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Enigmatic Enceladus

Geoscientist Online 21 May 2007

NASA’s Saturn orbiter Cassini has observed watery eruptions on icy moon Enceladus since it made several close flybys in early 2005. Two groups of scientists have formulated complementary ideas on how this venting occurs, reports Emily Baldwin.

Saturn's moon Enceladus, as seen by the orbiter Cassini. NASA/Cassini Imaging Team. The moon is c. 500km in diameter.The orbiter Cassini has observed plumes - composed mostly of water - apparently arising from a set of near-parallel cracks that run for about 130km across the moon’s southern polar region. They have a higher temperature than the surrounding terrain, implying that heat is being produced along fault lines. Two new studies, published as two separate papers in the May 17 edition of Nature, provide complementary models of how gravitational forces might help to form the plumes of vapour spouting from these cracks. Nimmo et al. show that a shearing motion along the tiger stripes - caused by tidal forces - provides the energy for the heating; while Hurford et al. consider how forces across the stripes might open and close the vents.

Enceladus is just 500km in diameter and orbits Saturn with a period of 1.37 days between neighbouring moons Mimas and Tethys. Its proximity to Saturn should have naturally brought Enceladus into a circular orbit; however, the influence of another nearby moon - Dione - has forced Enceladus into an eccentric orbit. This forces the moon first closer to Saturn and then further away, so that the gravitational attractions it feels change over time. This eccentric orbit is thought to be driving the whole process that leads to the formation of the plumes – a process similar to that affecting Jupiter’s moon Io, the most volcanically active body in the Solar System. In the case of Enceladus, however, the tidal heating is concentrated around the moon’s southern pole, with predictable patterns of stress cycling across its surface on each orbit.

The daily tides act on the tiger stripes and cause the sides of the faults to rub back and forth against each other laterally for about 0.5 metres, producing enough heat to transform some of the ice into plumes of water vapour. The faults are essentially being squeezed together and then stretched apart so that they alternate between being in a state of tension or compression during the two halves of the moon’s orbit. Other tiger stripes are thought to experience different rates of long-term heating due to the spatial variation in tidal stresses and the stripes' orientation.

The plume activity may even inject material into one of Saturn’s rings – the E ring, affecting its formation and structure over time. There are also implications for Enceladus's internal structure: the stresses controlling the eruptions imply that the shell behaves as a thin elastic layer, perhaps only a few tens of kilometres thick, with a liquid water ocean below the icy "crust". This configuration allows the ice shell to deform enough to promote the movement observed along the fault lines. Nimmo et al. speculate that if the ice lay directly on top of a rocky interior then the tidal forces would not be able to produce enough movement to generate the observed temperatures.

Both ideas presented in these papers provide new directions for understanding the enigmatic Enceladus with separate, but complimentary investigations of the forces acting on the tiger stripes. Many questions remain unanswered questions, and the models only describe how the satellite is behaving – rather than explaining why. One particular mystery that still remains is why Enceladus is so dynamic, in direct contrast to its neighbour Mimas, which has a closer orbit to Saturn and even larger eccentricity, but yet has remained totally inactive.

Refs: Shear heating as the origin of the plumes and heat flux on Enceladus. F. Nimmo, J R Spencer, R T Pappalardo & M E Mullen. Eruptions arising from tidally controlled periodic openings of rifts on Enceladus. T A Hurford, P Helfenstein, G V Hoppa, R Greenberg & B G Bills