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Scientists propose alternate model for plume on Enceladus
James E.
Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@uiuc.edu
Released
12/14/06
|
CHAMPAIGN, Ill. —
What’s causing all the commotion on Enceladus?
Last year,
when the Cassini spacecraft discovered an enormous plume erupting on
Enceladus, one of Saturn’s moons, scientists
speculated that liquid water lay at shallow depths beneath the
icy surface.
Now, as reported in the Dec. 15 issue of the journal Science, researchers
have proposed an alternate model to account for this spectacular plume.
“With a diameter of only 300 miles, Enceladus is a tiny moon;
it would fit easily between Los Angeles and San Francisco,” said
Susan Kieffer, a geology professor
and planetary scientist at the University of Illinois at Urbana-Champaign,
and lead author of the Science paper. “This tiny satellite should
be cold and inactive, like our own moon. But it isn’t.”
The surface of Enceladus is composed of water ice with traces of carbon
dioxide. Part of this surface does appear old and cratered like Earth’s
moon, Kieffer said. “The south polar region, however, is geologically
active, with many surface features, indicating current activity.”
Kieffer, who holds a Charles R. Walgreen Jr. Chair at the U. of I.,
has studied geysers and volcanoes on Earth; on Io, a satellite of Jupiter;
and on Triton, a satellite of Neptune.
Instruments on the Cassini spacecraft revealed a gigantic plume of
gas, water vapor and ice particles erupting from Enceladus’ surface.
Some of the ice escapes the moon’s feeble grasp and replenishes
a ring of ice particles around Saturn, called the “E ring.”
Initial reports speculated that chambers of liquid water lay close
to the moon’s surface and erupted in a giant geyser. The water would
be near freezing, so scientists dubbed the model “Cold Faithful,”
after the familiar, but hotter, Old Faithful geyser in Yellowstone
National Park.
“A problem with this model,” Kieffer said, “is that
10 percent of the plume consists of the gases carbon dioxide, nitrogen
and methane. You might get a carbon dioxide-driven liquid geyser there,
but you can’t put this much nitrogen and methane into liquid
water at the low pressures found inside Enceladus.”
Nitrogen and methane are nearly insoluble in liquid water, but highly
soluble in frozen water – in an ice phase called clathrate. When
clathrate is exposed to a vacuum, the gas molecules burst out, ripping
the ice lattice to shreds and carrying the fragments away.
Kieffer and colleagues have proposed an alternate model to explain
the plume on Enceladus. The gases in the plume, they propose, are dissolved
in a reservoir of clathrate under the water ice cap in the south polar
region. The clathrate model allows an environment that would be 80
to 100 degrees Celsius colder than liquid water, with a “Frigid Faithful”
plume emanating from clathrates, rather than from liquid water reservoirs.
“Exposed to near-vacuum conditions by fractures at the south pole,
the clathrates decompose violently, spewing out nitrogen, methane and
carbon dioxide gases, and ice particles; as well as leaving fracture
walls coated with water ice,” said Kieffer, who is also a professor
in the university’s Center for
Advanced Study, one of the highest forms of campus recognition.
“Some ice particles and ice coatings evaporate to produce the
water vapor observed with the other gases,” she said.
Active tectonic processes at the south pole cause continuous formation
of cracks in the ice, through which many separate vents create a plume.
The total discharge is comparable to that of Old Faithful, but the plume
is enormously bigger because it is erupting at very low gravity into
the near vacuum of space.
“We propose that cracks in Enceladus’ ice cap may be opening
and closing continuously, producing the spectacular plume we see reaching
high above Enceladus’ surface,” Kieffer said. “Even
if conditions are as cold as our model suggests, there is no problem
launching ice particles into Saturn’s E-ring.”
The other authors of the paper besides Kieffer are postdoctoral researcher
Xinli Lu and geologists Craig Bethke and Steve Marshak at the U. of
I., planetary scientist John Spencer at the Southwest Research Institute,
and chemist Alexandra Navrotsky at the University of California at Davis.
The work was funded by the National Aeronautics and Space Administration.
Editor’s note: To reach Susan Kieffer, call 217-244-6206; e-mail: skieffer@illinois.edu.
