Other
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.3007h&link_type=abstract
American Astronomical Society, DPS meeting #38, #30.07; Bulletin of the American Astronomical Society, Vol. 38, p.538
Other
Scientific paper
Europa's icy shell has experienced extensive resurfacing due to extensional tectonic events arising from stresses >1; MPa for a nominal shell thickness of 10 km. However, known stress sources - including diurnal tidal flexing and non-synchronous rotation (NSR) - may not be sufficient to generate such large stresses. Although the accumulation of NSR may generate relatively large stresses of 0.1 MPa per degree of rotation, this state is not dynamically viable if Europa's shell possesses even a tiny mass asymmetry. We calculate the size of this minimum asymmetry (the quadrupole moment) to be α0 ≡ (B - A)/C ≈ 10-5 for a McDonald ("tidal lag") model, and α0 ≈ 10-6 using a Darwin (Fourier decomposition) tidal model. An asymmetry of this size may be approximated by two diametrically opposed (rigidly supported) mass anomalies of 1012 kg. As a comparison, this is five orders of magnitude smaller than recently reported mass anomalies on Ganymede. On the other hand, if the global asymmetry is expressed as long-wavelength shell thickness variations, isostatic compensation produces membrane stresses, which we calculate to be 1 MPa for a nominal 10 km shell thickness. These are predicted to persist in the brittle upper layers on a timescale greater than the surface age of 30-80 Myr. Thus, the extensional features observed on Europa's highly fractured surface may be generated by diurnal tidal stresses operating in concert with membrane stresses, which have similar geometries. Europa's actual rotation state and history are constrained by comparing known (relative) topography and the predictions of published thermal models with the above considerations. We predict that if Europa's rigidly-supported asymmetry is greater than α0, the shell rotates synchronously. Otherwise, thermal re-equilibration of the shell causes NSR with a period >107 yr, modulated by the influence of minor anomalies.
Hayne Paul O.
Lissauer Jack . J.
Sleep Norman H.
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