January 9, 2013
New Cassini Images Indicate Floating Ice On Saturn’s Moon Titan
April Flowers for redOrbit.com - Your Universe Online
Scientists at NASA's Cassini mission revealed today that blocks of hydrocarbon ice might cover the surface of existing lakes and seas of liquid hydrocarbon on Saturn's moon Titan. Cassini's mixed readings from the reflectivity of the surfaces of the lakes on Titan might be explained by the presence of ice floes.
Besides Earth, Titan is the only other body in our solar system with stable bodies of liquid on its surface. Unlike Earth's cycle of precipitation and evaporation which involves water, Titan's cycle involves hydrocarbons like ethane and methane, which are organic, carbon-based molecules. Scientists think these molecules could potentially serve as building blocks for the more complex chemistry that leads to life. Cassini has observed a vast network of hydrocarbon seas covering Titan's northern hemisphere, along with a more sporadic set of lakes along the southern hemisphere.
Prior to this study, scientists assumed that Titan lakes would not have floating ice because — unlike water — the solid form of methane is denser than liquid methane, meaning the solid would sink. The new model, however, considers the interaction between the lakes and the atmosphere, which would result in different mixtures of compositions, pockets of nitrogen gas and changes in temperature. The scientists found that winter ice will float in Titan's methane-and-ethane-rich lakes and seas if the temperature is below the freezing point of methane — minus 297 degrees Fahrenheit. All the varieties of ice they considered would float, the scientists realized, if they were composed of at least 5 percent "air," which is an average composition for young sea ice on Earth. Titan's air has significantly more nitrogen than air on Earth, and almost no oxygen.
A temperature drop of only a few degrees would cause the ice to sink due to the relative proportions of nitrogen gas present in the liquid versus the solid. Floating and sinking ice — a hydrocarbon ice crust above the liquid and blocks of hydrocarbon ice on the bottom of the lake bed — could be the result of temperatures close to the freezing point of methane. The ice would most likely be clear, such as on Earth, or perhaps lightly tinged reddish-brown by Titan's atmosphere.
“We now know it's possible to get methane-and-ethane-rich ice freezing over on Titan in thin blocks that congeal together as it gets colder — similar to what we see with Arctic sea ice at the onset of winter," explained Jason Hofgartner, a Natural Sciences and Engineering Research Council of Canada scholar at Cornell. "We'll want to take these conditions into consideration if we ever decide to explore the Titan surface some day."
By watching what happens to the reflectivity of the surface of these lakes and seas, Cassini's radar instrument will be able to test this model. The reflective nature of a lake might increase as ice rises to the surface in the early spring thaw on Titan, as the northern lakes have begun to do. The rougher surface quality reflects more radio energy back to Cassini, making it look brighter. The lake surface will be pure liquid and will appear to darken as the weather turns warmer and the ice melts.
"Cassini's extended stay in the Saturn system gives us an unprecedented opportunity to watch the effects of seasonal change at Titan," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory. "We'll have an opportunity to see if the theories are right."