Ice may float on the lakes of Saturn's moon Titan
Sen—Blocks of hydrocarbon ice may be floating on the seas and lakes of Saturn's largest moon, Titan, according to new research.
Titan, Saturn's largest moon and the second largest in the Solar System, is unique because it has a dense atmosphere and many Earth like characteristics including seas, lagoons, rivers, mountains - and rain. However, the liquid that forms the cycle of precipitation and evaporation is not water, but hydrocarbons such as methane and ethane. Titan is the only other world in our Solar System known to have liquid on its surface.
The Cassini space probe, which arrived at Saturn in 2004, has observed a vast network of hydrocarbon seas in the northern hemisphere and lakes in the southern hemisphere. Ethane and Methane are organic molecules which scientists believe can be the building blocks for more complex chemistry from which life can arise, and Titan is therefore of great interest to astrobiologists.
"One of the most intriguing questions about these lakes and seas is whether they might host an exotic form of life," said Jonathan Lunine, co-author of the research paper and a Cassini Titan scientist at Cornell University. "And the formation of floating hydrocarbon ice will provide an opportunity for interesting chemistry along the boundary between liquid and solid, a boundary that may have been important in the origin of terrestrial life."
Scientists previously assumed that, because of its density, solid methane would sink on liquid methane. However, they have constructed a new model that takes into account the atmosphere and which adds pockets of nitrogen gas in the ice blocks, and which also takes into account changes in temperature. The results 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 (90.4 Kelvins). Provided the ice had 5% or more "air" (which on Titan has a significantly higher percentage of nitrogen), which is what young sea ice on Earth has, then it would float.
If the temperature falls by a few degrees the ice will sink because of the relative proportions of nitrogen gas in the liquid versus the solid. The paper suggests that where the temperature is close to the freezing point of methane there could be both floating and sinking ice – a hydrocarbon ice crust above the liquid and blocks of the ice on the bottom of the lake or sea.
"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," said Jason Hofgartner, lead author of the research and a Natural Sciences and Engineering Research Council of Canada scholar at Cornell University. "We'll want to take these conditions into consideration if we ever decide to explore the Titan surface some day."
This Cassini image of Titan shows its thick atmosphere as a blue haze. Credit: NASA/JPL/Space Science Institute
Scientists now hope to use Cassini's radar instrument to test their model by monitoring reflectivity off the surface of the lakes during seasonal changes. A sea or lake with floating ice should reflect more than during a warmer season when the lake's surface would be pure liquid and appear darker to Cassini's radar. Cassini project scientist Linda Spilker commented "Cassini's extended stay in the Saturn system gives us an unprecedented opportunity to watch the effects of seasonal change at Titan. We'll have an opportunity to see if the theories are right."
Cassini, which continues to provide much information and stunning images of Saturn, its rings and its moons, was launched in 1997 and entered Saturn's orbit in 2004.
Cassini also carried the European Space Agency's Huygens probe which landed successfully on Titan on January 14, 2005. The Huygens team were relieved and delighted when the small spacecraft landed and started sending back the first data about Titan's surface. The European Space Agency have released a video celebrating the 8th anniversary of the landing: watch Huygens video.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory (JPL) based in Pasadena, California, a division of the California Institute of Technology, manages the overall mission for NASA.