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| Titel |
Effect of obliquity on viscoelastic deformation and stresses at Europa's surface |
| VerfasserIn |
Hermes Miguel Jara Orue, Bert L. A. Vermeersen |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052553
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| Zusammenfassung |
| In this study we present a semi-analytical Maxwell-viscoelastic representation of surface
diurnal deformations and stresses at Europa’s surface. Europa’s shell experiences forces on a
diurnal scale as a result of the orbit’s eccentricity, the spin axis’ obliquity and physical
librations. Here we take into account the first two effects. We show that a small
obliquity (< 1°) can have serious effects on deformations and stresses, mainly
by disrupting the symmetric distribution of deformation and stress patterns with
respect to the equator. The asymmetry in radial deformation is caused by the periodic
displacement of the tidal bulge in north-south direction, which is largest when Europa is
located 90° away from the nodes formed by the intersection of Europa’s orbital and
equatorial planes. As an additional deformation mechanism, the obliquity tide also
leads to an increase of the amplitude of surface deformations. For obliquities larger
than 0.7°, tidal deformations might become 20% larger in amplitude (~ 35 m)
than in the zero-obliquity case (~ 29 m). Although the effect on the amplitude
becomes rather small for obliquities smaller than ~ 0.25°, local differences in
magnitude of nearly 3 meters can take place even for an obliquity of 0.1°. Therefore, it
will be important to accuraletely determine the position of Europa’s spin axis in
space before using future altimetric measurements of Europa’s tidal deformation
to retrieve some information about the thickness, rigidity and viscosity of the icy
shell.
The influence of a small, but non-zero, obliquity on tidal deformations directly affects the
diurnal stress field at the surface. Tensile and compressive regions on the surface are shifted
in north-south direction, thereby changing the usual orientation of diurnal stress patterns. Due
to the asymmetric distribution of diurnal stresses with respect to the equator, the possible
existence of a non-zero obliquity has been recently used to improve the modeling of Europa’s
cycloidal features and strike-slip faults. This modeling, however, does not take into account
the effect of viscoelasticity in the plausible case that the lowest portion of Europa’s ice shell
shows strong dissipation.
In order to model the effect of viscoelasticity, we assume that Europa’s interior is
differentiated in five homogeneous material layers: a liquid metallic core, a silicate mantle, a
subsurface water ocean, a low-viscous and warm ice layer (asthenosphere, viscosity
1012 - 1017 Pa-
s), and a high-viscous and cold upper ice layer (lithosphere, viscosity 1021
Pa-
s). Viscoelasticity only affects the surface stress patterns through the viscoelastic response
of the interior to tides. The importance of the viscoelastic response on surface stresses is
proportional to the ratio between the relaxation time (Ïj) of a given viscoelastic mode and the
period of the tidal excitation force (i.e. one orbital period). On a diurnal timescale, the fast
relaxation of transient modes related to the low viscosity of the asthenosphere can alter the
magnitude and phase shift of the diurnal stress field at Europa’s surface. The effects
are largest, up to 20% in magnitude and 7° in phase for ice rigidities lower than
3.487 GPa, when the value of Ïj corresponding to the aforementioned transient
modes approaches the inverse of the average angular rate of Europa’s orbit (i.e.
1-n). |
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