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Modeling stresses on satellites due to nonsynchronous rotation and orbital eccentricity using gravitational potential theory |
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| Authors: | John Wahr Zane A Selvans Amy C Barr Michelle M Selvans |
| Institution: | a Department of Physics, University of Colorado, UCB 390, Boulder, CO 80309-0390, USA b Laboratory for Atmospheric and Space Physics and the NASA Astrobiology Institute, University of Colorado, UCB 392, Boulder, CO 80309-0392, USA c Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA d Department of Physics and Astronomy, Wheaton College, 26 East Main Street, Norton, MA 02766-2322, USA e Division of Geological and Planetary Sciences, California Institute of Technology, MC 252-21, Pasadena, CA 91125, USA f Jet Propulsion Laboratory, California Institute of Technology, M/S 183-301, 4800 Oak Grove Dr., Pasadena, CA 91109, USA |
| Abstract: | The tidal stress at the surface of a satellite is derived from the gravitational potential of the satellite's parent planet, assuming that the satellite is fully differentiated into a silicate core, a global subsurface ocean, and a decoupled, viscoelastic lithospheric shell. We consider two types of time variability for the tidal force acting on the shell: one caused by the satellite's eccentric orbit within the planet's gravitational field (diurnal tides), and one due to nonsynchronous rotation (NSR) of the shell relative to the satellite's core, which is presumed to be tidally locked. In calculating surface stresses, this method allows the Love numbers h and ?, describing the satellite's tidal response, to be specified independently; it allows the use of frequency-dependent viscoelastic rheologies (e.g. a Maxwell solid); and its mathematical form is amenable to the inclusion of stresses due to individual tides. The lithosphere can respond to NSR forcing either viscously or elastically depending on the value of the parameter  , where μ and η are the shear modulus and viscosity of the shell respectively, and ω is the NSR forcing frequency. Δ is proportional to the ratio of the forcing period to the viscous relaxation time. When Δ?1 the response is nearly fluid; when Δ?1 it is nearly elastic. In the elastic case, tensile stresses due to NSR on Europa can be as large as ∼3.3 MPa, which dominate the ∼50 kPa stresses predicted to result from Europa's diurnal tides. The faster the viscous relaxation the smaller the NSR stresses, such that diurnal stresses dominate when Δ?100. Given the uncertainty in current estimates of the NSR period and of the viscosity of Europa's ice shell, it is unclear which tide should be dominant. For Europa, tidal stresses are relatively insensitive both to the rheological structure beneath the ice layer and to the thickness of the icy shell. The phase shift between the tidal potential and the resulting stresses increases with Δ. This shift can displace the NSR stresses longitudinally by as much as 45° in the direction opposite of the satellite's rotation. |
| Keywords: | Europa Satellites general Tides solid body Tectonics Geophysics |
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