We've known for more than a decade that the largest of Saturn's moons plays hosts to lakes and seas of liquid hydrocarbons, but the exact make-up of these reservoirs has remained a mystery. Until now that is, with scientists from the European Space Agency (ESA) using data from eight years worth Cassini flybys to confirm that Ligeia Mare – one of the largest seas on the moon – is made up largely of liquid methane.

In some ways, Titan isn't all that different from Earth, with a thick atmosphere and expansive reservoirs of liquid on its surface. However, while both our own planet and the distant moon have nitrogen-heavy atmospheres, Titan's contains little oxygen, instead exhibiting high levels of methane. It's also much more distant from the Sun than Earth, allowing methane to – in theory at least – exist as a liquid on the body's surface.

Since arriving at Saturn in 2004, the Cassini-Huygens mission has revealed that more than 1.6 million sq km of Titan (618,000 sq miles) is covered in liquid – equal to roughly two percent of the moon's surface. Now, a new study, making use of Cassini data collected between 2007 and 2015, has confirmed the make up of one of the discovered liquid bodies, known as Ligeia Mare.

Several thermal emissions observations were used, alongside microwave wavelength readings and data from a radio sounding experiment conducted in 2013. The latter readings revealed details of the depth of the sea, finding that it extends down 160 m (525 ft) at its most extreme point.

When looking at the data, the team expected to find that the sea was mostly made up of ethane, which is produced when sunlight breaks up methane molecules, but the readings instead revealed that the body is mostly pure methane. While the reason for the unexpected composition isn't yet known, scientists have suggested two possibilities – either some unknown process is removing ethane from the sea, or fresh methane rainfall is constantly replenishing it.

"This study has pinned down, for the first time, the basic properties of one of Titan's seas, improving our understanding of climate and circulation processes on this fascinating world," said ESA's Cassini-Huygens project scientist Nicolas Altobelli.

Full details of the new study are published in the Journal of Geophysical Research: Planets.

Source: ESA

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