Just one lake has been found in the southern hemisphere.

The exact make-up of these liquid reservoirs remained elusive until recently. A new study using scans from Cassini’s radar during flybys of Titan between 2007 and 2015 confirms that one of the largest seas on the moon, Ligeia Mare, is mostly liquid methane.