NASA scientists believe that icebergs might float on the lakes and seas of Saturn's icy moon Titan - and the ice could harbour strange new forms of life.
Unexplained readings from the Cassini probe, which orbits Saturn, hint that blocks of ice might bob on the surface of the lakes of liquid hydrocarbons on Titan.
Titan is the only other body besides Earth in our solar system with stable bodies of liquid on its surface.
If life does lurk in icebergs on the moon, it will be very different from life on Earth - the lakes are composed of liquid ethane and methane. Titan's surface temperature is thought to be around -180 degrees Centigrade.
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"One of the most intriguing questions about these lakes and seas is whether they might host an exotic form of life," said Jonathan Lunine, a paper co-author and Cassini interdisciplinary Titan scientist at Cornell University, Ithaca, N.Y. "
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."
Ethane and methane are organic molecules, which scientists think can be building blocks for the more complex chemistry from which life arose.
Cassini has seen a vast network of these hydrocarbon seas cover Titan's northern hemisphere, while a more sporadic set of lakes bejewels the southern hemisphere.
Up to this point, Cassini scientists assumed that Titan lakes would not have floating ice, because solid methane is denser than liquid methane and would sink.
The result, scientists found, is 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.
Scientists haven't worked out what color the ice would be, though they suspect it would be colorless, as it is on Earth, perhaps tinted reddish-brown from 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," said Jason Hofgartner, first author on the paper and 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."
Cassini's radar instrument will be able to test this model by watching what happens to the reflectivity of the surface of these lakes and seas.
A hydrocarbon lake warming in the early spring thaw, as the northern lakes of Titan have begun to do, may become more reflective as ice rises to the surface.
As the weather turns warmer and the ice melts, the lake surface will be pure liquid, and will appear to the Cassini radar to darken.
"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, Pasadena, Calif. "We'll have an opportunity to see if the theories are right."