The Bizarre Lakes of Titan

This image of the north polar region of Saturn's moon Titan was obtained on Sept 13, 2013, using the Cassini Imaging Science Subsystem (ISS). A number of seas and lakes, consisting of very cold hydrocarbons, show up as dark patches. The image spans about 2000 km from top to bottom, and has a resolution of about 500 m/pixel.
Click here to see an annotated version of the image.
Image Credit: NASA/JPL-Caltech/SSI/JHUAPL/Univ. of Arizona

Besides Earth, Saturn's moon Titan is the only other planetary body in our Solar System that we know has stable liquid seas and lakes at its surface. Planets like Mars may see liquids occasionally erupt from beneath their surfaces, but such liquids tend to quickly evaporate into the atmosphere, and so don't stay long on the surface. But on Titan, scientists have discovered a number of lakes and seas that appear to be stable over long geologic time periods.

The surface of Titan is believed to be made predominantly of water ice. But the temperature at Titan's surface is a very cold 90 Kelvin (about -300 degrees Fahrenheit or -180 degrees C). At that temperature, water ice does not easily melt and so acts more like the planet's bedrock. Titan's seas are, therefore, not made of water, but rather of hydrocarbons, like ethane and methane, which at these temperatures are liquid.

Scientists have been studying the ultra-cold hydrocarbon lakes of Titan for several years, most recently with the help of the Cassini mission to Saturn.  This past September, the spacecraft flew by Titan's north pole and obtained exciting new imagery of these intriguing liquid features using the near-infrared instrument on Cassini's Imaging Science Subsystem (ISS). These images show the lakes as dark patches with very distinctive shapes, having rounded scallop-like edges and steep sides. The surrounding material is also unusual, being much brighter than the rest of Titan's surface, which tends to be dark grey in colour.

The majority of Titan's seas and lakes are found at the north pole, with only a few lakes near the southern pole. It was originally thought that dark terrains at the equator were also liquid hydrocarbon seas. But Cassini images have shown that these are large plains covered in long, linear dunes. Thus, the polar lake areas are truly unusual on Titan. It is thought that their unique environment holds clues to how they were formed, but the exact process is not yet known. Scientists have suggested two possible scenarios: 1) they are Karst terrains, that were formed when the liquids dissolved the underlying rock (or water ice in this case), making surface holes and underground caves in the bedrock, or 2) they were formed by volcanic processes, where magma chambers, which were emptied by volcanic eruptions, collapsed leaving large holes at the surface. In both scenarios, the liquid hydrocarbons would simply have filled up the resulting holes.

The largest liquid body on Titan is Kraken Mare at the north pole.  A sea on planetary bodies is referred to as a mare, the latin word for sea. This dates back to the time of Galileo, who thought the large dark areas on the Moon were seas. We now know that the lunar maria (plural for mare) are not liquid at all, but are solid basalt rocks. However, the name stuck and has been passed on to the liquid seas of Titan. Kraken Mare is quite large by terrestrial standards, spanning 400,000 square kilometers. This is roughly equivalent to the combined size of the Caspian Sea and Lake Superior on Earth, a truly spectacular size!

This radar image of Ontario Lacus, the largest lake in Titan's southern hemisphere, was obtained on Jan. 12, 2010. The lake is about 15,000 square kilometers (6,000 square miles) in size, which is slightly smaller than its terrestrial namesake, Lake Ontario in North America.
Image Credit:  NASA/JPL-Caltech/ASI

Prior to the most recent flybys of Titan, the north polar region hadn't been imaged very well, with only distant, oblique, or partial views being obtained. Part of the problem was that when Cassini arrived at Saturn 9 years ago Titan was experiencing a northern winter, so the north pole was in complete darkness. Since then, summer has been approaching and the northern pole is finally receiving sunlight.  A number of factors have lined up to make the most recent flybys particularly conducive to collecting very good imagery. For one, the sunlight and flyover trajectory have provide a much improved viewing geometry over previous opportunities. Also, with the approach of summer, the thick cap of winter haze that hung over Titan's north pole has dissipated.  And finally, Titan's weather has been unusually cooperative, providing almost cloudless and rain-free skies.

These conditions have also allowed scientists to collect data using the visual and infrared mapping spectrometer (VMS) on board Cassini. By analyzing data collected at a variety of wavelengths, the composition of the surface materials can be inferred. While most of Titan's surface seems to be composed of water ice, sections of the north polar region appear to contain materials that are interpreted to be evaporites. On Earth, evaporites form when shallow seas evaporate, leaving thick deposits of salts behind. Titan's evaporites are thought to consist of haze particles. Liquid methane in the atmosphere dissolves the atmospheric haze particles, which are then rained down to the surface and left behind when the shallow methane lakes evaporate.

We have been referring to these interesting methane lakes and seas as liquid. However, it should be noted that these bodies may not be liquid quite the way that terrestrial seas and lakes are liquid. Radar imagery of these features shows that they are extremely smooth, even at the millimeter scale. This means that they have no waves on their surfaces, not even small ripples. But, scientists calculate that even the slightest breeze should produce substantial waves, because the mixtures of ethane and methane that make up these bodies are less viscous than water.  So, it may be that these lakes also contain other hydrocarbons which make the ethane/methane mixture much more viscous, giving it a thick consistency, like that of tar or mud.

Bizarre lakes, indeed!

Sources:
JPL's Cassini Featured Image, 2013, Bird's Eye View of the Land of Lakes.

JPL's Photojournal, 2013, Titan's Northern Lakes: Salt Flats?

Hecht, 2011, Ethane lakes in a red haze: Titan's uncanny moonscape, NewScientist, 2820.

Lorenz, 2010, Winds of Change on Titan, Science,  V329 (5991), 519–20, doi:10.1126/science.1192840.