Titanic Methane Seas

Even after a decade of interloping among the Saturnian system, the Cassini spacecraft is still doing great science.  It helps that there are lots of places to visit, since Saturn has 62 moons and the largest ring system of the gas giants.  Arguably the best science has come from Saturn’s largest moon Titan, second largest moon in the solar system (behind Ganymede) and the only moon known to have an atmosphere.

Sunlight glints off of Titan’s northern seas this near-infrared, color mosaic from NASA’s Cassini spacecraft. Credit: NASA/JPL/Univ. Arizona/Univ. Idaho

Since Cassini has been in orbit around Saturn and it’s system of moons, it’s been revealed that over 1.6 million square kilometers of Titan’s surface are covered in liquid lakes.  These lakes are far different from the freshwater lakes found on Earth.  With Titan – they are made of liquid hydrocarbons.  Titan is far from the Sun’s warmth, and so Methane and Ethane, normally gases on Earth, are cold enough to stay in liquid form.  Titan’s atmosphere is mostly Nitrogen, like Earth’s, but there is no Oxygen to be found.

For years, scientists have wondered what Titan’s lakes are made of.  But in 2014, Cassini’s radar instrument took a closer look at Ligeria Mare, a northern hemisphere lake as large as lakes Michigan and Huron combined.  The results showed that the lake is rich in Methane.

“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart. Instead, this sea is predominantly made of pure methane,” said Alice Le Gall, a Cassini radar team associate at the French research laboratory LATMOS, Paris, and lead author of the new study.

The result was a bit of a surprise, as the lake was expected to be mostly Ethane, since it can be made in abundance as sunlight breaks down Methane molecules in Titan’s atmosphere.  Le Gall explains “Either Ligeia Mare is replenished by fresh methane rainfall, or something is removing ethane from it. It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare, but that will require further investigation.”

Even though we end up with more questions, the discovery to this point is incredible.  There is another world in our own solar system that has a liquid cycle, the only alien world in the universe known to have such a system.  It produces lakes of liquid gas that rival the largest lakes on Earth.  and we have studied it with a robot that has spent ten years flying billions of kilometers through space.


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