There’s a good chance you’ve heard of Lake Ontario. You may know that it’s one of the five Great Lakes, and you may even know that it’s the easternmost Great Lake. But did you know that Lake Ontario has a sister lake on one of the moons of Saturn? Ontario Lacus is a lake on the southern side of Titan, the largest moon of Saturn. Ontario Lacus is named after its terrestrial Great Lake counterpart due to their similar size and shape, but that is where their similarities end. Unlike any lake you may have seen on Earth, Ontario Lacus is filled not with water, but with hydrocarbons like ethane and methane. These are compounds that are typically gaseous at the temperatures and pressures on the surface of the Earth, but the conditions on Titan are so cold that these compounds occur in their liquid form on the surface in large enough quantities to form lakes, rivers, and seas.
Titan in front of Saturn (image from nasa.gov)
Methane is more commonly known as natural gas—that’s right, the liquid that fills seas and lakes on this distant moon is the same as the gas we use to heat our homes. While the lakes of Titan are not made of water, Titan does have an abundance of water. The body of Titan on which these lakes of ethane and methane lay is made mostly of water ice and rock. Titan is the only other body in our Solar System besides Earth known to host stable bodies of liquid on its surface. Just as explorers and scientists have covered our Earth documenting its natural wonders, scientists are now probing the corners of our solar system and documenting what they discover. Scientists first glimpsed the lakes and seas of Titan in 2004 from infrared images taken by the Cassini probe that was launched in 1997. More detailed radar images of Ontario Lacus were obtained by flybys in 2008 and 2009. Recently, scientists discovered that seabed echoes in radar could be used to image the seabed of Ligeia Mare, a larger body of liquid hydrocarbons in the northern hemisphere. This method has been applied to the data obtained in 2008 and 2009 to reveal new details about the seabed and composition of Ontario Lacus, as described in a new paper published in the January 2018 issue of Icarus.
Image of the surface of Titan and its many lakes (image form nasa.gov)
The features of Titan are described with a naming scheme developed by the scientists at NASA. Features are described using terms from Latin—the largest bodies of liquid are “mare” which is Latin for seas, and smaller bodies of liquid are “lacus” or lakes. While there are three larger bodies of liquid in Titan’s northern hemisphere– Kraken Mare, Ligeia Mare, and Punga Mare– Ontario Lacus is the largest body of liquid in the southern hemisphere. As the recent paper by Mastrogiuseppe and others explains, the different abundances of liquid ethane and methane in the northern and southern hemispheres may be due to transport of those compounds on the surface of the moon driven by orbital insolation— essentially the sun hitting the surface of the moon and causing the evaporation and raining of ethane and methane onto the northern hemisphere, similar to the water cycle we observe on Earth.
These dynamics also mean that bodies of liquid in the southern hemisphere have higher concentrations of particulates and denser hydrocarbons, which can be observed by the reflectance at the surface. All of this data indicates that Ontario Lacus has also experienced a declining lake level—which would be bad news for lakeside properties if there were any. The authors of this research compare the declining lake level at Ontario Lacus to the dynamics observed in some lake beds on Earth—Racetrack Playa in Death Valley, or Etosha pan in Namibia. The authors of the study determined that Ontario Lacus has a maximum depth of 50 meters, about as high as the Arc de Triomphe in Paris. This is much shallower than its sister lake on Earth which has a maximum depth almost five times greater at 244 meters. The scientists also estimated the composition of Ontario Lacus in greater detail—it is 51% methane, 38% ethane, and 11% nitrogen by volume. The details of lake composition are important for future decisions about what instruments to send to Titan for further investigation. The similarity of surface processes on Titan to those on Earth, and the presence of stable surface liquids make it a prime target in the search for new kinds of life.
Mastrogiuseppe, Marco, et al. “Bathymetry and composition of Titan’s Ontario Lacus derived from Monte Carlo-based waveform inversion of Cassini RADAR altimetry data.” Icarus300 (2018): 203-209.