Titan, Saturn’s largest moon, is the only world besides Earth known to have stable liquid on its surface. Almost all of it lies around the north pole, where Cassini’s 2013 infrared flybys finally gave scientists a clear, sunlit look at a land of methane lakes and hydrocarbon seas.
Titan: a moon with lakes and seas
Titan has a radius of about 2,575 kilometres, making it the second-largest moon in the solar system after Jupiter’s Ganymede. A thick, hazy nitrogen atmosphere hides its surface from ordinary cameras, and the ground sits at about −179 °C. At that temperature water ice behaves like rock, while methane and ethane can exist as liquids that rain from clouds, carve river channels and pool in lakes.
Cassini’s radar first revealed dark, smooth patches near the north pole during a flyby on 22 July 2006. In January 2007 scientists announced definitive evidence of methane-filled lakes. Today hundreds of lakes are mapped, and nearly all of them lie in the northern high latitudes; the south polar region has one large lake, Ontario Lacus, and a few smaller ones.
Kraken Mare, Ligeia Mare and Punga Mare
Titan’s largest liquid bodies are called maria, or seas, and the International Astronomical Union names them after sea monsters from world mythology. Smaller lacus carry the names of lakes on Earth.
- Kraken Mare is the giant. It covers about 500,000 square kilometres, slightly more than the Caspian Sea, stretches roughly 1,170 kilometres at its longest and is at least 100 metres deep in its main body. It is the largest known lake in the solar system.
- Ligeia Mare covers about 126,000 square kilometres, a little more than Lake Michigan and Lake Huron combined. Cassini’s radar bounced off its floor at a depth of about 170 metres, and a 2013 analysis found a mix of methane, ethane and nitrogen.
- Punga Mare is the smallest of the three seas and lies closest to the pole.
Cassini’s 2013 infrared views of the north pole
When Cassini reached Saturn in 2004, Titan’s north pole was in winter darkness under a thick haze cap. A Saturn year lasts about 30 Earth years, so the spacecraft had to wait for the seasons to turn. By 2013 northern summer was approaching, the haze had cleared and the weather during the flybys of 10 July, 26 July and 12 September was nearly cloudless and free of rain.
Radar can see through Titan’s haze and had already mapped much of the region. The new mosaics added two other instruments. The visual and infrared mapping spectrometer (VIMS) produced a false-colour image that separates materials by composition, while the imaging science subsystem (ISS) captured near-infrared images that showed a bright terrain unit around the lakes for the first time. NASA’s Jet Propulsion Laboratory released the results on 23 October 2013.
“Titan’s northern lakes region is one of the most Earth-like and intriguing in the solar system,” said Linda Spilker, Cassini project scientist at JPL. VIMS scientist Jason Barnes of the University of Idaho added that the north pole was “even more interesting than we thought.”
Evaporites: Titan’s version of salt flats
The VIMS data suggested that parts of some lakes and seas have evaporated, leaving deposits behind in the way salt flats form on Earth. On Titan the residue is thought to be organic compounds that fall from the haze and dissolve in liquid methane. In the false-colour mosaic these evaporites appear orange against greenish water-ice bedrock.
Many of the lakes have rounded, steep-sided “cookie-cutter” outlines. Scientists have proposed two ways they could form: collapse after volcanic activity, or karst-style dissolution of soluble bedrock, similar to the sinkhole lakes of limestone country on Earth. Models of the lake liquid suggest a typical mixture of mostly ethane with methane and propane, although composition likely varies from lake to lake.
After Cassini
Cassini launched in 1997 and ended its mission in September 2017 with a deliberate plunge into Saturn’s atmosphere. Its radar, infrared and imaging data still underpin every map of Titan’s seas. The northern lakes remain the clearest example anywhere of a familiar landscape – shorelines, bays, rivers and islands – built from completely unfamiliar materials. For two very different frozen lakes on Earth, see a large terrestrial lake in winter, or explore another data visualisation whose colours need a legend.
Related reading
- Antarctica’s Coldest Snow Surfaces: Satellite Estimates and Weather Records
- The 2013 Arctic Melt Pond Photograph: What the Camera Actually Showed
- Lake Hodgson: Microbial Evidence from Antarctic Sediments
- Greenland’s Buried Canyon: Reading the Landscape Through Radar
- Antarctic Sea Ice: Understanding an Older Growth Story
- Svalbard Global Seed Vault: What the Arctic Backup Actually Does
Sources
- NASA JPL: Cassini Gets New Views of Titan’s Land of Lakes (23 October 2013)
- NASA Science: Titan facts
- Wikipedia: Lakes of Titan
- Wikipedia: Kraken Mare
- 2013 story by Jason Major, Universe Today.
- Images: NASA / JPL / Cassini imaging teams; lake illustration © Ron Miller.
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