Icy satellite shell thickening: consequences for non-synchronous rotation rates and stresses

Details Icy satellite shell thickening: consequences for non-synchronous rotation rates and stresses Icy satellite shell thickening: consequences for non-synchronous rotation rates and stresses

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p22a..05n&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P22A-05

Mathematics

Logic

5450 Orbital And Rotational Dynamics (1221), 5455 Origin And Evolution, 5475 Tectonics (8149)

Scientific paper

Satellites with subsurface oceans have likely experienced shell thickening over time [e.g. 1]; on the basis of geological observations, shell thickening may have occurred on Europa [2]. This shell thickening can have direct tectonic consequences [3]; less obviously, the rate of non-synchronous rotation (NSR) will also be affected. Our approach unifies two previous treatments of NSR [4,5] and shows that the NSR rate will first decrease, and then increase, as the shell progressively thickens. The thickness of the brittle layer in which failure occurs due to NSR stresses will increase monotonically with time; such behaviour is potentially detectable using geological observations. Whether or not a floating ice shell undergoes NSR depends on lateral variations in shell thickness [4,5]. These variations occur as a result of spatial variations in tidal heating [4] but will be smoothed out by lateral flow of the ice [6]. Both tidal heating and lateral ice flow depend on the viscosity of the ice at the base of the shell. At low shell thicknesses, the ice will be conductive and static; lateral ice shell thickness variations will result as long as tidal heating in the shell dominates other energy sources. Under these circumstances, the shell will undergo NSR on a timescale controlled by the conductive timescale of the shell [4]; the rate of NSR will decrease as the shell thickens. At higher shell thicknesses, the ice will convect, but long-wavelength lateral flow of ice will still be relatively slow [6] and lateral thickness variations may remain. Finally, at slightly higher shell thicknesses, lateral flow of ice will be rapid, and NSR will also be rapid and dependent on this relaxation time, the tidal dissipation factor and orbital eccentricity [5]. For a Maxwell viscoelastic material, the stresses generated by NSR are proportional to the rotation rate and are sufficiently high to make brittle failure likely. The depth to which brittle failure extends depends on both the rotation rate and the ice shell temperature structure. For a nominal Europan ice grain size of 1mm, a 10 km thick conductive shell will result in an NSR timescale of ~3 Myr and a 25 km thick convecting shell in an NSR timescale of ~0.1 Myr. The resulting stresses and brittle layer thicknesses are in both cases ~1 MPa and 1-2 km, respectively. These NSR stresses exceed the diurnal stresses and are comparable to those due to the shell-thickening effect [3]; the stress patterns will depend on the superimposition of these different mechanisms. An increase in brittle layer thickness with time is consistent with Europa's inferred geological history [2]. However, the theory is applicable to any satellite with a surface shell, decoupled from the interior, that has thickened (or thinned) with time. [1] H. Hussmann, T. Spohn, LPI Contrib. 1195, 7012, 2004. [2] P. Figueredo, R. Greeley, Icarus 167, 287-312, 2004. [3] F. Nimmo, JGR 109, E12001, 2004 [4] G.W. Ojakangas, D. Stevenson, Icarus 81, 220-241, 1989. [5] R. Greenberg, S.J. Weidenschilling, Icarus 58, 186-196, 1984. [6] Stevenson, D.J., LPSC 31, 1506, 2000.

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