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| Geological Evidence that Enceladus Librates about Synchronous Rotation |
| Presentation Time: Monday, 9:10 a.m. - 9:20 a.m. |
Terry Hurford1, B. G. Bills2, P. Helfenstein3, D. P. Hamilton4, G. V. Hoppa5, R. Greenberg6, C. M. Purser7
1NASA-GSFC, 2NASA-GSFC; SCRIPPS Institute of Oceanography, 3Cornell University, 4Univ. of Maryland, 5Raytheon, 6LPL. - University of Arizona, 7Eleanor Rooosevelt High School. |
| Presentation Number: 02.05 |
Enceladus is thermally active even though radiation from its surface was expected to have cooled it long ago. Radiogenic heating (Schubert et al., 1986) and tidal heating (Porco et al., 2006), resulting from its small orbital eccentricity, appear inadequate to explain observations of heat flow from its surface (Spencer et al., 2006). Additional heat can be generated if Enceladus librates, or departs from a uniform rate of rotation, due to resonant enhancement of its response to gravitational torques from Saturn (Wisdom, 2004). This forced libration has not yet been directly observed, but an upper limit of 1.5 degrees has been placed on its amplitude (Porco et al., 2006).
Libration would affect the diurnal stresses produced by tides on its surface. Tidal stresses due to orbital eccentricity should generate strike-slip displacements, which, in the absence of a forced libration, would be right-lateral in sense for all faults in the south polar region according to the tidal walking theory (Hoppa et al., 1999). Strike-slip displacement generated in the presence of a forced libration allows for left-lateral strike-slip displacement there, which is consistent with our observations of left-lateral strike-slip displacements, thus providing evidence that Enceladus may librate. In order to produce the observed left-lateral displacement in the south polar region the librational amplitude must be at least 0.54 degrees.
Tidal stress, incorporating Enceladus' libration, may have impacted the formation and subsequent evolution of other features in the tectonic record. Moreover, tidal stresses due to both orbital eccentricity and the libration may control the timing and location of jets from rifts near the south pole (Hurford et al., 2007), and may generate at least some heat along these faults (Nimmo et al., 2007). Until direct observations of Enceladus' libration are possible, geologic observations combined with tidal theory provide the strongest evidence for its existence. |
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