Here on Earth as we know it, we have tectonic plates, grinding and gnashing their way to become earthquakes, volcanoes, and tsunamis. On Jupiter's closest moon Io, there are volcanoes due to the intense tidal effects of Jupiter. Io's interior tidal heating provides enough heat energy to cause volcanic activity on that moon even though it is relatively small.
But Saturn's moon, Enceladus, continues to intrigue scientists, because it may contain enough interior friction to heat ice, creating huge geysers that shoot far into space. But where and how do these geysers of ice originate?
Scientists studying the Cassini spacecraft images have observed a series of 120-kilometer (75-mile) long cracks in the south polar region of Enceladus, which were nicknamed "tiger stripes" because they resemble a tiger's distinctive marks. Cassini observations show that the stripes are much warmer than their surroundings, so scientists believe they are the source of the eruptions. The Cassini observations also show the plumes consist of water vapor, so there is evidence for heating and perhaps melting of the ice. Since liquid water is necessary to support known forms of life, Enceladus has become a promising place to look for extraterrestrial life.
Enceladus has an egg-shaped orbit, so the moon's distance from Saturn changes regularly as it travels in its orbit. When Enceladus is closer to Saturn, the pull of Saturn's gravity is stronger, creating a larger tide; and when Enceladus is farther away, the pull is weaker, creating a smaller tide. The moon also rocks back-and-forth slightly as it orbits. These two effects combine to produce an ever-changing stress field that continuously affects the tiger stripes.
According to Dr. Terry Hurford of NASA's Goddard Space Flight Center, the way the tiger stripes are oriented on the moon's surface when Enceladus is farthest from Saturn pull most of them open, and when Enceladus is closest to Saturn, the stresses force most of them to close, which exposes water vapor to the vacuum of space.
Furthermore, the Imaging Science Subsystem (ISS) camera onboard the Cassini spacecraft has also revealed that the 4 tiger stripes are very young in age because virtually no craters have been found on or even near the stripes. And, compared to the rest of the icy moon, Cassini's Composite Infrared Spectrometer (CIRS) instrument shows that they are hot.
The moon's "hottest measured temperatures are at the tiger stripes," says planetary scientist Dr. Robert Pappalardo of NASA's Jet Propulsion Laboratory in Pasadena, California.
According to Pappalardo and his colleagues, Enceladus' exaggerated elliptical orbit causes the moon's icy crust to flex at these fault lines. As huge pieces of crust rub together, they generate enough heat to evaporate the ice lurking within the faults, in the same way that comets sprout tails when they veer too close to the sun's heat.
In addition, Enceladus might have a vast, underground ocean that allows Enceladus to be more easily stretched by tidal forces so that it would produce enough heat to sprout geysers.
Currently, JPL scientists Dr. Bridget Smith-Konter and Pappalardo are creating a 3D deformation model that maps tidally induced tiger stripe fault stresses over an entire orbital cycle of Enceladus.
Calculations from this model indicate that while the entire tiger stripe fault system likely remains "locked" (unable to move) throughout most of the tidal cycle, conditions for shear failure may exist for a brief period of time when Enceladus is farthest from Saturn. The sequence of stress build-up and release primarily depends on the orientation of the tiger stripe faults, the depth of the faults, and friction along the faults. The scientists plan to compile a suite of time-dependent models that simulate these stress behaviors to investigate possible failure scenarios and stress interactions among the tiger stripe fault system.
Because of the possible presence of liquid water, geologic activity, and the possibility for tidally driven faulting at the tiger stripes, Enceladus will intrigue scientists in solar system exploration for decades to come.
T. A. Hurford, P. Helfenstein, G. V. Hoppa, R. Greenberg & B. G. Bills (2007), Eruptions arising from tidally controlled periodic openings of rifts on Enceladus, Nature, 447, 292-294.
F. Nimmo, J. R. Spencer, R. T. Pappalardo & M. E. Mullen (2007), Shear heating as the origin of the plumes and heat flux on Enceladus, Nature, 447, 289-291.
Porco, C. C. et al. Cassini observes the active south pole of Enceladus. Science, 311, 1393-1401 (2006).
Spencer, J. R. et al. (2006), Cassini encounters Enceladus: Background and the discovery of a south polar hot spot, Science, 311, 1401-1405.
Bridget R. Smith-Konter, Zane Crawford, and Robert T. Pappalardo (2007), Tidally driven fault deformation and stress accumulation at Enceladus's tiger stripes, Workshop on Ices, Oceans, and Fire, Lunar and Planetary Institute, Houston, TX, abstract #6062.
Last Updated: 7 June 2010