Titan’s liquid lakes
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 31, 2008

Cassini scientists have presented unrivalled evidence for the presence of ethane lakes on Saturn’s moon Titan, the first definitive example of liquid surface reservoirs on a planetary body other than the Earth.

The idea of liquid reservoirs on Titan’s surface is not a new one, and it was long believed that global oceans of ethane and other light hydrocarbons covered its surface as result of billions of year’s worth of solar radiation impingement on the methane-rich nitrogen atmosphere, which sparks chemical reactions that break the methane component down into complex organic material. But even after 40 close flybys of Titan by the Cassini spacecraft, it soon became clear that no such oceans exist.

The Imaging Science System aboard NASA's Cassini orbiter took the image, left, of Ontario Lacus in June 2005. Cassini's Visual and Infrared Mapping Spectrometer took the image, right, of Ontario Lacus in December 2007. This view, taken at 5-micron wavelengths from 1,100 kilometers, shows the part of the lake that is visible on Titan's sunlit side. What appears to be a beach is seen at the lower right of the image, below the bright lake shoreline. Images: left: NASA/JPL/Space Science Institute; right: NASA/JPL/University of Arizona.

Despite the lack of global oceans however, numerous features similar to terrestrial lakes, seas and shorelines, along with associated erosional features, were identified by Cassini’s radar passes, and finally, evidence for surface liquid ethane has been found in the shape of Titan’s 235 kilometre long Ontario Lacus. Named for its comparable size to North America’s Lake Onatario, it covers an area of roughly 20,000 square kilometres, although it is not yet known how far it extends in depth.

"Detection of liquid ethane in Ontario Lacus confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," says Larry Soderblom of the U.S. Geological Survey in Flagstaff, Arizona.

The discovery was made using the visual and infrared mapping spectrometer (VIMS), an instrument controlled by planetary scientists at The University of Arizona. VIMS identified the chemical signature of ethane by studying the way it reflected light from the surface. However, this was no easy task given Titan’s vast distance from the Sun and the seemingly impenetrable hydrocarbon haze of the atmosphere that masks the view to the moon’s surface. VIMS viewed the dimly lit surface at a highly slanted angle through Titan's thick atmosphere at transparent atmospheric windows at certain infrared light wavelengths of 2, 2.7 and 5 microns. The scientists saw the specific signature of ethane as a dip at the precise wavelength – 2 microns - that ethane absorbs infrared light, raising expectations for future ethane lake discoveries using this method.

The ethane is thought to be in a liquid solution with methane, nitrogen and other low-molecular weight hydrocarbons and the measurements rule out the presence of other materials that also show spectral features at similar wavelengths such as water-ice, ammonia and carbon dioxide.

"This is the first observation that really pins down that Titan has a surface lake filled with liquid," says VIMS principal investigator Robert Brown. "We know the lake is liquid because it reflects essentially no light at 5-micron wavelengths. It was hard for us to accept the fact that the feature was so black when we first saw it. More than 99.9 percent of the light that reaches the lake never gets out again. For it to be that dark, the surface has to be extremely quiescent, mirror smooth. No naturally produced solid could be that smooth."

Titan’s methane/ethane cycle. Large stores of methane appear to exist within Titan in the form of methane hydrates (clathrates), which are released to the atmosphere via volcanic outgassing and evaporation. Ethane is formed by the chemical breakdown of methane in the atmosphere by light, which eventually replenishes the surface lakes, bearing many organic species in the process. Image: Raulin, Nature 454, 31 July 2008.

The Cassini scientists’ observations also suggest that tiny ethane particles – almost as fine as cigarette smoke – appear to be filtering out of the atmosphere and into the lake. But the observations of a dark ‘tide’ mark around the lake’s ‘beach’ suggest that the pool is also evaporating.

"We can see there's a shelf – a beach – that is being exposed as the lake evaporates," says Brown. The beach’s dark appearance could mean that the ‘sand’ is saturated or covered with a film of liquid organics.

Precipitation and evaporation is very reminiscent of the water cycle that maintains the levels of water on Earth, and like Earth, Titan has its very own cycle: the methane/ethane cycle. The interaction between surface fluids and the atmosphere was first revealed by the Huygens probe that landed on Titan’s surface in January 2005, which showed that methane saturates and drains from Titan's atmosphere, creating river-like and lake-like features on the surface.

It has also been suggested that these organic reservoirs could provide the perfect soup to cook up life, and these lakes will undoutedly be priority targets for future landing missions.

The full report on the observations at Ontario Lacus appear in this week’s issue of Nature, published today.