A new study analyzing data from NASA's Cassini spacecraft suggests that the lake, known as Ontario Lacus, behaves most similarly to what we call a salt pan on Earth. A salt pan is formed where liquid pools were. A salt pan would be a lake or a pond if it were located in a climate where the rate of evaporation were not faster than the rate of water precipitation, i.e., if it were not in a desert. If the water is unable to drain into the ground, it remains on the surface until it evaporates, leaving behind whatever minerals were dissolved. A group led by Thomas Cornet of the Université de Nantes, France, a Cassini associate, found evidence for long-standing channels etched into the lake bed within the southern boundary of the depression. This suggests that Ontario Lacus, previously thought to be completely filled with liquid hydrocarbons, could actually be a depression that drains and refills from below, exposing liquid areas ringed by materials like saturated sand or mudflats.
"We conclude that the solid floor of Ontario Lacus is most probably exposed in those areas," said Cornet, whose paper appears in a recent issue of the journal Icarus.
These characteristics make Ontario Lacus very similar to the Etosha salt pan on Earth, which is a lake bed that fills with a shallow layer of water from groundwater levels that rise during the rainy season. This layer then evaporates and leaves sediments like tide marks showing the previous extent of the water.
The Etosha pan is a large salt pan, forming part of the Kalahari Basin in the north of Namibia. The 75-mile long dry lakebed and its surroundings are protected as Etosha National Park, one of Namibia's largest wildlife parks. The pan is mostly dry but after a heavy rain it will acquire a thin layer of water, which is heavily salted by the mineral deposits on the surface of the pan, which most of the year is dry mud coated with salt.
Ontario Lacus is a lake composed of methane, ethane and propane near the south pole of Saturn's moon Titan. Its character as a hydrocarbon lake was confirmed by observations from the Cassini spacecraft, published in the 31 July 2008 edition of Nature. Ontario Lacus has a surface area of about 5,800 square miles, about 20% smaller than its terrestrial namesake, Lake Ontario in North America.
"Some of the things we see happening in our own backyard are right there on Titan to study and learn from," said Bonnie Buratti, a co-author and Cassini team member based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "On Earth, salt pans tend to form in deserts where liquids can suddenly accumulate, so it appears the same thing is happening on Titan."
While the liquid on Titan is methane, ethane and propane rather than water, the cycle appears to work in a very similar fashion to the water cycle on Earth. Beyond Earth, Titan is the only other world known to bear stable liquids on its surface. There, the full hydrocarbon cycle is based on hydrogen, carbon and nitrogen, and takes place between the atmosphere, the surface and the subsurface. Titan's lakes are an integral part of this process.
This latest paper is part of an ongoing study of Ontario Lacus, the largest lake in Titan's south polar region. Cassini has been observing the lake with multiple instruments and employing multiple methods of analysis to see if Titan, like Earth, changes with the seasons. During the time Cassini has been exploring the Saturn system, Titan's southern hemisphere has transitioned from summer to fall.
For further information see Ontario Lacus
Titan Lake image via NASA