Frictional Heating Explains Plumes on Enceladus
- From: baalke@xxxxxxxxxxxxx
- Date: 16 May 2007 18:23:01 -0700
http://www.jpl.nasa.gov/news/news.cfm?release=2007-060
Frictional Heating Explains Plumes on Enceladus
Jet Propulsion Laboratory
May 16, 2007
Pasadena, Calif.--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 (see
earlier press release at http://press.ucsc.edu/text.asp?pid=878). 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.
More information on the Cassini mission is available at
http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .
------------------------------------------------------------------------
Media contact: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
Tim Stephens 831-459-2495
University of Santa Cruz, Calif.
2007-060
.
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