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Frictional
Heating Explains Plumes on Enceladus
05/16/07
For
Hi-Res and complete caption
Shear
Heating on Enceladus
Credit:
NASA / JPL
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Rubbing your
hands together on a cold day generates a bit of heat, and the
same process of frictional heating may be what powers the geysers
jetting out from the surface of Saturn's moon Enceladus.
Tidal
forces acting on fault lines in the moon's icy shell cause the
sides of the faults to rub back and forth against each other,
producing enough heat to transform some of the ice into plumes of
water vapor and ice crystals, according to a new study published
in the May 17 issue of the journal Nature.
Francis Nimmo,
assistant professor of Earth and planetary sciences at the
University of California, Santa Cruz, and his co-authors
calculated the amount of heat that could be generated by this
mechanism and concluded that it is the most likely explanation
for the plumes and other features observed in the south polar
region of Enceladus. This region is warmer than the rest of the
frozen surface of Enceladus and has features called "tiger
stripes" that look like tectonic fault lines.
"We
think the tiger stripes are the source of the plumes, and we made
predictions of where the tiger stripes should be hottest that can
be tested by future measurements," Nimmo said.
Driving
the whole process is the moon's eccentric orbit, which brings it
close to Saturn and then farther away, so that the gravitational
attraction it feels changes over time.
"It's getting
squeezed and stretched as it goes around Saturn, and those tidal
forces cause the faults to move back and forth," Nimmo
said.
Unlike some other proposals for the origin of the
plumes, this mechanism does not require the presence of liquid
water near the surface of Enceladus, noted co-author Robert
Pappalardo of NASA's Jet Propulsion Laboratory in Pasadena,
Calif.
"The heat is sufficient to cause ice to
sublimate, like in a comet -- the ice evaporates into vapor, and
the escaping vapor drags particles off into space,"
Pappalardo said.
The study does suggest, however, that
Enceladus has a liquid ocean lying deep beneath the ice. That
allows the ice shell to deform enough to produce the necessary
movement in the faults. If the ice shell sat directly on top of
the moon's rocky interior, tidal forces would not produce enough
movement in the faults to generate heat, Nimmo said.
The
frictional, or "shear heating," mechanism is consistent
with an earlier study by Nimmo and Pappalardo which proposed that
Enceladus reoriented itself to position its hot spot at the south
pole . In that study, the researchers described how the
reorientation of Enceladus would result from a lower density of
the thick ice shell in this region.
In the new paper, the
researchers estimated the thickness of the ice shell to be at
least 5 kilometers (3 miles) and probably several tens of
kilometers or miles. They also estimated that the movement along
the fault lines is about half a meter over the course of a tidal
period.
In addition to Nimmo and Pappalardo, the
co-authors of the paper include John Spencer of the Southwest
Research Institute in Boulder, Colo., and McCall Mullen of the
University of Colorado, Boulder. This study was funded by NASA's
Planetary Geology and Geophysics and Outer Planets research
programs.
Enceladus has sparked great interest among
scientists, particularly since the discovery more than a year ago
by NASA's Cassini spacecraft of the geysers shooting off its
surface. This is one of two papers about Enceladus appearing in
the May 17 issue of Nature. In the other paper, scientists
explain how cracks in the icy surface of Enceladus open and close
under Saturn's pull. Saturn's tides could control the timing of
the geyser's eruptions, researchers suggest.
The
Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. NASA¿s
Jet Propulsion Laboratory, a division of Caltech, manages the
Cassini-Huygens mission for NASA's Science Mission Directorate,
Washington. The Cassini orbiter and its two onboard cameras were
designed, developed and assembled at JPL.
For Related
Article:
Cracks
on Enceladus Open and Close Under Saturn's Pull
Source:
NASA / JPL / University of Santa Cruz
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