Frictional heating explains plumes on Saturn's moon Enceladus(Forwarded)

University of California-Santa Cruz

Contact:
Tim Stephens, (831) 459-2495

May 16, 2007

Frictional heating explains plumes on Saturn's moon Enceladus

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 coauthors 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 coauthor Robert Pappalardo of NASA's Jet Propulsion
Laboratory in Pasadena.

"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, in which they proposed that Enceladus
reoriented itself to position the hot spot at the south pole (see earlier
press release, 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 coauthors of the paper include
John Spencer of the Southwest Research Institute in Boulder, Colorado, 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
geysers' eruptions, researchers suggest.

The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency, and the Italian Space Agency. More information on the
Cassini mission is available online from NASA and JPL.

Note to reporters:
You may contact Nimmo at (831) 459-1783 and Pappalardo at (818) 354-5837.