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| This artist's concept envisions what hydrocarbon ice forming on a liquid hydrocarbon sea of Saturn's moon Titan might look like. Image credit: NASA/JPL-Caltech/USGS |
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."
The Cassini-Huygens mission is a cooperative project of NASA,
the European Space Agency and ASI, the Italian Space Agency. NASA's Jet Propulsion
Laboratory, a division of the California Institute of Technology in Pasadena,
manages the mission for NASA's Science Mission Directorate, Washington.
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