Cassini Data Suggests Blocks of Hydrocarbon Floating on Titan's Lakes
It's not exactly icing on a cake, but it could be icing on a lake. A new
paper by scientists on NASA's Cassini mission finds that blocks of
hydrocarbon ice might decorate the surface of existing lakes and seas of
liquid hydrocarbon on Saturn's moon Titan. The presence of ice floes
might explain some of the mixed readings Cassini has seen in the
reflectivity of the surfaces of lakes on Titan.
"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. "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."
Titan is the only other body besides Earth in our solar system with
stable bodies of liquid on its surface. But while our planet's cycle of
precipitation and evaporation involves water, Titan's cycle involves
hydrocarbons like ethane and methane. 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. But the new model considers the interaction between the
lakes and the atmosphere, resulting in different mixtures of
compositions, pockets of nitrogen gas, and changes in temperature. 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 -- minus 297 degrees Fahrenheit (90.4
Kelvins). The scientists realized all the varieties of ice they
considered would float if they were composed of at least 5 percent
"air," which is an average composition for young sea ice on Earth.
("Air" on Titan has significantly more nitrogen than Earth air and
almost no oxygen.)
If the temperature drops by just a few degrees, the ice will sink
because of the relative proportions of nitrogen gas in the liquid versus
the solid. Temperatures close to the freezing point of methane could
lead to both floating and sinking ice - that is, a hydrocarbon ice crust
above the liquid and blocks of hydrocarbon ice on the bottom of the
lake bed. Scientists haven't entirely figured 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 some day."
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. This would provide a rougher surface quality that
reflects more radio energy back to Cassini, making it look brighter. 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."